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Home Freezing of Fruits and Vegetables

PNW 214 • Reprinted September 2008
A Pacic Northwest Extension Publication
Oregon State University • Washington State University • University of Idaho
Freezing fruits
(See chart on pages 3 and 4 for specic directions.)
Select fully ripe fruit that is not soft or mushy. Most fruit has
the best avor, color, and food value if it has ripened on the tree
or vine.
Carefully wash and sort fruit. Trim and discard parts that are
green or bruised.
Peel, trim, pit and slice fruit as directed.
Prepare fruit for freezing by packing with or without sugar (or
syrup). Use ascorbic acid to retard browning of light-color fruit.
(See Methods of freezing.)
Pack prepared fruit in suitable containers as directed.
Store in freezer as directed.
To serve, thaw fruit at room temperature or in the refrigerator.
Serve while a few ice crystals remain.
Methods of freezing
Without sugar
Any fruit can be frozen without sugar. However, the texture
may be softer than that of fruit frozen with sugar.
Some fruits such as berries, cherries, and grapes may be frozen
in a single layer on cookie sheets before packing in containers.
This prevents them from sticking together. Serve them frozen as
snacks or thaw and use as a topping for salads or desserts.
If desired, a water pack (without sugar) can be used for fruit
such as peaches. Fruit juice (either extracted from the fruit or
purchased) can be used. Orange and berry juices are suitable.
Syrup pack
Fruits that will be served uncooked are often packed in syrup.
The syrup may be prepared from either cane or beet sugar. If
desired, part of the sugar may be replaced by corn syrup or honey.
Select the strength of syrup that will give the desired avor. (This
Prepared by Carolyn A. Raab, Extension food and nutrition specialist,
Oregon State University, and reviewed by Extension food safety
specialists at Washington State University and the University of Idaho.
Freezing is one of the simplest and least time
consuming methods of food preservation.
For best quality, it is important to follow directions
careful ly. Color, flavor, and nutritive value can
be affected by freshness of the pro duce selected,
method of preparation and packaging, and
conditions of freezing.
depends on the sweetness of the fruit, personal preference, and
intended use.) Allow about 23 cup of syrup for each pint of fruit
and 113 cups for each quart of fruit. Dissolve sugar in hot or cold
water. If hot, cool before using.
Sugar pack
Juicy fruits and those that will be used for pies or other cooked
products are often packed in sugar. Use about 1 cup of sugar for
each 2 to 3 pounds of fruit. Sugar and fruit should be gently but
thoroughly mixed until the sugar has dissolved in the juice.
Retarding browning
Ascorbic acid When freezing light-color fruit, ascorbic acid
can be added to inhibit browning. Ascorbic acid in powder or
crystal form is available at pharmacies. Ascorbic acid tablets
also can be used. Crush nely before use. (Three crushed 500-
milligram vitamin C tablets equal ½ teaspoon of ascorbic acid.)
For syrup or liquid packs, add ½ teaspoon powdered or crushed
ascorbic acid to each quart (4 cups) of cold syrup.
For sugar or sugarless (dry) packs, dissolve ½ teaspoon of
ascorbic acid in 3 tablespoons cold water and sprinkle over 4 cups
of fruit just before adding sugar.
Commercial antidarkening mixture Follow manufacturer’s
Freezing vegetables
(See chart on pages 4 and 5 for specic directions.)
Select top-quality vegetables. If possible, harvest them in the
early morning or early evening when it is cool. Prepare them
as soon as possible to avoid loss of quality. If there is a delay,
store them in the refrigerator.
Wash and sort vegetables in cold running water.
Peel, trim, and cut into pieces as directed.
Prepare vegetables for freezing by blanching. This short heat
treatment stops enzymes that otherwise cause undesirable
changes in avor, texture, color and nutritive value during
storage. (See Methods of blanching.) Unless otherwise
indicated, times in the chart refer to blanching in boiling water.
Steam-blanching takes longer.
Pack prepared vegetables in suitable containers as directed.
