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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 ... – PowerPoint PPT presentation

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Title: 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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