Chemical Evaluations

The chemical methods used for evaluating spoilage basically embody the assumption that as foods undergo spoilage, some new product or products are created by spoilage. Some of the specific components for which analyses have been undertaken and related to spoilage include: chitin, ergosterol, D-amino acids, D-lactate, tyramine, volatile basic nitrogen, trimethylamine, hydroperoxides, thiobarbi-turic acid-reactive substances, and headspace volatiles.

Chitin (N-acetyl-glucosamine) is an important component of fungal cell walls and is assessed by hydrolysis of chitin to glucosamine, followed by deamination to an aldehyde that is measured colorimetrically. Chitin, unfortunately, is also a major cuticular component of grain storage insects. Because insect debris is common in stored grain, the levels of chitin may not always be related to the level of spoilage.

Ergosterol (ergosta-5-7, 22-trienol) is another predominant sterol found in most fungi belonging to the classes Ascomycetes and Deuteromycetes. The amount of ergosterol produced by the fungi, however, is influenced by substrate composition, extent of aeration, and growth phase of mycelium; thus, it may not always be a true indicator of the extent of spoilage.

D-amino acids are important components of bacterial peptidoglycans. The presence of > 1 ppm D-alanine in a variety of fruit juices indicated bacterial as opposed to yeast contamination (21). Thus, the potential exists for this compound to be used as a spoilage indicator. Similarly the Disomer of lactate is not found in fresh foods and can be a diagnostic feature for bacterial spoilage.

Biogenic amines (putrescine, spermine, spermidine, histamine, tyramine) are produced by a number of bacteria and represent a hazard due to their psychoactive or vasoactive effects. In terms of their usefulness as spoilage indicators, only tyramine has been detected prior to the appearance of a faint putrid smell (initial stage of putrefaction). Using a sensor, composed of a tyramine oxidase-immobilized column and an oxygen electrode, tyramine levels have been found useful for estimating bacterial spoilage in aging beef. (22)

Both trimethylamine (TMA) and total volatile basic nitrogen (TVBN) levels have been routinely measured during iced and refrigerated storage of fish products. A recent comprehensive study, however, using 115 specimens of marine teleosteans and examination after three, five, and eight days of chilled storage revealed a strong relationship of organoleptically detected spoilage with TMA content, but not with TVBN content (23).

Food spoilage due to lipid oxidation has commonly been monitored by analyzing foods for the presence of hydro peroxides (primary reaction products) and for thiobarbi-turic acid-reactive substances (TBARS, secondary reaction products). Which compound is targeted depends on the food being analyzed. For instance, in frying oils, hydroperoxides are the compound of choice, whereas in muscle foods, TBARS has been the most common substance measured. The latter compound is preferred in muscle because hydroperoxides tend to break down quickly from interaction with the iron in the muscle.

At the root of many of the spoilage problems in food is the presence of off-flavors and off-odors. It therefore stands to reason that these compounds would be the target for identification and quantification. One of the most common means to measure the food for off-flavors and off-odors has been to sample the headspace of the food for its volatile content and subject it to gas chromatography (GC), oftentimes in combination with mass spectrometry (MS). In this manner, the dominant volatiles, 3-methyl-l-butanol, 1-octen-3-ol and 3-octanone, have been used as indicators of spoilage in stored grain (24). Another food item where GC has been used to correctly identify spoilage samples has been with shrimp. In this case, the data from 9 to 11 volatiles were required for identification of rancidity in shrimp meat (25). Similarly, GC/MS in combination with principal component analysis of the data was proven useful in rapid differentiation of control reduced-fat milk samples from reduced-fat milk samples abused by light, heat, copper, and microbial (P. fluorescens, P. aureofaciens, or P. pu-trefaciens) contamination (26).

Another instrumental method to analyze volatiles that just came on the scene in the early to mid 1990s is the electronic nose. This instrument consists of an array of nonspecific electronic chemical sensors that react differentially with the headspace volatiles of the food. Similar to the human nose, the electronic nose recognizes odor patterns rather than specific patterns. While the human brain processes the signals from our nose, a pattern recognition routine analysis program analyzes the sensor responses in the electronic nose. Thus, in conjunction with sensory panels, the instrument can be trained to recognize products that have changed significantly in aroma and flavor. The potential of electronic noses to serve as spoilage indicators has been demonstrated with products such as grain, ground meat, vacuum-packaged beef, and freeze-stored chicken (27). To further the use of this instrument for spoilage measurements, however, improvements will need to be made in the sensors' stability, susceptibility to humidity, and lifetime.

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