To serve, cook frozen vegetables in a small amount of salted
water until tender. Corn-on-the-cob should be partially thawed
before cooking. To cook in a microwave oven, refer to the
instruction manual.
of syrup
Light 4 1 434
Medium 4 1345
Heavy 4 234
Methods of blanching
In boiling water
Put water in a large kettle with a tighttting lid and bring
to a rolling boil. (Allow 1 gallon of water for each pound of
vegetables except for leafy greens, which require 2 gallons per
Put a small quantity of vegetables in a wire basket, strainer, or
cheesecloth bag. Immerse in water
Cover kettle and boil at top heat for the required length of
time (see chart). Begin counting time as soon as vegetables are
placed in the water.
Cool immediately in cold running water (or ice water) for about
the same length of time used for blanching. When thoroughly
cool, drain and pack.
In steam
Put 1 inch of water in a kettle and bring to a rolling boil.
Put a small quantity of vegetables in a steamer basket or in a
colander with legs. Suspend over boiling water.
Cover kettle and heat vegetables for the required length of time.
Cool immediately in cold running water (or ice water) for about
the same length of time used for blanching. When thoroughly
cool, drain and pack.
In microwave oven
For recommended blanching times in the microwave, check the
oven’s instruction manual.
Freezing steps
Packing in containers
Food must be packed in suitable containers for freezing. These
should be moisture-vapor-resistant, durable, and easy to handle.
Select containers that have a shape that will pack well in the
freezer and a good size (to feed your family for one or two meals).
Some common types are:
Plastic freezer bags
Vacuum packaging designed for freezing
Rigid plastic containers
Glass canning jars with wide mouths
Pack foods tightly into containers. Allow ample headspace
between the packed food and the lid to allow room for expansion
during freezing. Products packed without added sugar or liquid
require ½ inch in pint containers with a wide top opening (1 inch
in quarts). Allow 34 inch in pint containers with a narrow top
opening (1 ½ inches in quart containers).
To keep fruit covered with liquid, put a crumpled piece of
waxed paper between the fruit and the lid. This will keep the
surface from darkening and drying out.
When food is packed in freezer bags, squeeze out as much air
as possible.
Label containers with name of product, type of pack (for fruit),
and date.
Loading the freezer
Freeze fruits and vegetables soon after they are packed. If there
will be a delay before freezing, keep packages in the refrigerator.
Foods that freeze too slowly may lose quality or spoil. Put no
more unfrozen food into a home freezer than will freeze within
24 hours. Usually this will be about 2 or 3 pounds of food for each
cubic foot of its capacity. For fastest freezing, place packages
against freezing plates or coils and leave a little space between
them so that air can circulate freely.
Storing frozen food
After freezing, packages may be stored close together.
Store them at 0°F (–18°C) or below. Foods lose quality and
nutritive value much faster at higher temperatures. Use a freezer
thermometer to check the temperature periodically.
Most fruits and vegetables maintain high quality for 8 to
12 months. (Unsweetened fruits lose quality faster than those
packed in sugar or syrup.) Storage for longer periods will affect
the quality of the frozen foods, but they will be safe to eat.
Keeping a freezer inventory and dating packages will help to
rotate the supply.
Freezing juices
A variety of fruit juices can be prepared including cherry,
grape, grapefruit, plum, raspberry, and strawberry. The procedure
is similar for each.
Select fully ripe, good quality fruit
Prepare as directed for freezing
Extract juice by crushing fruit and straining through a jelly bag.
(Heat fruit slightly to start ow of juice if necessary.) A steam
juicer also can be used.
Sweeten juice if desired
Pour into containers, allowing adequate headspace. (Refrigerate
grape juice overnight before packing, to allow sediment to sink
to the bottom.)
Seal and freeze
Tomato juice can be extracted by simmering quarters or
eighths for 5 to 10 minutes. (Skipping this step causes the juice
to separate.) Then press through a sieve or food mill. Season with
salt, if desired. Pour into containers and freeze.
