Eating Habits and Sustainable Food Production in the Development of Innovative “Healthy” Snacks (Running Title: Innovative and “Healthy” Snacks)

Abstract

In recent years, science about nutrition and food technology has grown enormously. These advances have provided information about the human body’s need for certain nutrients and the impact of human nutrition on quality of life and health. New technologies enable the production of many new products that meet the expectations of food consumers. To meet the challenges posed by consumers, food producers are developing new food products that are included in the next generation food. Changing nutritional trends force the food industry and technologists to look for innovative products that are not only ready for immediate consumption, but are also unique in terms of nutritional value and contain a minimum number of additives. Existing research trends are intended to develop innovative products, which can be considered a healthy snack that can help in the fight against obesity, especially among children. Such products are freeze-dried fruit or vegetable gels, fruit skins or edible films. The aim of the work is to present a review of the problem of increasing childhood obesity, the place of snacks in the daily diet and the possibility of replacing unhealthy, high-calorie snacks with alternative products with beneficial properties, in which balanced production is used. For example, the use of freeze-drying and the addition of only natural hydrocolloids provides an “clean label” healthy snack that is appreciated by conscious consumers.

Full text

sustainability
Review
Eating Habits and Sustainable Food Production in the
Development of Innovative “Healthy” Snacks
(Running Title: Innovative and “Healthy” Snacks)
Agnieszka Ciurzy ´nska , Piotr Cie´sluk, Magdalena Barwi ´nska, Weronika Marczak,
Agnieszka Ordyniak, Andrzej Lenart and Monika Janowicz *
Department of Food Engineering and Process Management, Faculty of Food Sciences, Warsaw University
of Life Sciences, SGGW, 02-787 Warszawa, Poland; agnieszka_ciurzynska@sggw.pl (A.C.);
piotrcieslukk@gmail.com (P.C.); magdalenabarwinska95@gmail.com (M.B.); wermar111@wp.pl (W.M.);
agnieszkajordyniak@gmail.com (A.O.); andrzej_lenart@sggw.pl (A.L.)
*Correspondence: monika_janowicz@sggw.pl; Tel.: +48-022-593-7566
Received: 27 April 2019; Accepted: 11 May 2019; Published: 16 May 2019


Abstract:
In recent years, science about nutrition and food technology has grown enormously. These
advances have provided information about the human body’s need for certain nutrients and the
impact of human nutrition on quality of life and health. New technologies enable the production of
many new products that meet the expectations of food consumers. To meet the challenges posed
by consumers, food producers are developing new food products that are included in the next
generation food. Changing nutritional trends force the food industry and technologists to look for
innovative products that are not only ready for immediate consumption, but are also unique in
terms of nutritional value and contain a minimum number of additives. Existing research trends are
intended to develop innovative products, which can be considered a healthy snack that can help in
the fight against obesity, especially among children. Such products are freeze-dried fruit or vegetable
gels, fruit skins or edible films. The aim of the work is to present a review of the problem of increasing
childhood obesity, the place of snacks in the daily diet and the possibility of replacing unhealthy,
high-calorie snacks with alternative products with beneficial properties, in which balanced production
is used. For example, the use of freeze-drying and the addition of only natural hydrocolloids provides
an “clean label” healthy snack that is appreciated by conscious consumers.
Keywords: sustainable food production; snacks; hydrocolloids; physical properties; freeze drying
1. Introduction
“We are what we eat.” This opinion appears not only in the “Philosophy of taste” of Anthelme
Brillat-Savarina but among many food researchers and technologists, and more and more often when
analyzing the impact of various products and food ingredients on the health of consumers [
1
,
2
].
Nutrition is a very important aspect of life because it affects its length, human health and productivity.
This is an extremely important issue if analyzing the specific group of recipients which are children.
Incorrect nutrition of children reduces immunity to diseases, inhibits physical and mental development,
while in adults it contributes to lifestyle diseases, e.g., obesity. Each organism requires a supply of
the right amount of energy, materials for building and regulating the metabolism, but if it is mainly
high-calorie food, our body will be undernourished despite feeling sated [3].
In 2016, the Institute of Food and Nutrition (a research and development institute established in
1963 that deals with the impact of food on human health and the determination of Polish standards and
nutritional recommendations) developed a new pyramid of “healthy nutrition and physical activity”
Sustainability 2019,11, 2800; doi:10.3390/su11102800 www.mdpi.com/journal/sustainability

Sustainability 2019,11, 2800 2 of 20
with physical activity located as the base. The second place is taken by vegetables and fruits, which
should provide up to 50% of consumed meals, while vegetables are recommended to be 75% and fruit
25%. The next segment is cereal products, among which the majority should be whole grain products.
The fourth place in the food pyramid is milk and dairy products. The fifth level includes meat products,
fish, legumes and eggs. Nutrition specialists indicate the need to reduce meat consumption in favor of
other products placed at the same level of the pyramid. The last place is taken by oils and nuts [4].
The Institute of Mother and Child is a research and development unit with a 60-year tradition,
which participates in solving health and social problems of mothers, children and adolescents who
depend on current social needs and state health policy, but at the same time are consistent with the
World Health Organization’s strategy in reproductive health and health and development of children
and youth and the public health policy of the European Union. According to the studies of the Institute
of Mother and Child carried out under the auspices of the WHO (World Health Organization), up to
31.2% of eight–year–olds in Poland have too high body mass, of which 12.7% are obese, and 18.5% are
overweight. On the other hand, underweight, mainly of a small degree, applies to 12.2% of children.
In the group of eight–year–olds, only 56.6% of children have normal body mass [
5
]. According to WHO
data in 2016, the number of children with severe and moderate overweight has increased tenfold in the
last 40 years (the trend continues), which may cause the generation of young children and adolescents
to be obese and at a high risk of disease. An important factor is the type and quality of food consumed.
The price of so-called “healthy food,” which is too high for many households, means that products that
are cheaper, but have a more adverse effect on health and the human body, are purchased more often.
One of the ideas to solve the problem of obesity is to reduce the consumption of cheap but processed
and nutrient-poor foods [6].
Research conducted by Mendyk et al. [
7
] showed that in the group of 114 pupils aged 13–18 years,
as much as 33.6% eat meals in places commonly named as “fast food” restaurants at least once a week
or more often, 61.1% do so several times a month, while those who do not eat there are, only 5.3%.
Most often they are boys.
Research conducted in a group of parents regarding the consumption of snacks by their children
showed that children most often eat sweets, such as chocolates, cookies, jellies, bars. The fruit came
in second with the indication of bananas and apples. Jelly, pudding, chips and fruit yoghurt was
mentioned in the last place [
8
]. Research shows that children also enjoy salty snacks, and the frequency
of their consumption increases with age. Children usually buy them themselves, however, parents
admit to the error of giving in to their requests when they shop together [9].
The aim of the work is to present the results of research on information about “healthy snacks”
presented in the literature and collected as part of surveys and to present the possibility of developing
a freeze-dried innovative healthy snack with the appropriately selected hydrocolloids whose addition
is to provide attractive physical and structural properties of the products obtained.
1.1. Definition of Snacks—Are There Any “Healthy Snacks”
Snacks are associated with products that are bad for health, but it is difficult to determine the effect
of “snacking” due to the problem of specifying the definition of “snacks” and “meal” [
10
]. There is no
single, coherent definition of snacks, so it is not clear whether an additional opportunity to eat should
be considered another meal or a snack [
11
]. Generally, a snack can be described as “snacking” on
small food products between main meals, or consumption of high-calorie beverages [
12
]. The product
defined in this way, seen against the background of a diet commonly considered healthy, which consists
of large meals consumed less often, is associated by the consumer with something harmful to health.
However, Rodriguez and Moreno [
13
] showed that there is no direct relation between the frequency of
consumption of food and harmful effects on human health, while the quality of small products treated
as snacks and consumed between large meals affects the feeling of satiety. It turns out that high-protein
products lengthen the satiety by 38 min in relation to high-carbohydrate foods [14].

