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.
Handling Raw Foods
Because of the effects of respiration, other enzymes, autoxidation, and desiccation (loss of water), the raw product temperature and the time between harvest and preservation are of utmost importance to preserving high-quality fruits and vegetables.