Refreezing frozen foods
Occasionally a home freezer stops running. The time that food
will stay frozen depends on the amount of food in the freezer and
the temperature of the food. A full load of food will stay frozen
for up to 2 days if the freezer is not opened. Dry ice can be used to
keep foods frozen longer. If the power will be off for a long period
of time, food should be transferred to a locker plant.
It is safe to refreeze fruits and vegetables that still have ice
crystals. For best quality, refreeze food in small quantities.
If the temperature has warmed above 40°F (5°C), foods may
not be t for refreezing. Use a freezer thermometer to determine
the temperature.
Thawed frozen fruit is safe to eat. However, it may have an
“off” avor if fermentation has begun.
Do not eat thawed vegetables that are above 40°F. The low
acidity of vegetables makes it possible for harmful bacteria to
grow. Unsafe products may not show signs of spoilage.
For information about the safety of particular products, call
your local county Extension ofce. Be prepared to give the
“history” of the food (length of time that the power was off and
temperature of the food before the freezer began to run again).
Store at 0°F (-18°C) for top quality
Freezing fruits
See page 1 for details on packing in sugar and syrup. Fruits also may be frozen as purées for making jam. Use ascorbic acid with
light-color fruits.
Select crisp, rm fruit. Wash, peel, core, and slice. To prevent browning during preparation, slice into salt water
(2 tablespoons salt to 1 gallon of water). Drain. Pack in syrup, or pack in sugar, or pack without sugar. Use ascorbic
acid to retard browning during storage.
Applesauce Wash apples, peel if desired, core and slice. Cook until tender in water (13 cup to each quart of slices). Cool and strain
if necessary. Sweeten to taste.
Apricots Select rm, ripe, uniformly yellow fruit. Wash, halve, and pit. Peel and slice if desired. (To loosen skins, dip in boiling
water for 15 to 20 seconds.) Pack in syrup or pack in sugar. Use ascorbic acid to retard browning.
Best frozen as purée (not whole or sliced). Select avocados that are soft and with rinds free from dark blemishes. Peel,
halve, and remove pit. Mash the pulp. For better quality, add 14 teaspoon of ascorbic acid per quart. Pack without sugar
if using for salads, dips, or sandwiches.
Bananas Select rm, ripe bananas. Peel and mash thoroughly. Add ½ teaspoon ascorbic acid per cup.
Blackberries Also boysenberries, loganberries. Select rm, fully ripe fruit with glossy skins. Wash and drain. Pack in syrup, or
pack in sugar, or pack without sugar.
Also huckleberries. Select ripe berries with tender skins. For syrup pack, wash and drain. (For dry pack without sugar,
wash before eating instead.) If desired, steam for 1 minute and cool immediately to tenderize skin. Pack in syrup or
pack without sugar.
Cantaloupe Also other melons. Select rm, well-colored, ripe melons. Cut in half, remove seeds and peel. Cut into slices, cubes,
or balls. Pack in syrup.
Select bright red, tree-ripened fruit. Stem and wash. Drain and pit. Pack in syrup, or pack in sugar, or pack without
Select tree-ripened red varieties. Stem and wash. Remove pits if desired. Pack in syrup or pack without sugar. Use
ascorbic acid to retard browning.
Currants Select fully ripe, bright red fruit. Wash and stem. Pack in syrup, or pack in sugar, or pack without sugar.
Figs Select tree-ripened, soft-ripe fruit. Wash and cut off stems. Peel if desired. Slice or leave whole. Pack in syrup or pack
without sugar. Use ascorbic acid to retard browning.
Gooseberries Select fully ripe (for pie) or slightly underripe (for jelly) berries. Wash and remove stems and blossom ends. Pack in
syrup or pack without sugar.
Also oranges. Select rm, tree-ripened fruit, heavy for its size and free from soft spots. Divide fruit into sections,
removing all membranes and seeds. Slice if desired. Pack in syrup (made with excess juice and water if needed) or
pack in water without sugar.