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The next definition of snacks qualifies these products as food with a high sodium, high energy
value and low nutrient content [
15
]. Some definitions classify snacks according to individual times
of the day [
16
], others due to the type of food consumed, its quantity and place of consumption [
17
].
The most general definition specifies that snacks are both “healthy” and “unhealthy” products, provided
they are consumed between meals [
18
]. The US Department of Agriculture has developed a list of
2500 grain-based snacks (corn, rice and wheat), potatoes (crisps), meat (dried meat, sausage fingers),
fruit and fruit candy products. Precise information on the composition and nutritional value are given
only for 171 of these products [11].
The changing pace of life means that more and more often consumers eat in solitude or in the
so-called “meanwhile,” reaching just for snacks. On the other hand, research shows that snacks are
much more often purchased by large, minimum five-person families who buy them by 16% more than
smaller households. This is because a snack does not take time to prepare for consumption, making it
the most effective way to satisfy hunger even if at the same time makes us forget about maintaining
the principles of healthy eating [19].
The term “healthy snack” is not entirely correct. There is no such thing as “healthy food” because
it would mean that every other food is harmful to our body. To distinguish between “healthy” and
“unhealthy” snacks, research was conducted to show whether different snacks could be part of a healthy
diet. The assessment was based on the concept of nutrients balance. Products containing especially
high content of sodium and saturated fatty acids were considered as snacks which should be avoided,
and meat snacks were definitely rated bad in the study, whereas protein or muesli bars with various
additives were indicated as “healthy snacks”, containing a lot of health-beneficial ingredients [
11
].
This is surprising because other studies indicate that cereals, which are added to bars (especially
wheat based bars) are harmful to health, causing, for example, abdominal obesity. After eating them,
our brain produces exorphines from gluten, which have addictive properties and stimulate the appetite,
so people feel the need to continue eating not only products rich in cereals, but also other foods. High
blood sugar level causes the pancreas to produce excess insulin, which affects the accumulation of
visceral fat [20].
Snacks in the form of concentrated bars belong to the group of convenient or functional food.
“Healthy foods” are usually cereal and protein bars, which mainly consist of carbohydrates from
additives (chocolate, glucose syrup, sweeteners) and proteins (hydrolysates or whey protein isolates).
The fat content is 6–16.8% [
21
], in breakfast cereals investigated in Mahesar et al. [
22
] fat content was
23.6–26.1%, but most of them are saturated fats, not recommended in the daily diet [21,23].
The concept of a “healthy snack” varies from country to country. In Greenland, a snack is an
additional opportunity to eat a meal and it is recommended to consist of fruit, vegetables, crispy bread
or dried fish [
24
]. Swedish nutrition specialists recommend bread and margarine sandwiches, fruit,
milk and occasional sweets, while the French consider “healthy snacks” fruit, bread with butter and
jam, and raw vegetables [11]. In Switzerland, the most detailed list of various snacks was developed,
in which products were divided into those that should be consumed almost daily, from time to time,
not recommended for consumption, and high fat [
25
]. In Poland, there is no such list of recommended
snacks, but Hess et al. [
11
] have developed a table for different countries, where they included the
need for particular nutrients important for health. They indicate that the development of snacks
beneficial for the human body will be an increasingly important area of cooperation between food
producers and dietitians [
11
]. Ruxton and Derbyshire [
26
] pointed out that tools are needed to facilitate
the introduction of healthy eating habits, in particular, to increase the consumption of vegetable
products. There is a need for a new snack on the market which will not only save time for the consumer,
(to prepare the product for consumption and shorten the time of eating snacks), but also introduce
positive nutritional changes by enriching the diet with vegetables, the more so because a conscious
consumer is looking for a healthy alternative to classic snacks, thus forcing food producers to develop
new products. The creation of innovative snacks should also focus on supplementing confirmed
deficiencies of vitamins, nutrients, micro and macroelements occurring in a given population [11].

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1.2. The Possibilities of Elaboration of Innovative Snacks in the Trend of Sustainable Food Production
Poland is one of the largest producers of fruit and vegetables in the European Union, but their
consumption in our country is still below the level recommended by specialists, despite the fact that
they provide nutrients, health benefits, reduce mortality and morbidity to diabetes, cancer and systemic
diseases circulation, protect cell DNA, have antioxidant activity, stimulate the immune system and order
the functions of hormones. These factors are the reason why fruits and vegetables were in second place
in the new food pyramid [
27
]. It turns out that the production of healthy snacks can be a way to increase
the fruit consumption. This is also a way to use fruits that often have a short shelf-life or manage
surplus of raw materials [
28
]. Such products are fruit leathers, which are eaten as candy or snacks.
Fruit leathers may be also added into beverages or into sauces, as well as used as ingredients in biscuits
or breakfast cereals [
29
]. They have novel and attractive appearance and normally do not require cold
storage to ensure microbiological safety [
30
]. Many investigations were conducted during the last
years for fruit leathers which were obtained with the use of e.g., mango [
31
,
32
], apple [
30
,
33
], pawpaw
and guava [
34
], pear [
35
], kiwifruit [
36
], grape [
37
,
38
], pineapple [
39
], papaya [
40
], jackfruit [
41
].
Fruit leathers are a confectionery product made by drying a thin layer of fruit puree or mixture of fruit
juice concentrate with other ingredients (e.g., acids, sugars, hydrocolloids) to produce flexible strips or
sheets with a texture similar to a soft leather [30,32,34].
Researchers were investigating the effect of drying methods and conditions, different equipment
solutions and the storage conditions on the quality of final product [
30
,
42
], but the composition of fruit
leathers is probably the most important. Conducted investigations shown that e.g., incorporation of soy
protein concentrate, skim milk powder and sucrose to mango leather significantly reduced the drying
rate, lowered the extensibility and weakening this product, effect on sensory properties and color
of mango leather [
31
]. Maltodextrin addition to the apple leathers and conditioning at low relative
humidity (RH) significantly decrease the adhesion force and the degree of cohesive failure. Whereas
glucose addition and conditioning at high RH caused a significant increase of the adhesion force [
43
].
Fruit leather composition also influences the glass transition temperature, which plays an important
role on the texture properties. For pear leathers a strong correlation of glass transition temperature
(Tg) with instrumental hardness, chewiness, and sensory attributes was shown at the surrounding
temperature (25–30
◦
C) above Tg, the sample softened and became rubbery [
35
]. Physicochemical
and sensory properties of fruit leathers may be also changed by hydrocolloid addition, e.g., pectin
(0.5–24%), which seems to be the most important ingredient that significantly affects textural properties
of the fruit leather. The reduction of pectin addition allows obtaining a softer, more appealing and
acceptable product [32,35,39].
1.2.1. Fruit Leathers and Edible Films as the Type of Healthy Snacks. The Possibilities of Increasing the
Quality of Such Products
Fruit leathers are a kind of edible film, which are used to extend the shelf life and quality of foods
by controlling mass transfer, preventing changes in aroma, taste, texture and appearance. They inhibit
the migration of moisture, oxygen, carbon dioxide, flavors and lipids [
28
]. The edible films are used as
primary packaging directly coated on food or formed into a film, as well as a food wrap [
44
] (Figure 1).

