Enzymes are ubiquitous in all living matter and are considered the essence of life. They catalyze reactions at ambient temperatures at rates of 102 to 107 moles of substrate converted to product per minute per mole of enzyme. In fruits and vegetables they are responsible for growth and maturation and for color, flavor, aroma, texture, and nutrient qualities; all these are desirable properties. However, they are also responsible for the production of unwanted antinutritive and toxic compounds. The enzymes continue to act in raw food products after the optimum level of color, flavor, aroma, texture, and nutritive quality has been achieved. They continue to act until all the substrate is exhausted or the pH changes to a value at which the enzymes are inactive or denatured.
Enzymatic reactions are chemical, biochemical, and physical reactions and affect the product quality during the processes before and/or after freezing, not during the freezing process. For example, a picked green banana becomes ripe only many days later. A pea in a pod detached from the vine continues to show many of the changes associated with ripening, and the pea removed from the pod is even more subject to these changes. The flesh of a bruised apple turns brown and softens. Also, meat continually changes in flavor and in tenderness, even though the animal is no longer living. All these alterations come from enzymatic reactions. Enzymes have the property of catalyzing or hastening chemical reactions. If these changes continue, they spoil the food.
Rancidity in foods is known as another type of chemical change that can take place in frozen products is the development of rancid, off flavors. This can occur when fat, such as in meat, is exposed to air over a period of time. Rancidity can be controlled by using a wrapping material which does not permit air to reach the product. It is also advisable to remove as much air as possible from the freezer bag or container to reduce the amount of air in contact with the product.