& peaches
Select rm, fully ripe, well-colored fruit. Wash, peel, and remove pit. Cut in halves, quarters, or slices. Pack in syrup or
pack in orange juice or in water without sugar. Use ascorbic acid to retard browning.
Pears Select wellripened, rm fruit. Wash and peel. Cut in halves or quarters and remove cores. Heat in boiling syrup for
1 to 2 minutes (depending on size of pieces). Drain, cool, and pack in syrup. Use ascorbic acid to retard browning.
Freezing vegetables
See pages 1 and 2 for details on blanching. Times refer to water blanching in most cases. Begin counting as soon as vegetables are
placed in water.
Asparagus Select young stalks with compact tips. Wash and sort by size. Leave whole or cut in 1- to 2-inch lengths. Blanch
small stalks 112 minutes, medium stalks 2 minutes, large stalks 3 minutes. Cool.
green (snap or wax)
Select young stringless beans. Wash, snip off tips. Cut or break into suitable pieces or slice lengthwise into
strips. Blanch 3 minutes. Cool.
lima, butter, or pinto
Harvest beans while seeds are green. Wash, shell, and sort according to size. Water-blanch small beans
2 minutes, medium beans 3 minutes, and large beans 4 minutes. Cool.
Beets Select beets less than 3 inches across. Sort by size. Remove tops and wash. Cook until tender (small beets, 25 to
30 minutes; medium beets, 45 to 50 minutes). Cool. Peel and slice or dice.
Select compact, dark green heads. Wash and trim leaves and woody ends. If necessary to remove insects, soak
30 minutes in salt brine (4 teaspoons salt to 1 gallon of water). Rinse and drain. Cut through stalks lengthwise,
leaving heads 1 inch in diameter. Blanch 3 minutes in water or steam-blanch 5 minutes. Cool. Pack heads and
stalks ends alternately in container.
Brussels sprouts Select green, rm, compact heads. Wash and trim outer leaves. Soak 30 minutes in salt brine (see broccoli).
Rinse and drain. Blanch medium heads 4 minutes, large heads 5 minutes. Cool.
Carrots Select tender carrots. Remove tops, wash, and scrape. Dice or slice 14 inch thick. Blanch 2 minutes. Cool.
Cauliower Select rm, white heads. Wash and trim. Split heads into pieces 1 inch across. If necessary to remove insects,
soak 30 minutes in salt brine (4 teaspoons salt to 1 gallon of water). Rinse and drain. Blanch 3 minutes. Cool.
cut or on the cob
Select ears with plump kernels and thin, sweet milk. Husk, remove silk, and wash. Whole kernel or cream style:
Blanch 4 to 5 minutes. Cool thoroughly. Drain, cut off cob. On-the-cob: Blanch small ears 7 minutes, medium
ears 9 minutes, large ears 11 minutes. Cool, drain. Wrap each ear separately or tightly pack desired number in
large freezer bags or containers.
Select orange, soft-ripe persimmons. Sort, wash, peel, and cut into sections. Press fruit through a sieve to make a purée.
To each quart of purée, add 18 teaspoon ascorbic acid. Purée made from cultivated varieties may be packed with or
without sugar.
Plums Also prunes. Select rm, tree-ripened fruit. Wash. Cut in halves and quarters or leave whole. Pack in syrup or pack
without sugar. Use ascorbic acid to retard browning.
Raspberries Select fully ripe, juicy berries. (Seedy berries are best for purées or juice.) Wash and drain. Pack in syrup or pack in
sugar or pack without sugar.
Rhubarb Select rm, well-colored stalks. Wash, trim and cut into 1- to 2-inch pieces. Pack in syrup or pack without sugar.
Strawberries Select rm, ripe red berries. Wash, drain, and remove hulls. Slice if desired. Pack in syrup, or pack in sugar, or pack
without sugar.
Freezing fruits—continued
Store at 0°F (-18°C) for top quality
Store at 0°F (-18°C) for top quality
Herbs—fresh Wash, drain, and pat dry with paper towels. Wrap a few sprigs or leaves in freezer wrap and place in a freezer
bag. Chop and use frozen herbs in cooked dishes.