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Figure 1. Fruit edible film with black currant (Source: own photos).
Dependent on the nature, characteristics, costs, specific needs, and expected benefits, the
appropriate emulsified layers are chosen for a specific product [44]. Edible films and coatings are
obtained with polysaccharides, proteins or lipids, but to obtain better functionality than films
produced with one component, a combination of these ingredients should be used [28,44,45]. Lipid
coatings protect food products (fruit or vegetables) before moisture loss [46–48]. Whey protein
isolate (WPI) addition and plasticisation with sucrose create a very good oxygen barrier, which is
flexible, tough and glossy. Authors compared selected properties of obtained edible films with films
plasticised with glycerol (no crystallisation) or plasticised with sucrose (crystallisation) plus a
crystallisation inhibitor (lactose and raffinose) and have shown that crystallisation in WPI/sucrose
films reduced tensile strength at break. Inhibitors addition hindered sucrose crystallisation, and
favorable properties were maintained longer. Raffinose was the more effective inhibitor and gives
the higher gloss [49]. Multicomponent films can be obtained as either a bi-layer composite system
(the lipid forms the second layer over the polysaccharide or protein layer) or emulsions (the lipid is
dispersed in the biopolymer matrix) [44]. Emulsified films in comparison to bilayer or multilayer
films require only one drying process which reduces the preparation time [50].
Even though many investigations were conducted for edible films and fruit leathers, there is
still a need to improve functionality and efficiency of such products to develop new genre of
materials that can better maintain the properties of the food products or give a product rich in
nutrients. Sothornvit and Pitak [51] pointed out that the use of fruit and vegetables in the form of
puree or flour, makes material preparation easier and eliminates the isolation of the film forming
compounds. Such product may be edible films or fruit leathers whose production may be also
treated as technology that allows using surplus of raw material or post-production waste, e.g., fruit
pomace from apple juice production (Figure 2) [52]. Press cake (pomace) generated in high amounts
during juice production (25% and 35% mass of the raw material) comprises fruit skins and seeds,
which contain fibres and bioactive substances [53]. We conducted our own preliminary research on
the use of dried fruit pomace or fruit with peel for the preparation of fruit leather , which indicated a
clear strengthening of the structure of the fruit leather , making the product more attractive in visual
assessment and increasing the nutritional value of the product (Figure 2). Such results confirmed
investigations of Viskelis et al. [52], who enriched apple and black currant fruit leather with
freeze-dried black currant and raspberry press cake powders, which significantly enhanced its
biochemical composition and reduced firmness of the final product.
Figure 1. Fruit edible film with black currant (Source: own photos).
Dependent on the nature, characteristics, costs, specific needs, and expected benefits,
the appropriate emulsified layers are chosen for a specific product [
44
]. Edible films and coatings
are obtained with polysaccharides, proteins or lipids, but to obtain better functionality than films
produced with one component, a combination of these ingredients should be used [
28
,
44
,
45
]. Lipid
coatings protect food products (fruit or vegetables) before moisture loss [
46
–
48
]. Whey protein isolate
(WPI) addition and plasticisation with sucrose create a very good oxygen barrier, which is flexible,
tough and glossy. Authors compared selected properties of obtained edible films with films plasticised
with glycerol (no crystallisation) or plasticised with sucrose (crystallisation) plus a crystallisation
inhibitor (lactose and raffinose) and have shown that crystallisation in WPI/sucrose films reduced
tensile strength at break. Inhibitors addition hindered sucrose crystallisation, and favorable properties
were maintained longer. Raffinose was the more effective inhibitor and gives the higher gloss [
49
].
Multicomponent films can be obtained as either a bi-layer composite system (the lipid forms the second
layer over the polysaccharide or protein layer) or emulsions (the lipid is dispersed in the biopolymer
matrix) [
44
]. Emulsified films in comparison to bilayer or multilayer films require only one drying
process which reduces the preparation time [50].
Even though many investigations were conducted for edible films and fruit leathers, there is still
a need to improve functionality and efficiency of such products to develop new genre of materials that
can better maintain the properties of the food products or give a product rich in nutrients. Sothornvit
and Pitak [
51
] pointed out that the use of fruit and vegetables in the form of puree or flour, makes
material preparation easier and eliminates the isolation of the film forming compounds. Such product
may be edible films or fruit leathers whose production may be also treated as technology that allows
using surplus of raw material or post-production waste, e.g., fruit pomace from apple juice production
(Figure 2) [
52
]. Press cake (pomace) generated in high amounts during juice production (25% and
35% mass of the raw material) comprises fruit skins and seeds, which contain fibres and bioactive
substances [
53
]. We conducted our own preliminary research on the use of dried fruit pomace or fruit
with peel for the preparation of fruit leather, which indicated a clear strengthening of the structure of
the fruit leather, making the product more attractive in visual assessment and increasing the nutritional
value of the product (Figure 2). Such results confirmed investigations of Viskelis et al. [
52
], who
enriched apple and black currant fruit leather with freeze-dried black currant and raspberry press
cake powders, which significantly enhanced its biochemical composition and reduced firmness of the
final product.

Sustainability 2019,11, 2800 6 of 20
Sustainability 2019, 11, x FOR PEER REVIEW 6 of 20
(a) (b)
(c) (d)
Figure 2. Fruit leather with: (a,b) apple pulp with apple peel, (c) apple pulp with dried apple
pomace, (d) apple and black currant pulp with dried apple pomace (Source: own photo).
Kadzińska et al. [54] argue that edible films with fruit and vegetable purees may be treated as
healthy snacks, edible oven bags, wraps for sushi, type of pancakes, tortillas or lavash in a
gluten-free diet. They mixed sodium alginate, apple puree and vegetable oils (rapeseed oil, coconut
oil and hazelnut oil), and glycerol as a plasticizer. The main component, common to all samples, was
sodium alginate. Addition of apple puree and vegetable oils had a significant effect on the visual
appearance of edible films, decreased the lightness and increased the greenness (except for apple
puree) and yellowness in comparison to pure sodium alginate films. The structure of edible films
was also modified (Figure 3). Additionaltly, Espitia et al. [55] showed that attractive colors and
flavors, as well as high nutritional value, of edible films with fruit and vegetable purees makes it that
they may be served as healthy snacks.
(a)
Figure 2.
Fruit leather with: (
a
,
b
) apple pulp with apple peel, (
c
) apple pulp with dried apple pomace,
(d) apple and black currant pulp with dried apple pomace (Source: own photo).
Kadzi´nska et al. [
54
] argue that edible films with fruit and vegetable purees may be treated as
healthy snacks, edible oven bags, wraps for sushi, type of pancakes, tortillas or lavash in a gluten-free
diet. They mixed sodium alginate, apple puree and vegetable oils (rapeseed oil, coconut oil and
hazelnut oil), and glycerol as a plasticizer. The main component, common to all samples, was sodium
alginate. Addition of apple puree and vegetable oils had a significant effect on the visual appearance
of edible films, decreased the lightness and increased the greenness (except for apple puree) and
yellowness in comparison to pure sodium alginate films. The structure of edible films was also modified
(Figure 3). Additionaltly, Espitia et al. [
55
] showed that attractive colors and flavors, as well as high
nutritional value, of edible films with fruit and vegetable purees makes it that they may be served as
healthy snacks.
Sustainability 2019, 11, x FOR PEER REVIEW 6 of 20
(a) (b)
(c) (d)
Figure 2. Fruit leather with: (a,b) apple pulp with apple peel, (c) apple pulp with dried apple
pomace, (d) apple and black currant pulp with dried apple pomace (Source: own photo).
Kadzińska et al. [54] argue that edible films with fruit and vegetable purees may be treated as
healthy snacks, edible oven bags, wraps for sushi, type of pancakes, tortillas or lavash in a
gluten-free diet. They mixed sodium alginate, apple puree and vegetable oils (rapeseed oil, coconut
oil and hazelnut oil), and glycerol as a plasticizer. The main component, common to all samples, was
sodium alginate. Addition of apple puree and vegetable oils had a significant effect on the visual
appearance of edible films, decreased the lightness and increased the greenness (except for apple
puree) and yellowness in comparison to pure sodium alginate films. The structure of edible films
was also modified (Figure 3). Additionaltly, Espitia et al. [55] showed that attractive colors and
flavors, as well as high nutritional value, of edible films with fruit and vegetable purees makes it that
they may be served as healthy snacks.
(a)
Figure 3. Cont.

