Pelleting properties and pellet quality of apple pomace

Abstract

Apple pomace (AP) is the main by-product of apple juice production. It is a residue after pressing of apples for juice extraction. AP is an organic material, with high moisture and sugar content, therefore its direct disposal on landfills or land spreading causes serious environmental pollution. One of the solutions for further exploitation of AP could be its utilization as animal feed. However, fresh AP is quite perishable, and it must be preserved in order to be stored and used over a long period of time. The aim of this research was to investigate the possibility to transform AP to a stabile form which would be acceptable for feed manufacturers in terms of stability, storage and handling. For this purpose, pelleting process was used and pelleting properties of AP were evaluated. Before pelleting, dried AP was divided into three batches, which were conditioned only by water addition in order to achieve different moisture contents of the material: 10% of moisture for the first batch, 13% for the second batch and 16% for the third batch. According to the obtained values for Pellet Durability Index (PDI) and pellet hardness, pellet quality for all examined AP batches was very high. Increased moisture content of AP led to the reduction in energy consumption of the pellet press, thus providing energy saving in the pelleting process. Pelleting process also caused strong increase of AP bulk density which is positive in terms of transportation and storage.

Full text

DOI: 10.5937/FFR1502147M UDK 634.10:636.085/.086
Original research paper
PELLETING PROPERTIES AND PELLET QUALITY OF APPLE POMACE
Marijana D. Maslovarić
1
*, Đuro M. Vukmirović
2
, Radmilo R. Čolović
2
, Nedeljka J. Spasevski
2
,
Rade D. Jovanović
1
, Nataša V. Tolimir
1
1
Institute for Science Application in Agriculture, 11000 Belgrade
Bulevar despota Stefana 68b, Serbia
2
University of Novi Sad, Institute of Food Technology, 21000 Novi Sad
Bulevar cara Lazara 1, Serbia
*Corresponding author:
Phone: +381113291443
Fax: +381112752959
E-mail address: mmaslovaric@ipn.co.rs
ABSTRACT: Apple pomace (AP) is the main by-product of apple juice production. It is a residue after
pressing of apples for juice extraction. AP is an organic material, with high moisture and sugar
content, therefore its direct disposal on landfills or land spreading causes serious environmental
pollution. One of the solutions for further exploitation of AP could be its utilization as animal feed.
However, fresh AP is quite perishable, and it must be preserved in order to be stored and used over a
long period of time. The aim of this research was to investigate the possibility to transform AP to a
stabile form which would be acceptable for feed manufacturers in terms of stability, storage and
handling. For this purpose, pelleting process was used and pelleting properties of AP were evaluated.
Before pelleting, dried AP was divided into three batches, which were conditioned only by water
addition in order to achieve different moisture contents of the material: 10% of moisture for the first
batch, 13% for the second batch and 16% for the third batch. According to the obtained values for
Pellet Durability Index (PDI) and pellet hardness, pellet quality for all examined AP batches was very
high. Increased moisture content of AP led to the reduction in energy consumption of the pellet press,
thus providing energy saving in the pelleting process. Pelleting process also caused strong increase of
AP bulk density which is positive in terms of transportation and storage.
Key words: apple pomace, animal feed, pelleting
INTRODUCTION
An increase in demand for livestock pro-
ducts implies an increase in animal feed
production. Animal feed industry is nowa-
days facing a shortage of quality feeds,
which reflects in a rather variable supply,
high prices and therefore in a need for
alternative feed resources. Many by-pro-
ducts of food processing have a potential
to be used as feedstuffs (Dhillon et al.,
2013).
Better utilization and evaluation of such
by-products would contribute to sustaina-
bility of both food and feed production.
Apple pomace (AP) is the main by-product
of apple juice production. It is a residue
after pressing of apples for juice ex-
traction. AP consists of apple peel, seeds,
core, stems and pulp, and represents
about 25-35% of the weight of the fresh
apple processed (Joshi and Attri, 2006;
García et al., 2009).
Apples are mostly consumed as fresh fruit,
but significant amounts are processed into
apple juice and other products, generating
several million tonnes of AP every year
(Bhushan et al., 2008). AP has been re-

Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
garded as a waste and disposed on land-
fills or treated by incineration and compo-
sting (Dhillon et al., 2013). The only utili-
sation currently carried out at the industrial
level is pectin recovery (Gullón et al.,
2007). Since AP is an organic material,
with high moisture (70-75%) and sugar
content, it is susceptible to microbial de-
composition and uncontrolled fermentation
(Bhushan et. al., 2008). Therefore, direct
disposal of AP on landfills or land sprea-
ding causes serious environmental pol-
lution. Furthermore, safe disposal of AP
involves substantial costs relating to its
treatment and transportation (Shalini and
Gupta, 2010; Dhillon et al., 2013). For
these reasons, it should not be considered
as a waste, but as a raw material for other
purposes. One of the solutions for further
exploitation of AP could be its utilization as
animal feed.
AP contains small amounts of protein and
fat, but it is a good source of sugars, crude
fiber, minerals and phytochemicals (Gab-
riel et al., 2013; Reis et al., 2014). Fresh
AP (obtained after pressing of apples) has
already been used locally as palatable
feed for cattle and sheep (Crawshaw,
2009).
However, since fresh AP is quite pe-
rishable, it must be preserved in order to
be stored and used over a long period of
time. Preservation of AP can be achieved
by ensiling or drying (Pirmohammadi et al.,
2006). When ensiled with other feedstuffs
(corn, alfalfa and wheat bran, sugar beet
pulp and brewery spent grain, tomato
pomace) AP silage can be successfully
used as ruminant feed (Toyokawa et al.,
1984; Antov et al., 2004,
2010).
Several authors demonstrated that dried
AP could be ground and used as a feed-
stuff in rations for both ruminants and non-
ruminants. Bae et al. (1994) observed that
cows fed total mixed ration containing 39%
AP showed increased protein content but
decreased lactose content in milk, when
compared with cows fed the control diet.
Milk fat and solid-not-fat were similar for
both diets. According to the research con-
ducted by Bowden and Berry (1958), when
dry AP was included in rations for fattening
pigs at the amount of 10%, no significant
change occurred in daily gain, carcass
quality and feed efficiency, compared to
the control group fed a standard diet.
Being a rich source of polyphenols, AP
has a positive impact on animal health.
Gutzwiller et al., (2005) demonstrated that
adding 8% of dry AP into weaner pigs’
diet, previously contaminated with Fusa-
rium toxins, counteracted the negative
effects of deoxynivalenol on pigs’ growth.
Dried AP can be potentially used as a sup-
plement in poultry feed. The results of the
study carried out by Zafar et al. (2005)
indicate that dry AP can be used safely as
an energy source in broiler rations re-
placing maize by 10% (w/w) without any
side-effects on broiler production.
Properties of dried AP could be further
enhanced, by processes of grinding and
pelleting. Pelleting increases bulk density
of the material allowing more efficient
transportation, by reducing handling and
transportation costs. Additionally, pelleting
enhances flow properties of material which
is important for transport in conveying
equipment, and discharging from silos
(Thomas and van der Poel, 1996).
Pelleting also decreases dustiness of ma-
terial (Abdollahi et al., 2013). AP in pel-
leted form could be used directly in
feeding of ruminant animals or could be
transported to the feed mills, where it
would be used as a standard feedstuff.
The aim of this research was to investigate
the possibility to transform AP, as a very
susceptible material, to a form which
would be acceptable for feed manu-
facturers in terms of stability and flowa-
bility. For this purpose, pelleting process
was used and pelleting properties of AP
were evaluated.
Presently, no data on AP pelleting pro-
perties are available in the literature but
future larger-scale utilization of AP would
require such data. Knowledge on pelleting
characteristics and quality of AP pellets,
such as bulk density, stability during trans-
portation and handling, would be impor-
tant to feed manufacturers, as well to im-
porters and exporters.

Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
MATERIAL AND METHODS
The research was conducted using AP
obtained from fruit processing factory ‘Vino
Ţupa’ in Aleksandrovac, Serbia. It was
dried to approximately 10% moisture in a
factory’s rotary drum dryer.
Technological processing of AP (grinding
and pelleting) and the analysis of the
physical quality of pellets were performed
at the pilot plant facility and at the
laboratories of the Institute of Food Tech-
nology in Novi Sad, Serbia.
Dried AP was ground using laboratory
hammer mill (Type 11, ‘ABC Inţenjering’,
Pančevo, Serbia) with 4 mm sieve ope-
nings.
All measurements and chemical analysis
were done in triplicates, except when
stated otherwise.
Chemical analysis
AP was analysed for moisture content
(AOAC 950.46), crude protein (Kjeldahl
method, AOAC 978.04), crude fat (AOAC
920.39), crude fibre (AOAC 962.10), crude
ash (AOAC 942.05), total sugars (Pra-
vilnik, 1988) and reducing sugars (SRPS
E.L8.019:1992).
Pelleting
Before pelleting, AP was divided into three
batches (30kg each), which were con-
ditioned only by water addition in order to
achieve different moisture content of the
material: 10% of moisture for the first
batch (AP10), 13% for the second batch
(AP13) and 16% for the third batch
(AP16). Water was added into a double-
shaft pedal mixer (Muyang SLHSJ0.2QA,
China) equipped with 6 nozzles positioned
above mixing pedals which ensures unif-
orm distribution of water.
All batches of AP were pelleted on the
laboratory flat die pellet press (model 14-
175, AMANDUS KAHL GmbH & Co. KG,
Germany), equipped with a 18 mm thick
die, having a 6 mm opening diameter (L/D
- 1:3). Material throughput was 15 kg/h in
all pelleting treatments. Specific energy
consumption of pellet press (kWh/t) was
calculated by using the following equation:
E-E
0
SEC = x 1000
Q
SEC–specific energy consumption (kWt/h)
E–energy consumption during pelleting of
the material (kW) (read out from pellet
press display)
E
0
–energy consumption during the idle
running of pellet press (kW)
Q–material throughput (t/h)
Temperature of pellet press die was read
out from pellet press display.
Before determination of physical quality,
pellets were cooled to room temperature
using fluidized bed cooler/drier (FB
500x200, Amandus Kahl, Germany) and
stored for 24 hours.
Physical quality of pellets
Pellet durability was determined by using a
New Holmen Pellet Tester, NHP 100
(TekPro Ltd, Norfolk, Great Britain), which
simulates rigorous treatment of pellets by
pneumatic handling. Sieved pellet samples
(100g) were introduced in a stream of air
for 30 s, as it was described by Thomas
and van der Poel (1996). In this way,
pellets and air were circulated through
right-angled bends, impinging repeatedly
on hard surfaces, which led to pellet
abrasion. After the treatment, the samples
were sieved again. The sieve with the 4.8
mm sieve openings (approximately 80% of
the pellet diameter) was used. Pellet du-
rability was expressed as the Pellet Du-
rability Index (PDI), and calculated by
using the following equation:
mass of pellets retained on the sieve after the test
Pellet Durability Index (%) = x100
mass of pellets before the test

Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
Pellet hardness was determined with ma-
nually operated compression pellet tester
“Pellet Hardness Tester“, AMANDUS
KAHL GmbH & Co. KG, Germany, by
measuring the force of the first fracture of
individual pellets. Twenty pellets of ap-
proximately equal length, from each batch
were tested. The results of the measure-
ments are expressed as Kahl hardness.
Bulk density of ground and pelleted AP
was measured with a bulk density tester
(Tonindustrie, West und Goslar, Ger-
many).
Statistical analysis
The one-way ANOVA analysis was per-
formed to evaluate data differences
between AP batches using Statistica soft-
ware version 12 (Statistica, 2013). Sig-
nificant differences were analyzed by
Tukey HSD tests.
RESULTS AND DISCUSSION
Chemical composition of AP varies
significantly, since it is affected by apple
cultivar, growing region, climate and
processing (Zafar et al., 2005). The results
of the chemical analysis of AP are given in
Table 1. As it can be seen, major con-
stituents of apple pomace are crude fibre
and sugars, while levels of protein and
crude fat are very low. These results are
comparable with the data presented by
Ganai et al. (2006), Joshi and Attri (2006)
and Dhillon et al. (2013).
During pelleting, the increased moisture
content caused a decrease in the
temperature of the pellet press die (Figure
1). According to Vukmirović et al. (2010),
this can be explained by lower friction in
die channels, since water has a lubricating
effect and decreases friction between
surface of die channels and pelleted
material. When looking at bulk density
values (Figure 2) it can be seen that
treatment AP10 had significantly lower
bulk density (p<0.05) compared to
treatments AP13 and AP16. On the other
hand, there was no statistically significant
difference in the bulk density of the
batches AP13 and AP16. However, much
more important was to compare bulk den-
sity before and after pelleting. Bulk density
of pelleted AP was almost twice as high
than it was before pelleting.
Temperature of pellet press die and spe-
cific energy consumption (SEC) can be
used as an indicator of the friction forces
in the die openings of the pellet press.
Similarly to results for die temperature, an
increase in moisture content of AP before
pelleting decreased the friction in die
channels due to lubricating effect of water,
which resulted in significantly lower energy
consumption of the pellet press (Table 2).
Similar results were presented by Fairchild
and Greer (1999) and Moritz et al. (2003)
who observed a decrease in pellet mill
energy consumption when moisture was
added to the pelleting mash at the mixer.
Lower friction during pelleting of AP with
higher moisture contents resulted in lower
pressures in the die openings. This
caused significant reduction of pellet hard-
ness with increasing the moisture content
of AP (Table 3). Thus, positive effect of
increased moisture content of AP was the
reduction in energy consumption of the
pellet press, which provides energy saving
in the pelleting process. Additionally, softer
pellets, obtained by pelleting AP with
higher moisture content, would be chewed
by ruminants more easily. Also, softer
pellets would be easier to grind in the pro-
cess of production of poultry or pig feed.
PDI is the most important quality para-
meter, with low pellet durability causing
major damage of pellets during trans-
portation and handling. Pellet quality for all
examined AP batches was very high,
since the obtained values for PDI were
above 99%, i.e. there was almost no
abrasion. Such results can be explained
by the high sugar content in AP. During
pelleting, sugars are solubilized. As a re-
sult of sugars’ recrystallization during co-
oling, binding between particles takes
place due to the formation of a solid bridge
(‘solid-solid interactions’) (Thomas et al.,
1998). Furthermore, high fibre content in
AP probably contributed to the pellet dura-
bility. Fibre may be useful in the pelleting
process, since fibre can entangle and fold
between different particles or strands of

Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
fibre (Rumpf, 1957). The highest PDI was
obtained for the batch AP16, which had
the highest moisture content. This is most
likely because sugars were solubilised to a
greater extent.
Figure 1. Temperature of the pellet die during AP processing
Results are given as mean ± standard deviation (n = 3)
Figure 2. Bulk density of the non-pelleted AP and AP pellets
Table 1.
Chemical composition of apple pomace (AP), (% w/w)*
Moisture
Crude
protein
Crude
fat
Crude
fibre
Crude
ash
Total
sugars
Reducing
sugars
AP
7.96±0.33
5.9±45
2.26±0.12
20.76±0.27
2.06±0.04
30.29±0.12
27.12±0.43
Results are given as mean ± standard deviation (n = 3)
*Wet weight basis
Table 2.
Specific energy consumption (SEC) in the pelleting process
AP10
AP13
AP16
SEC*, kWh/t
34.15±4.76
a
25.87±4.03
b
22.71±3.52
c
Results are given as mean ± standard deviation (n=25)
a,b,c
Means with different letters in the same row for each sample are statistically significantly different
*SEC for the batch AP10 was statistically significantly different compared to the SEC for the batches AP13 and
AP16, at the level p<0.01. Differences in SEC between batches AP13 and AP16 were statistically significant at
the level p<0.05

Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
Table 3.
Pellet Durability Index (PDI) and pellet hardness
AP10
AP13
AP16
PDI, %
99.13±0.24
а
99.55±0.27
а
99.74±0.01
а
Pellet hardness, KH*
12.22±2.28
a
9.66±1.34
b
6.06±1.07
c
Results are given as mean ± standard deviation (n=15)
a,b,c
Means with different letters in the same row for each sample are significantly different (p<0,01)
*Kahl hardness
According to Wood (1987) and Vukmirović
(2015), there is a positive correlation
between pellet hardness and pellet dura-
bility. Consequently, it could be expected
that an increase in moisture content of AP,
which decreases pellet hardness, could
also cause decrease of PDI. Nevertheless,
in this research PDI was very high in all
pelleting treatments, regardless of mois-
ture content before pelleting. This was pro-
bably because low moisture content
caused higher friction and pressure in the
die holes. This resulted in decrease of
distances between particles thus enhan-
cing particle binding by solid-solid interact-
tions (Thomas and van der Poel, 1996;
Vukmirović, 2015). On the other hand,
higher moisture content of AP provided
more bonds between particles due to ca-
pillary sorption and caused better com-
pression characteristics of material due to
chemical changes of AP components
(Thomas and van der Poel, 1996). Con-
sequently, there were no significant dif-
ferences between PDI values when AP
batches with different moisture contents
were pelleted.
CONCLUSIONS
From the obtained results it can be seen
that AP is characterized by high pelle-
tability, since it was shown to have very
good pelleting properties and high quality
of pellets was obtained. Addition of water
in liquid form is sufficient for the successful
pelleting of AP, i.e. there is no need for
steam conditioning of AP before pelleting.
This will decrease production costs com-
pared to situation when water is added in
the form of steam. Initial moisture content
of 10% would be sufficient for producing
high quality AP pellets, but in terms of
energy saving, higher moisture content of
the initial material (up to 16%) would be
recommended. Moisture content of dried
AP is usually below 10%. In order to de-
crease pellet press energy consumption,
drying to higher moisture levels (approx.
16%) could be recommended. This will re-
sult in decrease of energy consumption for
drying. Increasing the moisture content of
AP resulted in softer pellets that would be
more easily consumed by ruminants or
more easily ground during preparation of
poultry or pig feed. Pelleting process
caused strong increase in bulk density of
AP which is positive in terms of trans-
portation and storage.
In addition to using pelleted AP as a stan-
dard feedstuff, good pelleting properties of
AP suggest that it could also act as bin-
ding agent with a potential to facilitate
pelleting process and to improve pellet
quality when AP is included in feed mix-
tures, but further research on this matter is
needed.
АCKNOWLEDGEMENTS
This paper is a result of the research
within the project III 46012 - Study of
modern biotechnological methods in the
production of animal feed in order to in-
crease competitiveness, quality and sa-
fety of the feed, financed by the Ministry of
Education, Science and Technological
Development, Republic of Serbia.
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Marijana Maslovarić et al., Pelleting properties and pellet quality of apple pomace,
Food and Feed Research, 42 (2), 147-154, 2015
ИСПИТИВАЊЕ МОГУЋНОСТИ ПЕЛЕТИРАЊА ЈАБУЧНОГ ТРОПА И
КВАЛИТЕТА ПРОИЗВЕДЕНИХ ПЕЛЕТА
Маријана Д. Масловарић
1
*, Ђуро М. Вукмировић
2
, Радмило Р. Чоловић
2
,
Недељка Ј. Спасевски
2
, Раде Д. Јовановић
1
, Наташа В. Толимир
1
1
Институт за примену науке у пољопривреди, Београд
11000 Београд, Булевар деспота Стефана 68б, Србија
2
Универзитет у Новом Саду, Научни институт за прехрамбене технологије у Новом Саду,
21000 Нови Сад, Булевар цара Лазара бр. 1, Србија
Сажетак: Јабучни троп представља споредни производ који остаје након пресовања
јабука у технолошком процесу производње сока. Директно одлагање јабучног тропа на
депонијама или земљишту представља еколошки проблем, с обзиром да је троп као
органски материјал, са високим садржајем влаге и шећера, погодан медијум за развој
микроорганизама. Један од начина даљег искоришћења овог споредног производа је његова
употреба у исхрани животиња. Будући да је јабучни троп у свежем стању нестабилан и
подложан кварењу, неопходно је извршити његово конзервисање, односно превођење у
форму у којој се ово потенцијално храниво може чувати у дужем временском периоду. Циљ
овог истраживања било је испитивање могућности превођења јабучног тропа у стабилан
облик применом технолошког процеса пелетирања, као и испитивање квалитета добијених
пелета. Јабучни троп у облику пелета био би прихватљивији као сировина у индустријској
производњи хране за животиње, са становишта стабилности, лакшег руковања и
складиштења. Непосредно пре процеса пелетирања, осушени јабучни троп подељен је у три
шарже у које су третиране само додавањем воде, ради постизања различитог почетног
садржаја влаге, односно 10% влаге у првој, 13% влаге у другој и 16% влаге у трећој шаржи.
На основу измерених вредности индекса отпорности, тврдоће и насипне масе пелета, може
се закључити да је постигнут веома добар квалитет пелета код све три шарже. Повећање
садржаја влаге довело је до смањења потрошње енергије у процесу пелетирања, чиме је
омогућена уштеда енергије. Процес пелетирања довео је и до значајног смањења насипне
масе јабучног тропа.
Кључне речи: јабучни троп, исхрана животиња, пелетирање
Received: 26 October 2015
Accepted: 25 November 2015