2009 Fat, Sugar Substitues - Enzyme-catalyzed...

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Enzyme-catalyzed discolorization of plant foods Enzyme : Polyphenoloxidase (phenolase, PPO) occurs exclusively in plant tissues Catalyzes oxidation of polyphenols to quinones Reaction requires oxygen Reaction is usually undesirable Reaction: First step: PPO O 2 R R orthopolyphenol (colorless) orthoquinone (colorless)
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Subsequent steps: Non-enzymatic rxns of orthoquinones to form polymerized, colored complexes orthoquinones further oxidation rxns (non-colored) cyclic carbonyl intermediates (reactive) polymerization High molec. wt. polymeric complexes " melanoidins " [ brown-black hues ]
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** Enzymatic browning does not occur in intact plant tissues: Compartmentalization of substrate and enzyme [different location in plant cell] Tissue Damage (bruising, slicing, dicing, juicing, freeze-thaw) Release of substrate, enzyme Browning **Enzymic browning occurs at different rates in different plant foods More obvious in light-colored plant foods: Banana, apple, pear, potato, eggplant, etc. Juices (pear, apple) (fresh apple juice is colorless)
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Plant tissues differ with respect to: 1. Substrate (polyphenol) concentration: high conc rapid browning 2. Polyphenoloxidase activity: high activity rapid browning 3. Chemical environment of plant tissue: presence of inhibitors of rxn
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Inhibition of enzyme-catalyzed browning 1. Thermal inactivation blanching : brief heat treatment [boiling water] denatures PPO enzyme 2. Limit oxygen concentration Vacuum packaging Lower-O 2 atmosphere around product (modified atmosphere) 3. Use of sulfites binds reactive substrates, intermediates 4. Ascorbic acid reduces quinones to phenols
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Oxidation of carbohydrates forming pigments and flavor compounds Proteins not required (as in Maillard browning) Examples: molasses chocolate, coffee caramelized fruit, vegetable (starch breakdown) Reaction: Complex, many possibilities, depending on the carbohydrate e.g.: glucose > 100 identified compounds heat Non-enzymatic browning reactions Maillard browning (already covered) Caramelization Caramelization :
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Can also cause undesirable
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2009 Fat, Sugar Substitues - Enzyme-catalyzed...

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