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(b)
Figure 3. The structure of edible films with: (a) sodium alginate, (b) sodium alginate + apple puree +
rapeseed oil. Scanning microscope (Source: own photo).
1.2.2. Dried Fruit and Vegetable Gels on the Basic of Hydrocolloid as the Innovative Healthy Snacks
During the last years, many investigations were also conducted with dried gels, which are in
the interest of different sectors [56–60] or dried foams [61–63]. Fruit gels after drying may be
innovative snacks. Such product may meet the acceptance and interest of consumers due to its
attractive structure. Addition of fruit pulp provides nutrients, which makes the product a healthy
snack [60]. Whereas, dried press cake powder added in the value of 3% to fruit pulp created
innovative freeze-dried healthy snacks (Figure 4).
Figure 4. Freeze-dried fruit snacks with dried apple press cake (Source: own photo).
When we analyze the role of vegetables more closely, it turns out that they have a high
nutritional value and are low in calories, which is of great importance in the prevention and
treatment of obesity and overweight. Vegetables provide minerals, vitamins, β–carotene, dietary
fiber, and also regulate the pH of the body, alkalizing it [64]. For example, broccoli belongs to the
Figure 3.
The structure of edible films with: (
a
) sodium alginate, (
b
) sodium alginate +apple puree +
rapeseed oil. Scanning microscope (Source: own photo).
1.2.2. Dried Fruit and Vegetable Gels on the Basic of Hydrocolloid as the Innovative Healthy Snacks
During the last years, many investigations were also conducted with dried gels, which are in the
interest of different sectors [
56
–
60
] or dried foams [
61
–
63
]. Fruit gels after drying may be innovative
snacks. Such product may meet the acceptance and interest of consumers due to its attractive structure.
Addition of fruit pulp provides nutrients, which makes the product a healthy snack [
60
]. Whereas,
dried press cake powder added in the value of 3% to fruit pulp created innovative freeze-dried healthy
snacks (Figure 4).
Sustainability 2019, 11, x FOR PEER REVIEW 7 of 20
(b)
Figure 3. The structure of edible films with: (a) sodium alginate, (b) sodium alginate + apple puree +
rapeseed oil. Scanning microscope (Source: own photo).
1.2.2. Dried Fruit and Vegetable Gels on the Basic of Hydrocolloid as the Innovative Healthy Snacks
During the last years, many investigations were also conducted with dried gels, which are in
the interest of different sectors [56–60] or dried foams [61–63]. Fruit gels after drying may be
innovative snacks. Such product may meet the acceptance and interest of consumers due to its
attractive structure. Addition of fruit pulp provides nutrients, which makes the product a healthy
snack [60]. Whereas, dried press cake powder added in the value of 3% to fruit pulp created
innovative freeze-dried healthy snacks (Figure 4).
Figure 4. Freeze-dried fruit snacks with dried apple press cake (Source: own photo).
When we analyze the role of vegetables more closely, it turns out that they have a high
nutritional value and are low in calories, which is of great importance in the prevention and
treatment of obesity and overweight. Vegetables provide minerals, vitamins, β–carotene, dietary
fiber, and also regulate the pH of the body, alkalizing it [64]. For example, broccoli belongs to the
Figure 4. Freeze-dried fruit snacks with dried apple press cake (Source: own photo).
When we analyze the role of vegetables more closely, it turns out that they have a high nutritional
value and are low in calories, which is of great importance in the prevention and treatment of obesity
and overweight. Vegetables provide minerals, vitamins,
β
–carotene, dietary fiber, and also regulate

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the pH of the body, alkalizing it [
64
]. For example, broccoli belongs to the family of cruciferous plants.
It is most often sold as a raw flower, free of leaves or in a frozen form, previously blanched. It is valued
not only for its taste, but also for its health-promoting properties, because of the content of many
vitamins and minerals [
65
]. Numerous studies also confirm the antioxidant properties of broccoli and
its usefulness in the fight against cardiovascular and neurological diseases. The discovery of chemical
compounds with chemopreventive properties in broccoli give hope for the possibility of using a new,
natural product to fight cancer. The antioxidants contained in broccoli (vitamin C, E, carotenoids and
polyphenols) support the natural defense of the body against reactive oxygen forms. They also have
the ability to scavenge free radicals by inhibiting the initiation of chain reactions and singlet oxygen
quenching, which protects the system from oxidative stress [66–68].
The pre-treatment processes that vegetables are most often subjected to before sale have a negative
impact on the content of healthy ingredients in cruciferous plants. That is why it is so important
to choose the right technological process, which will make it more accessible and attractive to the
consumer, but will not significantly reduce the high nutritional value of this raw material [
69
]. Such
a process may be freeze-drying, considered a drying process that allows obtaining a higher quality
product compared to traditional dehydration techniques. It consists of removing water from frozen
material by sublimation, i.e., direct water transition into a gaseous state, by passing the liquid phase and
taking place under reduced pressure. Studies on the use of freeze-drying for preserving broccoli provide
information on retaining of 25% chemopreventive ingredients and 18% polyphenols in freeze-dried
broccoli [70].
Vegetables may be used successfully in the development of an innovative, healthy snack in the
form of bars preserved in the process of freeze-drying as a single layer or multilayer snack. Waste
resulting from the sorting of frozen vegetables may be used as a raw material for production (Figure 5).
Sustainability 2019, 11, x FOR PEER REVIEW 8 of 20
family of cruciferous plants. It is most often sold as a raw flower, free of leaves or in a frozen form,
previously blanched. It is valued not only for its taste, but also for its health-promoting properties,
because of the content of many vitamins and minerals [65]. Numerous studies also confirm the
antioxidant properties of broccoli and its usefulness in the fight against cardiovascular and
neurological diseases. The discovery of chemical compounds with chemopreventive properties in
broccoli give hope for the possibility of using a new, natural product to fight cancer. The
antioxidants contained in broccoli (vitamin C, E, carotenoids and polyphenols) support the natural
defense of the body against reactive oxygen forms. They also have the ability to scavenge free
radicals by inhibiting the initiation of chain reactions and singlet oxygen quenching, which protects
the system from oxidative stress [66–68].
The pre-treatment processes that vegetables are most often subjected to before sale have a
negative impact on the content of healthy ingredients in cruciferous plants. That is why it is so
important to choose the right technological process, which will make it more accessible and
attractive to the consumer, but will not significantly reduce the high nutritional value of this raw
material [69]. Such a process may be freeze-drying, considered a drying process that allows
obtaining a higher quality product compared to traditional dehydration techniques. It consists of
removing water from frozen material by sublimation, i.e., direct water transition into a gaseous state,
by passing the liquid phase and taking place under reduced pressure. Studies on the use of
freeze-drying for preserving broccoli provide information on retaining of 25% chemopreventive
ingredients and 18% polyphenols in freeze-dried broccoli [70].
Vegetables may be used successfully in the development of an innovative, healthy snack in the
form of bars preserved in the process of freeze-drying as a single layer or multilayer snack. Waste
resulting from the sorting of frozen vegetables may be used as a raw material for production (Figure
5).
Figure 5. Freeze-dried vegetable bars (Source: own photos).
To obtain such products, it is necessary to use hydrocolloids which are long-chain polymers,
mostly of polysaccharides and proteins (Table 1). They are characterized by the ability to form sticky
suspensions and gels in water. These properties are used in the production of functional and dietetic
foods, low-calorie products with reduced fat and sugar content, gluten-free and high in fiber [71].
The properties of the gel and the ease of obtaining it depend on the type of hydrocolloid, the pH of
the environment and the proportion of other substances in the solution. When using hydrocolloids
for the development of new products, an important factor is their interaction with ions, small
molecules (fatty acids, pigments, vitamins) and large particles (protein, saccharides), which are also
components of the product [72]. Among the hydrocolloids are distinguished, among others:
vegetable secretions and extracts, seaweed, seed or tuber extracts, microcolloids of microbiological
origin, natural modified and synthetic [73].
Figure 5. Freeze-dried vegetable bars (Source: own photos).
To obtain such products, it is necessary to use hydrocolloids which are long-chain polymers,
mostly of polysaccharides and proteins (Table 1). They are characterized by the ability to form sticky
suspensions and gels in water. These properties are used in the production of functional and dietetic
foods, low-calorie products with reduced fat and sugar content, gluten-free and high in fiber [
71
].
The properties of the gel and the ease of obtaining it depend on the type of hydrocolloid, the pH of the
environment and the proportion of other substances in the solution. When using hydrocolloids for the
development of new products, an important factor is their interaction with ions, small molecules (fatty
acids, pigments, vitamins) and large particles (protein, saccharides), which are also components of the
product [
72
]. Among the hydrocolloids are distinguished, among others: vegetable secretions and
extracts, seaweed, seed or tuber extracts, microcolloids of microbiological origin, natural modified and
synthetic [73].

