_by Gerald D. Kuhn and Louise W. Hamilton_[2]
All methods of preserving food will alter, if not lower, the quality of
fresh fruits and vegetables, but these changes can be minimized. For
practical understanding of how to minimize quality changes you need to
know something about: (1) natural differences in raw food, (2) natural
causes of quality deterioration, and (3) how various food handling,
preparation, packaging and storage techniques and conditions affect
quality retention.
Because of insufficient research information on home preserved foods,
some research findings of commercially processed food have been used to
foster an increased awareness of factors affecting the quality of home
preserved food.
Natural differences in quality and nutrient content in raw food often
exceed losses caused by preserving food. Extreme variations (tenfold or
more) in vitamin A and C content have been found in some fruits and
vegetables. These natural differences are known to be caused singly or
collectively by differences between varieties, climate changes between
seasons in the same region, and between regions in the same year. They
are also influenced by some cultural practices, and maturity of crops at
harvest.
Because of color, texture and especially flavor qualities, some
varieties are more suited for canning; others are more suitable for
freezing. Few varieties possess the all-purpose criteria needed to
preserve them as either high-quality canned or frozen products.
There is no single, ideal maturity for harvesting or preserving fruits
and vegetables. Obviously, the quality of canned or frozen products made
from any maturity other than ideal for that specific commodity lacks the
ultimate potential of satisfaction for eating.
Three natural causes account for most quality changes in freshly
harvested fruits and vegetables--respiration, enzymes and oxygen. Their
effects generally increase with the time between harvest and
preservation, and at higher temperatures.
Freshly harvested fruits and vegetables are living organisms. They are
sustained by cellular reactions known as "respiration," in which native
enzymes use oxygen and components in food and give off heat, water and
carbon dioxide. The energy needed to sustain respiration is obtained
from a storehouse of natural components in food. As a result, natural
flavors and vitamins are diminished, sugars and sometimes acids
decrease, and texture may soften. The speed of respiration and the onset
of its effects differs among various fruits and vegetables, and is
generally lowered by refrigerator temperatures.
Besides enzymes relating to respiration, fruits and vegetables sometimes
contain other cellular enzymes associated with reducing food quality.
These enzymes act with oxygen to cause rapid discoloration of bruised,
peeled and sliced tissue; oxidation of flavors, and excessive softening.
The roles of oxygen in respiration and other enzyme reactions have been
noted. Oxygen may react still another way to lower food quality:
autoxidation, where sensitive flavor, color and vitamin components are
altered by oxygen. This causes fading of colors, off-flavors and,
reduced vitamin retention. These reactions occur quickly during canning
and drying of foods, and also account for slow changes during storage of
preserved food.