Select edible mushrooms free of spots or decay. Wash and remove stem base. Freeze small mushrooms whole;
cut large ones into four or more pieces. When blanching, add 1 teaspoon citric acid (or 3 teaspoons lemon juice
or 12 teaspoon ascorbic acid) per quart of water to prevent darkening. Blanch medium or small whole mushrooms
5 minutes, cut pieces 3 minutes. Cool. Or: Slice mushrooms 14 inch thick and sauté in butter until almost done.
Cool by setting pan in cold water.
Onions Select fully mature onions. Peel, wash, and chop. Blanch 112 minutes. Cool. May also be frozen unblanched.
edible pod
Select young, tender pods. Wash. Remove stems, blossom ends, and any strings. Blanch small pods 1 minute,
large pods 112 to 2 minutes. Cool.
Peas—green Select bright green, plump, rm pods. Shell. Blanch l12 to 2 minutes. Cool.
sweet (green)
Select rm, crisp peppers. Wash, cut out stem, and remove seeds. Halve, slice, or dice. Blanch halved peppers
3 minutes; sliced or diced 2 minutes. Cool. May also be frozen unblanched.
hot (green chile)
Select rm, smooth peppers. Wash and dry. Broil for 6 to 8 minutes to loosen skin. (First make small slit in each
to allow steam to escape.) Cool. Remove peel, seeds, and stems before or after freezing. Protect hands with
rubber gloves.
Wash, pare; remove deep eyes, bruises, and green surface coloring. Cut in 14- to 12-inch cubes. Blanch
5 minutes. Cool. For french fries: Pare and cut in thin strips. Fry in deep fat until light brown. Drain and cool.
To serve, bake at 400°F for 10 to 20 minutes.
Select medium to large sweet potatoes. Wash and cook until almost tender. Peel, cut in halves, slice, or mash.
To prevent browning, dip 5 seconds in a solution of 1 tablespoon citric acid or 12 cup lemon juice to 1 quart of
water. To keep mashed sweet potatoes from darkening, mix 2 tablespoons orange or lemon juice with each quart
of mashed sweet potatoes.
Pumpkin Also other winter squash. Select full-color, mature pumpkin. Cut or break into fairly uniform pieces. Remove
seeds. Bake at 350°F or steam until tender. Cool, scoop pulp from rind, and mash or put through ricer.
Also other greens. Select young, tender leaves. Remove tough stems. Wash. Blanch most leafy greens
2 minutes. Blanch collards and stem portions of Swiss chard 3 to 4 minutes. Blanch very tender spinach
112 minutes. Cool.
Tomatoes Best frozen stewed or puréed. Select ripe tomatoes free from blemishes. Remove stem ends, peel, and quarter.
Cook until tender. Cool by setting pan in cold water.
Zucchini Also other summer squash. Select young squash with small seeds and tender rind. Wash and slice. Blanch
14-inch slices 3 minutes; 112-inch slices 6 minutes. Cool.
Freezing vegetables—continued
© 2007 Oregon State University
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... During freezing process, bulk water in fruit tissue is transformed to ice crystals. The ice crystals expand the tissue matrix leading to cell volume changes, resulting in cellular structural damage (De Ancos, Sánchez-Moreno, De Pascual-Teresa, & Cano, 2012;Silva, Gonçalves, & Brandoão, 2008). Freezing damage of cellular structure due to ice crystals usually occurs in plant tissues because of the semirigid nature of cells which leads to loss of tissue firmness, and a subsequent loss of textural quality in frozen fruits (Chassagne-Berces et al., 2009;Khan & Vincent, 1996;Zaritzky, 2012). ...
... Osmotic dehydrofreezing technique has been successfully used in freezing of fruits with minimal damage to cellular integrity and texture. This technique aims to reduce the water content in food materials by immersion in aqueous solution before freezing (Ando, Kajiwara, Oshita, & Suzuki, 2012;Li & Sun, 2002;Silva et al., 2008). The disadvantage of an osmotic dehydrofreezing technique is that it is a type of an aqueous solution that affects the sensory characteristics of products (Dixon & Jen, 1977). ...
