Mechanisms of Food Additives, Treatments, and Preservation Technology 297
are (1) enzymatic browning reaction during peeling, cutting, mechanical damage,
etc.; (2) lipoxygenase oxidation during the frozen storage of fruit and vegetables;
and (3) degradation of pectin in squeezed citrus juices.
The most important endogenous enzymes and their effects on produce are
summarized in Table 10.3.
In intact plant tissues enzymes are localized in the cytoplasm, while their sub-
strates are in vacuoles (e.g., phenols), or in membranes (e.g., fatty acids). Enzymatic
reactions proceed within the metabolic pathways of anabolic or catabolic processes
ongoing in fruit or vegetable cells or tissues. The undesirable changes start when
enzymes come in contact with substrates. The substrates are fruit or vegetable
components or, in the case of oxidoreductases, oxygen. Thus, one possible way to
prevent undesirable enzymatic changes is to reduce the contact of enzymes with
substrates. Examples of how this is achieved involve limiting oxygen and/or mini-
mizing the concentration of substrates using preventative processes such as using
sharp knives or machine blades that make clean clean cuts without damaging plant
tissue. Other general prevention methods include acidification (or changing the pH
out of the enzyme’s optimum range) and use of various inhibitors, especially thermal
inactivation of enzymes (coagulation of protein parts of the enzyme).
TABLE 10.2
Maximum Tolerable Levels of Carbon
Dioxide and Minimum Levels of Oxygen for
Storage of Selected Fruits and Vegetables
Produce CO 2 (%) O 2 (%)Apple (Golden Delicious) 2 2
Asparagus (5°C) 10 10
Avocado 5 3
Banana 5 —
Broccoli 15 1
Cabbage 5 2
Carrot 4 3
Cauliflower 5 2
Citrus fruits — 5
Cucumber 10 3
Lettuce 1 2
Onion 10 1
Pea 7 5
Pear (Barlett) 5 2
Potato 10 10
Strawberry 20 2
Tomato 2 3
Source:Fellows, P., Food Processing Technology, Principle
and Practice, CRC Press, Boca Raton, FL, 2000c, Part 4.