b. Cause. The putrid odor is caused by the anaerobic decomposition of protein
by bacteria, mainly the Clostridium species. The bacteria cannot utilize proteins as they
are found in a food source. Therefore, the bacteria secrete an enzyme which breaks
the protein into simpler compounds, such as indole, hydrogen sulfide, ammonia, and
free amino acids. The bacteria cell wall is then able to utilize these compounds as food.
Since many of these compounds are very odoriferous, they give the characteristic
c. Summary. Putrid odor may be summarized by the following diagram.
1-17. ENZYMATIC DEGRADATION
a. Results of Enzyme Action. Deterioration of foods resulting from the
catalytic action of enzymes is easy to observe.
(1) An example is the ripening process of the banana wherein it becomes
sweeter, softer, less astringent in taste, and more odorous. One of the reactions in this
process is the hydrolysis (degradation) of the starch (essentially tasteless and insoluble
in the water of the banana) to simple water-soluble sugars.
(2) Another example is the overripe tomato. Here the softening is due
largely to the hydrolysis of the pectins to their simpler carbohydrate building blocks.
Pectin is a water-soluble carbohydrate found in ripe fruits and has strong gelling
properties which are used in cooking.
b. Changes in Proteins and Fats. Proteins in such foods as cheese, meat,
and fish may be hydrolyzed to simpler compounds by the enzymes naturally present.
Fats in some foods may also be hydrolyzed by naturally present enzymes. Such
chemical changes are often manifested as changes in taste, odor, texture, and so forth.
c. Oxidative Enzymes. When freshly harvested products are processed for
eating, the normal cellular organization of the tissues may be disrupted, with the result
that residual enzymes may initiate degradative changes at a very rapid rate. One of the
most common examples of such changes is the rapid darkening of freshly peeled
potatoes, apples, peaches, and pears. Here the oxidative enzymes of the freshly
exposed tissues use oxygen from the air to change many naturally occurring colorless
compounds (phenols) to colored compounds (quinones). Other oxidizing enzymes
induce the common and sometimes intense "hay" flavor of vegetables such as lima
beans, corn, and broccoli if they are not cooked soon enough after harvesting.