Sustainability 2019,11, 2800 9 of 20
Table 1.
Classification of selected hydrocolloids due to their origin (Source: own study based on [
74
]).
Origin Examples
Natural
Animal origin Gelatin
Kasein
From land plants
Plant secretions Gum arabicus
Guma Karaya
Extracts from seeds or tubers
Locust bean gum
Pectins
Tara gum
Konjac gum
Guar gum
Seaweed extracts
Agar
Carrageenans
Alginians
Microbiological origin Xanthan gum
Gellan gum
Modified Chemical Modified starches
Physical Cellulose derivatives
Synthetic Through chemical synthesis Polymers of ethylene oxide
Pectins (heteropolysaccharides of plant origin) are one of the most popular hydrocolloids used
as food additives. Their skeleton is composed of 65% galacturonic acid residues that combine into
long chains connected by
α
-1,4 glycosidic bonds and are partially esterified with methanol. The most
commonly used pectin function is gelling. It depends mainly on the length of bonds and chemical
properties of pectins, as well as external conditions, such as temperature, pH value, presence of
additional substances [
73
,
75
]. Low-methoxyl pectins in the presence of ammonia form amidated
pectins. They gel in a wide range of pH 2.8–7, in the presence of 0.01–0.1% divalent metal ions, and
at a temperature that rises at a lower pH level of the environment. Gels are more flexible, and the
lack of necessity to use high concentrations of saccharides allows them to be used for production of
diet products with reduced sugar content [
76
,
77
]. Pectins also show many pro-health features, e.g.,
they positively affect the absorption of glucose, cholesterol and lipids, increase the bioavailability of
microelements, improve intestinal peristalsis and have anti-cancer properties [78].
Another hydrocolloid often used in the food industry is sodium alginate. The alginate group are
copolymers of various amounts of
β
-d-mannuronic acid (M) and
α
-l-guluronic acid (G), connected
in a block manner by 1,4-glycosidic linkages. They are present in brown algae or produced by some
bacteria. To obtain a gel with appropriate stiffness, alginates are used in the presence of calcium ions,
and the resulting molecular structure is called the eggs-in-box model [
75
,
79
]. In the food industry, these
hydrocolloids perform many functions, but are mainly used as a gelling substance. Gelation occurs
in a wide range of pH and temperature, and the appropriate concentration of alginate and calcium
ions determines gel stability, its viscosity and gelation rate [
80
]. The restructuring process for meat,
fruit and vegetables [
81
] can be successfully applied using alginates for production of freeze-dried
vegetable snacks.
Xanthan gum is a microbial origin polysaccharide of high molecular weight. It is widely used in
the food industry, e.g., as a substitute for fat, reducing the caloric value of the product and improving
its texture. In the reconstitution of fruit and vegetable dry matter, it provides the right texture and
allows for limiting losses of dry matter components [
75
]. To imbue gels with new features and to
increase their stability, xanthan gum is often combined with other hydrocolloids, e.g., guar gum or
locust bean gum. The structure of these two substances is compatible with xanthan gum, because
their structure includes regions where galactose binds to the side chain with a bond that also occurs in
xanthan gum. These hydrocolloids also include regions that are not linked to galactose, meaning that

Sustainability 2019,11, 2800 10 of 20
they can combine with xanthan gum, and the more there are, the stronger the synergy. The combination
of xanthan gum with locust bean gum provides better results than in the synergy with guar gum, and
the formed gels have the appropriate flexibility and hardness [82].
1.2.3. Properties of Dried Gels and the Effect of Different Ingredients on Them
Especially for snack texture, crispness and hardness are very important criteria affecting consumer
acceptance and freeze-dried fruit or vegetable gels meet this condition. There is a need to investigate
dried gel systems as they act as effective ingredients. Freeze-drying allows gel shape and volume
to be better maintained, decreasing the occurrence of shrinkage [
59
]. Different hydrocolloids may
differentiate the properties of freeze-dried snacks. Therefore, properly selecting the right hydrocolloids
is invariably important in designing a product with specific characteristics. Such investigations were
conducted by Ciurzy´nska et al. [
83
–
85
] for freeze-dried strawberry gels on the basic of low-methoxyl
pectin, the mixture of xanthan gum and locust bean gum, as well as mixture of xanthan gum and
guar gum. The authors showed that hydrocolloids created the structure of freeze-dried samples,
which is what influenced the physical and organoleptic properties. Ciurzy ´nska et al. [
83
] showed that
freeze-dried gels with low-methoxyl pectin seem to be the best way to obtain an innovative strawberry
product with the designed structure. Pectin added to this sample caused the freeze-dried gels to
have the best rehydration properties, what was related to the structure, the lowest shrinkage and real
density. Samples with a mixture of hydrocolloids were more compact, shrinkage and real density were
higher and the rehydration properties were the lowest. Yousefi and Jafari [
86
] investigated the effect of
different biopolymers including polysaccharides and proteins or both, on dairy formulations, their
texture, rheology, physicochemical properties, and sensory attributes. They showed that hydrocolloids
can improve the texture of dairy products by increasing their viscosity, but also that they may interact
with different components of products, which changes the properties of dairy products. Similar effects
may be obtained in freeze-dried vegetables or fruit bars as an effect of sample composition, different
ingredients and pH, acid and sugar content [83].
Additionally, the ability to aerate the gel matrix while mixing the ingredients further increases the
attractiveness of the product structure. Investigations have shown that aeration is becoming more
and more popular in food production not only affecting its texture and firmness, but also changing
the appearance, color and sensory experience [
87
,
88
]. This is also the way to produce the low caloric,
dietetic food, because air supplies no calories and extends the sensation of satiety by slowing down the
rate of breakdown in the gastrointestinal tract [89].
In the literature there are few publications about dried fruit or vegetable gels. Strawberry pulp
added to the low-methoxyl pectin, the mixture of xanthan gum and locust bean gum or mixture of
xanthan gum and guar gum allowed obtaining a porous structure attractive for consumers, what was
confirmed by e.g., sensory analysis (Figure 6) [
60
,
83
]. The porous structure of gel matrix was also
created by different aeration time (3, 5, 7 and 9 min). Conducted investigations have shown that the
type of hydrocolloid has an important influence on freeze-dried gels properties while aeration time
was in most cases insignificant. It was shown that samples obtained on the basis of low-methoxyl
pectin were characterized by the best physical properties, the most attractive structure and were the
most crispy and fragile [60,83].

