Seven rapid analytical tests (color value, thiobarbituric acid number, extract release volume, pH, “tyrosine” value, pHt, and redox potential) were evaluated as possible indicators of bacterial contamination in intact meat. Color value is a reflectance value related to hedonic acceptance of the meat. Comparisons of results from these seven tests with determination of bacterial load (plate count) and with time of storage were analyzed statistically to determine the relative contributions of bacterial action and of intrinsic reactions. The color values and “tyrosine” values were the most effect monitors of bacterial contamination. Although the thiobarbituric acid test effectively monitored changes in meat quality, it was not specific for those produced by bacteria. The remaining tests were ineffective under conditions employed.

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INTRODUCTION THE COLOR of fresh meat is determined mainly by the relative proportions of three meat pigments: purple reduced myoglobin (Mb), red oxymyoglobin (MbOz) and brown metmyoglobin (MetMb). Under normal marketing conditions reduced myoglobin in the presence of O2 is completely changed to Mb02 within a few hours (Van den Oord and Wesdorp, 1971a). The bright red of the MbOz is the preferred color of fresh beef packaged for retail @es. The change of Mb& to MetMb can be effected by bacterial growth or oxygen tension leyels, resulting in slow oxidation of the heme iron to its ferric state. As this change occurs the meat becomes less acceptable to consumers. The relative proportions of Mb& and MetMb are a prime factor in the acceptability of-fresh beef color to consumers. At approximately 50% conversion to MetMb the meat is unacceptable to most consumers and therefore unsuitable for retail sale (Van den Oord and Wesdorp, 1971a).The preferred method for measuring consumer acceptability is color evaluation by a panel of trained observers This method has several serious disadvantages for continuous evaluation of meat color changes. Panel measurements are time consuming, prone to subjective errors and limited in the number of evaluations which can be made at one time.Reflectance measurement is the instrumental technique of choice because it measures the color on the surface of the meat as observed by the consumer and it is nondestructive. Two types of instruments can be used in reflectance studies: calorimeters and spectrophotometers with reflectance attachments. Previous investigators have employed a variety of techniques for meat studies. Haas and Bratzler (1965) used Munsell disk calorimetry and a Gardner color difference meter to follow oxygenation rates in beef, pork and lamb. Snyder (1965) used the Gardner Rd, a, b, and a/b values to indicate changes taking place in intact beef samples. The "Rd" value is a measure of the total light reflected while "+a" is a measure of redness, " -a" is a measure of greenness, "+b" is a measure of yellowness and "-b" is a measure of blueness in the reflected light. He found that a high a/b value indicated a high concentration on the surface of the meat of either Mb or MbOz, while a low a/b value indicated a high concentration of MetMb. Romans et al. (1965) attempted correlation of Munsell disk calorimetry values with myoglobin concentration but found low correlation coefficients (r = -0.5). Jeremiah et al. (1972) related color difference values to consumer acceptability of beef color. Their study was designed to measure the preferred intensity of color from MbOz. They correlated the values from the Macbeth-Munsell disk calorimeter, the Gardner color difference meter, and the Photovoltreflectance meter. Dean and Ball (1960) used the Gardner aL value as a measure of redness or bloom in beef.Other reflectance methods which could be used for estimating consumer acceptability of beef color measure the concentration of the myoglobin pigments with a reflectance attachment to a s...

Chemical and instrumental methods for determination of nitrogen and protein are reviewed for their mode of action and utility in analysis of meat proteins and products. Although the Kjeldahl digestion method is satisfactory for determining total nitrogen, it is imprecise for determining total protein content. Presence of variable amounts of nonprotein nitrogenous components and of connective tissue proteins such as collagen and elastin produces error if the formula (N x 6.25) is used to calculate crude protein. Such fibrous proteins have higher nitrogen levels (over 18%) than other muscle proteins (about 16%), and a higher than actual protein value will be determined unless a lower conversion factor is used to correct for their content. To determine meat protein content more accurately, a combination of Kjeldahl determination with one or more additional tests to correct for nonprotein and fibrous protein content is recommended. The choice of the additional method(s) is based on the user's requirement for protein characterization, available time, type of meat product, and sample size.