Title: FST 151
1FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY
151 Food Freezing Quality Aspects Lecture
Notes Prof. Vinod K. Jindal (Formerly
Professor, Asian Institute of Technology) Visiting
Professor Chemical Engineering
Department Mahidol University Salaya,
Nakornpathom Thailand
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10Effect of freezing on plant tissues (a) slow
freezing (b) fast freezing.
11Changes in foods
- Effect of freezing
- The main effect of freezing on food quality is
damage caused to cells by ice crystal growth.
Freezing causes negligible changes to pigments,
flavors or nutritionally important components,
although these may be lost in preparation
procedures or deteriorate later during frozen
storage.
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15The main changes in frozen foods during storage
are
- Degradation of pigments. Chloroplasts and
chromoplasts are broken down and chlorophyll is
slowly degraded to brown pheophytin even in
blanched vegetables. In fruits, changes in pH due
to precipitation of salts in concentrated
solutions change the color of anthocyanins.
16- Loss of vitamins. Water-soluble vitamins (for
example vitamin C and pantothenic acid) are lost
at sub-freezing temperatures . - Vitamin C losses are highly temperature
dependent a 10ºC increase in temperature causes
a sixfold to twentyfold increase in the rate of
vitamin C degradation in vegetables and a
thirtyfold to seventyfold increase in fruits.
Losses of other vitamins are mainly due to drip
losses, particularly in meat and fish (if the
drip loss is not consumed).
17- Residual enzyme activity. In vegetables which are
inadequately blanched or in fruits, the most
important loss of quality is due to
polyphenoloxidase activity which causes browning,
and lipoxygenases activity which produces
off-flavors and off-odors from lipids and causes
degradation of carotene. Proteolytic and
lipolytic activity in meats may alter the texture
and flavour over long storage periods. - Oxidation of lipids. This reaction takes place
slowly at -18ºC and causes off-odors and
off-flavors.
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23CROSS-SECTION OF ASPARAGUS SPEARS FROZEN BY THREE
METHODS(A) Immersion freezing(B) Contact
freezing(C) Still air freezing
24The ice crystal location in cod muscle, blast
frozeen at -30OC
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29Recrystalization of ice in beef tissues. Effect
of time and storage tempetarures on mean
equivalent diameters of te ice crystals.? -5OC,
o -10OC, ? -15OC, ? 20OC
30Change in ice crystal size distribution in
vanilla ice cream during accelerated
recrystallization with and without oscillations
(a and b) in the storage temperature.
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