Sustainability 2019,11, 2800 11 of 20
Sustainability 2019, 11, x FOR PEER REVIEW 11 of 20
(a) (b)
(c)
Figure 6. Freeze-dried strawberry gels with different hydrocolloids: (a) low-methoxyl pectin, (b)
mixture of xanthan gum and locust bean gum, (c) mixture of xanthan gum and guar gum (Source:
own photos).
Broccoli waste from the sorting of frozen material was used to produce freeze-dried bars with
addition of xanthan gum (0.5%) and locust bean gum (0.5%). The use of a combination of
hydrocolloids allowed obtaining a delicate and glassy broccoli gel, that showed a high degree of
aeration. The internal structure of the freeze-dried broccoli gel was delicate, porous, which is
certainly influenced by the air bubbles introduced into the gel mass while mixing the gel
components (Figure 7). Similar results were obtained Ciurzyńska et al. [83] and Ciurzyńska et al.
[84], who studied the structure of freeze-dried gels based on a mixture of xanthan gum and locust
bean gum of various compositions, and also showed that structure is porous, with variable sized
pores due to the cracking of numerous air bubbles.
Figure 7. Internal structure of freeze-dried broccoli gels based on a mixture of xanthan gum and
locust bean gum powder. Scanning microscope (magnification 100 ×) (Source: own photo).
Porosity is very important for the quality and the texture of dry foods, because has a significant
effect on the physical, and mechanical properties of foods [90]. The measurement of textural porosity
Figure 6.
Freeze-dried strawberry gels with different hydrocolloids: (
a
) low-methoxyl pectin, (
b
) mixture
of xanthan gum and locust bean gum, (
c
) mixture of xanthan gum and guar gum (Source: own photos).
Broccoli waste from the sorting of frozen material was used to produce freeze-dried bars with
addition of xanthan gum (0.5%) and locust bean gum (0.5%). The use of a combination of hydrocolloids
allowed obtaining a delicate and glassy broccoli gel, that showed a high degree of aeration. The internal
structure of the freeze-dried broccoli gel was delicate, porous, which is certainly influenced by the air
bubbles introduced into the gel mass while mixing the gel components (Figure 7). Similar results were
obtained Ciurzy ´nska et al. [
83
] and Ciurzy ´nska et al. [
84
], who studied the structure of freeze-dried gels
based on a mixture of xanthan gum and locust bean gum of various compositions, and also showed
that structure is porous, with variable sized pores due to the cracking of numerous air bubbles.
Sustainability 2019, 11, x FOR PEER REVIEW 11 of 20
(a) (b)
(c)
Figure 6. Freeze-dried strawberry gels with different hydrocolloids: (a) low-methoxyl pectin, (b)
mixture of xanthan gum and locust bean gum, (c) mixture of xanthan gum and guar gum (Source:
own photos).
Broccoli waste from the sorting of frozen material was used to produce freeze-dried bars with
addition of xanthan gum (0.5%) and locust bean gum (0.5%). The use of a combination of
hydrocolloids allowed obtaining a delicate and glassy broccoli gel, that showed a high degree of
aeration. The internal structure of the freeze-dried broccoli gel was delicate, porous, which is
certainly influenced by the air bubbles introduced into the gel mass while mixing the gel
components (Figure 7). Similar results were obtained Ciurzyńska et al. [83] and Ciurzyńska et al.
[84], who studied the structure of freeze-dried gels based on a mixture of xanthan gum and locust
bean gum of various compositions, and also showed that structure is porous, with variable sized
pores due to the cracking of numerous air bubbles.
Figure 7. Internal structure of freeze-dried broccoli gels based on a mixture of xanthan gum and
locust bean gum powder. Scanning microscope (magnification 100 ×) (Source: own photo).
Porosity is very important for the quality and the texture of dry foods, because has a significant
effect on the physical, and mechanical properties of foods [90]. The measurement of textural porosity
Figure 7.
Internal structure of freeze-dried broccoli gels based on a mixture of xanthan gum and locust
bean gum powder. Scanning microscope (magnification 100 ×) (Source: own photo).
Porosity is very important for the quality and the texture of dry foods, because has a significant
effect on the physical, and mechanical properties of foods [
90
]. The measurement of textural porosity
of the freeze-dried broccoli bars confirmed microscopic observations, which were characterized by a
high porosity of ~96% (Table 2).

Sustainability 2019,11, 2800 12 of 20
Table 2.
The selected physical properties of broccoli gel. n=3
±
SD (standard deviation) (Source:
own investigations).
Sample Porosity (%) Water Activity (aw) Water Content (%)
Gel before freeze-drying - 0.938 ±0.005 92.16 ±0.15
Freeze-dried gel 95.83 ±0.22 0.073 ±0.007 2.07 ±0.27
Freeze-dried gel after rehydration - - 92.66 ±1.00
This value is typical for freeze-dried samples described in the literature, whereas the differences
may be due to the type of raw material, the different structure of plant products and gels, as well as the
parameters of the freezing and drying process [
91
]. The high porosity of freeze-dried food is created as
an effect of the absence of capillary forces during sublimation of the frozen water [
92
]. A similar value
was obtained for freeze-dried gels with a combination of xanthan gum and locust bean gum [
84
,
85
],
freeze-dried apple and potato [
93
]. In the case of freeze-dried strawberry gels, the porosity was in
the range of ~88–91% [
94
]. Reduction of the porosity of the gel due to the addition of dry matter
components was also demonstrated by Nussinovitch et al. [
95
], who found that the addition of banana
puree to the agar solution reduced the porosity to 94%, while the addition of orange puree reduced the
tested parameter by about 10%.
The structure of dried matter has a significant impact, among others, on the ability to rehydrate,
the course of which is a determinant of quality, because it indicates the range of physical and chemical
changes that occurred during freeze-drying. The freeze-dried broccoli bar showed a high degree
of rehydration and a slight loss of dry matter content, which was confirmed by comparing the
water content of the gel before freeze-drying and after rehydration of the freeze-dried gels (Table 1).
It was shown that the difference is 0.5%, and the tested parameter is greater for the freeze-dried
gel after rehydration. This indicates not only the high degree of water binding in the freeze-dried
sample structure, but also the attachment of water by hydrocolloids being components of the gel,
as demonstrated by Ciurzy´nska et al. [
83
]. In the case of freeze-dried broccoli gel, rehydration is most
effective in the first 15 min (Figure 8), which confirms the observation during the study of freeze-dried
strawberry gels [83] and freeze-dried strawberries [96].
Sustainability 2019, 11, x FOR PEER REVIEW 12 of 20
of the freeze-dried broccoli bars confirmed microscopic observations, which were characterized by a
high porosity of ~96% (Table 2).
Table 2. The selected physical properties of broccoli gel. n = 3 ± SD (standard deviation) (Source: own
investigations).
Sample Porosity (%) Water Activity (aw) Water Content (%)
Gel before freeze-drying - 0.938 ± 0.005 92.16 ± 0.15
Freeze-dried gel 95.83 ± 0.22 0.073 ± 0.007 2.07 ± 0.27
Freeze-dried gel after rehydration - - 92.66 ± 1.00
This value is typical for freeze-dried samples described in the literature, whereas the differences
may be due to the type of raw material, the different structure of plant products and gels, as well as
the parameters of the freezing and drying process [91]. The high porosity of freeze-dried food is
created as an effect of the absence of capillary forces during sublimation of the frozen water [92]. A
similar value was obtained for freeze-dried gels with a combination of xanthan gum and locust bean
gum [84,85], freeze-dried apple and potato [93]. In the case of freeze-dried strawberry gels, the
porosity was in the range of ~88–91% [94]. Reduction of the porosity of the gel due to the addition of
dry matter components was also demonstrated by Nussinovitch et al. [95], who found that the
addition of banana puree to the agar solution reduced the porosity to 94%, while the addition of
orange puree reduced the tested parameter by about 10%.
The structure of dried matter has a significant impact, among others, on the ability to rehydrate,
the course of which is a determinant of quality, because it indicates the range of physical and
chemical changes that occurred during freeze-drying. The freeze-dried broccoli bar showed a high
degree of rehydration and a slight loss of dry matter content, which was confirmed by comparing
the water content of the gel before freeze-drying and after rehydration of the freeze-dried gels (Table
1). It was shown that the difference is 0.5%, and the tested parameter is greater for the freeze-dried
gel after rehydration. This indicates not only the high degree of water binding in the freeze-dried
sample structure, but also the attachment of water by hydrocolloids being components of the gel, as
demonstrated by Ciurzyńska et al. [83]. In the case of freeze-dried broccoli gel, rehydration is most
effective in the first 15 min (Figure 8), which confirms the observation during the study of
freeze-dried strawberry gels [83] and freeze-dried strawberries [96].
Figure 8. Relative weight gain during the rehydration of freeze-dried broccoli gel with a mixture of
xanthan gum and locust bean gum (Source: own investigations).
To be safe, food products should have water activity (aw) below 0.6 [97]. Katz and Labuza [98]
have shown that the increase of aw decreased the sensory acceptability. The critical value of this
parameter was determined in the range of 0.35–0.5, where the product became unacceptable from a
sensory point of view, because in simple sugar food systems transformations occur from amorphous
to crystalline form, and mobilization of soluble food constituents begins. Similar values were
Figure 8.
Relative weight gain during the rehydration of freeze-dried broccoli gel with a mixture of
xanthan gum and locust bean gum (Source: own investigations).
To be safe, food products should have water activity (a
w
) below 0.6 [
97
]. Katz and Labuza [
98
]
have shown that the increase of a
w
decreased the sensory acceptability. The critical value of this
parameter was determined in the range of 0.35–0.5, where the product became unacceptable from a
sensory point of view, because in simple sugar food systems transformations occur from amorphous to
crystalline form, and mobilization of soluble food constituents begins. Similar values were obtained by
Lewicki et al. [
99
], who showed a
w
=0.306 for crackers and 0.538 for corn-rye bread. Above these

