FST 151

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FST 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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Effect of freezing on plant tissues (a) slow
freezing (b) fast freezing.
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Changes 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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The 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.

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• 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).

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• 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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CROSS-SECTION OF ASPARAGUS SPEARS FROZEN BY THREE
METHODS(A) Immersion freezing(B) Contact
freezing(C) Still air freezing
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The ice crystal location in cod muscle, blast
frozeen at -30OC
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Recrystalization of ice in beef tissues. Effect
of time and storage tempetarures on mean
equivalent diameters of te ice crystals.? -5OC,
o -10OC, ? -15OC, ? 20OC
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Change 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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