... Reduction of freezing damage caused by ice crystal is a key factor in the preservation of frozen fruits because fruits contain a large amount of water (Silva et al., 2008), their cellular structure could be destroyed by ice crystal during freezing. A procedure to reduce the amount of bulk water before transformation to ice crystal could reduce freezing damage. ...
... Like drying (Alabi, Olaniyan, & Odewole, 2016;Pragati & Preeti, 2014;Sagar & Suresh Kumar, 2010) and cooling (McDonald & Sun, 2000;Olunloyo, Olunloyo, & Ibiyeye, 2019), freezing is a widely used technique to preserve some quality attributes of fruits and vegetables, such as color, minerals, and vitamins. However, freezing can also cause physical and chemical changes, which could lead to a loss in quality (Chassagne-Berces et al., 2009;Silva, Gonçalves, & Brandão, 2009). This loss is more apparent in tissues of fruits and vegetables due to high water content and ice crystals formation (Li, Zhu, & Sun, 2018). ...
Full-text available
Osmotic dehydration (OD) is a process of soaking products in an aqueous solution containing salt or sugar, which is normally applied to fruits and vegetables. The combination of OD pretreatment with freezing, or osmotic dehydrofreezing (ODF), is a novel technology to shorten the freezing process and prolong the preservation of fruits and vegetables. However, the effectiveness of ODF is affected by process parameters and nature of the product, thus information on freezing characteristics and quality of osmotically dehydrated frozen fruits and vegetables is useful to the food industry. This review intends to provide an overview of the effects of OD pretreatment on freezing characteristics such as freezing rate, thermal properties, and quality of frozen fruits and vegetables. Fundamentals of ODF technology, including significance of OD to freezing, and mechanism and factors affecting ODF are summarized. In addition, hurdle technologies comprising of ODF and other innovative nonthermal techniques, such as ultrasound and pulsed electric field (PEF) are presented, and future trends of the combined technology are briefly discussed. ODF can accelerate the freezing process and enhance the quality of osmotically dehydrated frozen fruits and vegetables. The novel ultrasound and PEF techniques, which can provide cryoprotection from in situ interference, were proposed for the production of product with many‐functional characteristics, by incorporating bioactive compounds like plants sterols, probiotics, and dietary fibers, into the matrix of cellular tissues during ODF process. However, these techniques can enhance the performance of the ODF to promote fast freezing, produce small ice crystals, and raise glass transition temperature of cellular tissues. The future trends of ODF technology should mainly focus on controlling the mass and heat transfer processes, improving quality stability during glassy state storage condition and development of product with many‐functional characteristics. Practical Applications Fruits and vegetables are subject to freezing damage, particularly tissue softening and drip loss when thawing, thus reducing their quality and market value. OD pretreatments to freezing or ODF has great potentials in preservation of fruits and vegetables, with the advantage of minimum quality loss due to the reduction in freezing loads. Currently, innovative studies have been carried out on the combined use of OD pretreatments and emerging freezing techniques to improve the freezing process, achieve better quality with extended shelf life, and produce products with many‐functional characteristics. However, the findings presented in this review work can provide detail insights on the quality of fruits and vegetables that were frozen by ODF and give some guidance for further developments of ODF technology.
... During thawing, a fraction of the water will not return to its original location resulting in high drip loss and poor food quality. On the other hand, fast freezing produces small ice crystals formed in the intra-and extracellular space, leading to superior quality (Petzold and Aguilera, 2009;Silva et al., 2008). ...
Abstract The quality of frozen food products is strongly dependent on the ice crystal size and shape. In ice cream, for example, the size distribution of ice crystals has a significant impact on smoothness, meltdown, and cooling properties. Therefore, controlling the size and shape of ice crystals in frozen foods is a primary concern to the food industry. This article gives a short review on ice crystal morphology and the factors influencing ice crystal shape and size. In addition some brief fundamentals about crystallization are given.