Sustainability 2019,11, 2800 13 of 20
values softening of snacks occurred. However, Gabas et al. [
100
] argue that the strongest relationship
between a
w
and mechanical properties occurs in the water activity range of 0.3–0.7. Freeze-drying of the
broccoli bars reduced this parameter to 0.073 (Table 1). Such a low value indicates the microbiological
safety of the freeze-dried gel, and with the activity of water below 0.2, enzymatic reactions are also
inhibited [
100
]. The obtained value is similar to the water activity of freeze-dried gels with a mixture of
xanthan gum and locust bean gum powder, where, depending on the aeration time, a
w
of 0.018–0.021
was obtained [84] and significantly lower than the water activity of freeze-dried agar gels with apple
pur
é
e (0.148–0.151) [
101
]. The addition of hydrocolloid to thawed vegetables should not reduce water
activity, as was demonstrated by Hoefler [
102
], who claimed that gums are substances that retain water,
but do not reduce its activity by its high molecular weight, while the addition of a larger amount
of hydrocolloid may affect the reduction of water activity of the product. Such observations were
confirmed by Jakubczyk et al. [
103
] by studying gels with agar, for which they obtained the value of
water activity of 0.782 and 0.899 respectively for concentrations of 40% and 0.5%. Additionally, Sankat
and Castaigne [
104
] claimed that foaming agents help to remove moisture during the drying process
which is what creates the porous structure.
The high quality of vegetable gels may be confirmed by the average water content similar to
water content of raw material (vegetables). For the broccoli gel 92.16% of water content (Table 1) is
typical value for broccoli, which varies in the range of 91.92–88.1% depending on the type of raw
material [
105
,
106
], and the process of freezing and thawing has little effect on this parameter [
107
].
The gels with xanthan gum and locust bean gum powder containing 87% addition of strawberry
pulp obtained a water content of about 84% [
83
], and agar gels with or without albumin added were
characterized by the water content between 91% and 98% [108].
For the consumer, the appearance of the food is a very important parameter determining the
choice of the product. The color similar to raw material decides that the product is received as natural.
Based on the coefficient value (L*), the product’s brightness can be determined in the range from 0
(black) to 100 (white), the color parameter (a*) defines the color contribution from green (
−
60 units) to
red (+60 units) and the yellow coefficient (b*) determines the color contribution from blue (–60 units)
to yellow (+60 units). Freeze-drying caused a statistically significant increase in lightness coefficient
of broccoli gel, which is related to the high porosity of the freeze-dried samples (Figure 9, Table 1).
For the color coefficient a* intensity of green pigment was shown, typical for broccoli, confirming that
such product is natural. Çali¸skan et al. [
109
] studied the effect of freeze-drying on the color of kiwi and
showed similar changes in the L*, a*, b* color parameters observed in the case of broccoli gel, which
indicates the protective nature of freeze-drying in the product color.
Sustainability 2019, 11, x FOR PEER REVIEW 13 of 20
obtained by Lewicki et al. [99], who showed aw = 0.306 for crackers and 0.538 for corn-rye bread.
Above these values softening of snacks occurred. However, Gabas et al. [100] argue that the
strongest relationship between aw and mechanical properties occurs in the water activity range of
0.3–0.7. Freeze-drying of the broccoli bars reduced this parameter to 0.073 (Table 1). Such a low value
indicates the microbiological safety of the freeze-dried gel, and with the activity of water below 0.2,
enzymatic reactions are also inhibited [100]. The obtained value is similar to the water activity of
freeze-dried gels with a mixture of xanthan gum and locust bean gum powder, where, depending on
the aeration time, aw of 0.018–0.021 was obtained [84] and significantly lower than the water activity
of freeze-dried agar gels with apple purée (0.148–0.151) [101]. The addition of hydrocolloid to
thawed vegetables should not reduce water activity, as was demonstrated by Hoefler [102], who
claimed that gums are substances that retain water, but do not reduce its activity by its high
molecular weight, while the addition of a larger amount of hydrocolloid may affect the reduction of
water activity of the product. Such observations were confirmed by Jakubczyk et al. [103] by
studying gels with agar, for which they obtained the value of water activity of 0.782 and 0.899
respectively for concentrations of 40% and 0.5%. Additionally, Sankat and Castaigne [104] claimed
that foaming agents help to remove moisture during the drying process which is what creates the
porous structure.
The high quality of vegetable gels may be confirmed by the average water content similar to
water content of raw material (vegetables). For the broccoli gel 92.16% of water content (Table 1) is
typical value for broccoli, which varies in the range of 91.92–88.1% depending on the type of raw
material [105,106], and the process of freezing and thawing has little effect on this parameter [107].
The gels with xanthan gum and locust bean gum powder containing 87% addition of strawberry
pulp obtained a water content of about 84% [83], and agar gels with or without albumin added were
characterized by the water content between 91% and 98% [108].
For the consumer, the appearance of the food is a very important parameter determining the
choice of the product. The color similar to raw material decides that the product is received as
natural. Based on the coefficient value (L*), the product’s brightness can be determined in the range
from 0 (black) to 100 (white), the color parameter (a*) defines the color contribution from green (−60
units) to red (+60 units) and the yellow coefficient (b*) determines the color contribution from blue
(–60 units) to yellow (+60 units). Freeze-drying caused a statistically significant increase in lightness
coefficient of broccoli gel, which is related to the high porosity of the freeze-dried samples (Figure 9,
Table 1). For the color coefficient a* intensity of green pigment was shown, typical for broccoli,
confirming that such product is natural. Çalişkan et al. [109] studied the effect of freeze-drying on
the color of kiwi and showed similar changes in the L*, a*, b* color parameters observed in the case
of broccoli gel, which indicates the protective nature of freeze-drying in the product color.
Figure 9. Color of the gel before and after freeze-drying of broccoli gel with a mixture of xanthan
gum and locust bean gum, in the CIE-Lab color system. The values are mean (n = 3) ± SE. Means
followed by the same letter (A, B, a, b) in the bar diagram are not significantly different according to
ANOVA and Tukey multiple comparison tests (Source: own investigations).
Figure 9.
Color of the gel before and after freeze-drying of broccoli gel with a mixture of xanthan gum
and locust bean gum, in the CIE-Lab color system. The values are mean (n=3)
±
SE. Means followed
by the same letter (A, B, a, b) in the bar diagram are not significantly different according to ANOVA
and Tukey multiple comparison tests (Source: own investigations).

