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Niven, Castellani, and Allanson (1949) described a new species of heterofermentative Lactobacillus that frequently is the cause of surface greening on cured meat items. This type of discoloration results from contamination of the meat surface after heat processing followed by holding conditions that allow extensive growth of these bacteria. The green color is the result of hydrogen peroxide produced by these bacteria reacting with the cured meat pigment -(Niven, 1951). Lactobacilli with identical serological, physiological, and nutritional characteristics also were isolated from sausages with green cores (Niven and Evans, 1950; Evans and Niven, 1951). This latter type of discoloration is caused by bacteria that survive the smoking and cooking process and multiply to a large population during subsequent storage of the product. The green core does not appear until the cut surface has been exposed to air for several hours, allowing the microorganisms in the core area to produce hydrogen peroxide (Niven, 1951). Thermal tolerance studies on some of these greening lactobacilli showed that strains isolated from green cores survived 6 to 10 times as long at 145 to 155 F as did strains isolated from cases of surface discoloration. These unexpected results suggested the possibility that conditions of manufacture and handling of these products in some plants are such that progressively more heat-resistant strains are "selected" or "trained". The heat resistance oflactobacilli from cheese has been studied by Slatter and Halvorson (1947) and Bassett and Slatter (1953) without reporting any similar wide variation in resistance among closely related strains. White (1953), studying the variations in heat resistance of a culture of Streptococcus faecalis in different stages of its life cycle, found that survivor curves of young cultures often show a change in slope in the middle of such curves. However, this could have been due to a residue of physiologically old cells in the young culture. An investigation of some of the factors concerned with variation in heat tolerance of strains of greening lactobacilli is described in the present report.

Thermal Tolerance Studies on the Heterofermentative Lactobacilli That Cause Greening of Cured Meat Products1