Four vegetables (green beans, yellow squash, purple hull peas, and mustard greens) were processed for freezing by steam, hot water, and microwave blanching. Blanched vegetables were chilled, packaged and stored at – 18°C for 3 months. Each vegetable was prepared and subjected to quantitative descriptive analysis and preference testing by a trained eight-member panel. The effects on texture, colour and flavour are described.
Ascorbic acid content was determined and compared for the vegetables: green beans, purple hull peas, yellow squash, and mustard greens, blanched by microwave and conventional methods. Generally, no difference existed among methods o f blanching except for steam blanched purple hull peas which were higher in ascorbic acid content (P≤ 0.05) than those blanched by the other methods. Excessive loss of ascorbic acid seemed to be related to high surface to mass ratio as evidenced by low retention values for sliced yellow squash and mustard greens.
This paper reviews new developments in methods of freezing (high-pressure freezing, dehydrofreezing and applications of antifreeze protein and ice nucleation protein) and thawing (high-pressure and microwave thawing, ohmic thawing and acoustic thawing) for foods. With a good understanding of the solid–liquid phase diagram of water, the effects of pressure on food freezing–thawing cycles are highlighted. High-pressure freezing promotes uniform and rapid ice nucleation and growth through the whole sample. Dehydrofreezing has been successfully used in freezing of vegetables and fruits with the advantage of less damage to plant texture because of partial water removal before freezing. Recently, studies have been carried out for the biotechnological use of antifreeze and ice nucleation proteins because of their uniqueness in directly improving freezing processes. Thawing under pressure can be achieved at lower temperature than that at atmospheric pressures. Finally microwave, ohmic and acoustic thawing are described. It is hoped that this paper will attract more research in novel freezing and thawing processes and methods.
The combined effect of simultaneous application of heat treatments, low frequency ultrasound (20 kHz) at different amplitudes, on Aspergillus flavus and Penicillium digitatum spore viability suspended in laboratory broth formulated at different aw (0.99 or 0.95) and pH (5.5 or 3.0), with or without vanillin or potassium sorbate were evaluated. An ultrasonic horn (13 mm) was submerged into the broth, heat and ultrasound was continuously applied. Survival curves were obtained, and D and z values calculated. For aw 0.99, increasing ultrasound amplitude and reducing pH resulted in reduced D values. At constant aw, D values decreased with pH reduction. At constant pH, D values were lower for aw 0.99 than for 0.95. In general, D values were lower for thermosonication treatments than for thermal treatments. When potassium sorbate or vanillin was added and thermosonication treatment was applied at increased amplitude, lower D values were obtained.
The effect of heat and the combined heat/ultrasound (thermosonication) treatment on the inactivation kinetics of peroxidase in watercress (Nasturtium officinale) was studied in the temperature range of 40–92.5 °C. In the heat blanching processes, the enzyme kinetics showed a first-order biphasic inactivation model. The activation energies and the rates of the reaction at a reference temperature for both the heat-labile and heat-resistant fractions were, respectively, Ea1 = 421 ± 115 kJ mol−1 and Ea2 = 352 ± 81 kJ mol−1, and . The initial relative specific activity for both isoenzyme fractions were also estimated, being C01 = 0.5 ± 0.08 μmol min−1 mg protein−1 and C02 = 0.5 ± 0.06 μmol min−1 mg protein−1, respectively. The application of thermosonication was studied to enable less severe thermal treatments and, therefore, improving the quality of the blanched product. In this treatment the enzyme kinetics showed a first-order model. The activation energy, the rate of reaction at a reference temperature and the initial relative specific activity were, respectively, Ea3 = 496 ± 65 kJ mol−1, and C03 = 1 ± 0.05 μmol min−1 mg protein−1, proving that the enzyme became more heat labile. The present findings will help to design the blanching conditions for the production of a new and healthy frozen product, watercress (Nasturtium officinale), with minimized colour or flavour changes along its shelf life.