Sustainability 2019,11, 2800 14 of 20
Texture of dried food products can be described by mechanical properties as crispness and hardness,
which influence consumer acceptance [
110
,
111
]. Barbosa-C
á
novas [
112
] defined texture as “all the
mechanical, geometrical and surface attributes of a food product perceptible by means of mechanical,
tactile and, where appropriate, visual and auditory receptors.” Mechanical properties, especially
hardness are very important for dried fruit [
113
] and may also apply to snacks. Marzec et al. [
114
]
showed that freeze-dried sour cherries had a delicate and porous structure, which resulted in a
lower mechanical strength in relation to convection dried dry matter. The strength of the sample
is significantly influenced by the method of its preparation, composition, choice of drying method
and parameters, pre-treatment and drying time. Freeze-dried gels obtained on the basis of xanthan
gum and locust bean gum powder at the same deformation obtained a maximum compression force
of about 5 N [
84
], and the addition of sugar to the same gels caused reinforcement of the structure
and increase in the compression force to 6–19 N depending on the time the sample was aerated [
88
].
In contrast, gels with the addition of 87% strawberry pulp based on low-methoxyl pectin or a mixture
of xanthan gum and guar gum obtained a maximum compression force of 40–50 N [
90
]. Freeze–dried
broccoli gel has a uniformly porous structure because the compression curve is smooth and deforms to
reach a maximum compression force of ~21 N after about 10 s (Figure 10).
Sustainability 2019, 11, x FOR PEER REVIEW 14 of 20
Texture of dried food products can be described by mechanical properties as crispness and
hardness, which influence consumer acceptance [110,111]. Barbosa-Cánovas [112] defined texture as
“all the mechanical, geometrical and surface attributes of a food product perceptible by means of
mechanical, tactile and, where appropriate, visual and auditory receptors.” Mechanical properties,
especially hardness are very important for dried fruit [113] and may also apply to snacks. Marzec et
al. [114] showed that freeze-dried sour cherries had a delicate and porous structure, which resulted
in a lower mechanical strength in relation to convection dried dry matter. The strength of the sample
is significantly influenced by the method of its preparation, composition, choice of drying method
and parameters, pre-treatment and drying time. Freeze-dried gels obtained on the basis of xanthan
gum and locust bean gum powder at the same deformation obtained a maximum compression force
of about 5 N [84], and the addition of sugar to the same gels caused reinforcement of the structure
and increase in the compression force to 6–19 N depending on the time the sample was aerated [88].
In contrast, gels with the addition of 87% strawberry pulp based on low-methoxyl pectin or a
mixture of xanthan gum and guar gum obtained a maximum compression force of 40–50 N [90].
Freeze–dried broccoli gel has a uniformly porous structure because the compression curve is smooth
and deforms to reach a maximum compression force of ~21 N after about 10 s (Figure 10).
Figure 10. The compression curve in the stress relaxation test of the freeze-dried broccoli gel with a
mixture of xanthan gum and locust bean gum. Deformation 50% (Source: own investigations).
2. Conclusions
The study explored the possibility of obtaining innovative, healthy snacks, which production
additionally allows the development of full-value waste from fruit and vegetable processing. The
possible solutions, such as fruit leathers and edible films for the production of which dried fruit
pomace or fruit pure can be added were indicated, as products which fit perfectly into the policy of
sustainable development. Thanks to such innovations, the nutritional value increases and the
product features improve.
Hydrocolloids may be used to produce fruit leathers or edible films, but also to develop an
innovative fruit or vegetables snack in the form of a freeze-dried bar with a minimum amount of
additives. The freeze-drying process allows to maintain high nutritional value of the product,
natural color, whereas the created structure is porous and fragile, which is particularly appreciated
by consumers when choosing snacks. Our own research on the possibility of using waste from the
sorting process of frozen vegetables in the production of freeze-dried broccoli bars showed that the
gel obtained on the basis of xanthan gum and locust bean gum was characterized by a color similar
to the color of raw broccoli, it was delicate but flexible and of adequate hardness. The freeze-dried
broccoli gel showed low water activity and water content guaranteeing microbiological safety. The
high porosity of the freeze-dried samples ensured its quick rehydration and provided the color
similar to the color of raw broccoli. The mechanical properties of the freeze-dried gels were typical
for a porous and delicate product.
Figure 10.
The compression curve in the stress relaxation test of the freeze-dried broccoli gel with a
mixture of xanthan gum and locust bean gum. Deformation 50% (Source: own investigations).
2. Conclusions
The study explored the possibility of obtaining innovative, healthy snacks, which production
additionally allows the development of full-value waste from fruit and vegetable processing.
The possible solutions, such as fruit leathers and edible films for the production of which dried
fruit pomace or fruit pure can be added were indicated, as products which fit perfectly into the policy of
sustainable development. Thanks to such innovations, the nutritional value increases and the product
features improve.
Hydrocolloids may be used to produce fruit leathers or edible films, but also to develop an
innovative fruit or vegetables snack in the form of a freeze-dried bar with a minimum amount
of additives. The freeze-drying process allows to maintain high nutritional value of the product,
natural color, whereas the created structure is porous and fragile, which is particularly appreciated
by consumers when choosing snacks. Our own research on the possibility of using waste from the
sorting process of frozen vegetables in the production of freeze-dried broccoli bars showed that the gel
obtained on the basis of xanthan gum and locust bean gum was characterized by a color similar to the
color of raw broccoli, it was delicate but flexible and of adequate hardness. The freeze-dried broccoli gel
showed low water activity and water content guaranteeing microbiological safety. The high porosity
of the freeze-dried samples ensured its quick rehydration and provided the color similar to the color

Sustainability 2019,11, 2800 15 of 20
of raw broccoli. The mechanical properties of the freeze-dried gels were typical for a porous and
delicate product.
Author Contributions:
Conceptualization, A.C., M.J., A.L.; methodology, A.C., M.J.; formal analysis, M.J., A.C.;
investigation, A.C., M.J., P.C., W.M., A.O.; resources, A.C., M.J., P.C., M.B.; data curation, A.C., M.J., P.C.; writing
original draft preparation, A.C., M.J.; writing review and editing, M.J., A.C.; visualization, M.J., A.C., W.M.;
supervision, A.C., M.J., A.L.; project administration, M.J.; funding acquisition, M.J.
Funding:
This work was founded by the National Center for Research and Development, as part of
the III BIOSTRATEG. “The development of an innovative carbon footprint calculation method for the
basic basket of food products“—task in the project “Development of healthy food production technologies
taking into consideration nutritious food waste management and carbon footprint calculation methodology“
BIOSTRATEG3/343817/17/NCBR/2018 and was also co-financed by a statutory activity subsidy from Polish
Ministry of Sciences and Higher Education for the Faculty of Food Sciences of Warsaw University of Life
Sciences 505-20-092600-Q00195-99.
Conflicts of Interest: The authors declare no conflict of interest.
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