The object of this study was to reevaluate speciation of Propionibacterium by using numerical taxonomy. Fifty-six cultures representing eight species were studied. Thirty-eight morphological and physiological features were employed in the calculation of matching coefficients which were used to sort the cultures into groups. The results suggest a need for species consolidation; for example, Propionibacterium shermanii could appropriately become P. freudenreichii var. shermanii. Close resemblances were found between P. rubrum, P. peterssonii, and P. jensenii. Propionibacterium arabinosum and P. pentosaceum formed another mutually similar group. Further studies using additional cultural, physiological, serological, genetic, and phage host-range characteristics should be instituted to establish an improved classification of propionibacteria.

This paper, the second of three sections of a review, deals with propionate and acetate formation, carboxylation, decarboxyiation, and transcarboxylation, the roles of biotin and vitamin B12, and the enzymes involved in the propionic-acid fermentation. Pathways for propionate, acetate, and carbon dioxide formation by Propionibacterium have been thoroughly investigated and established. These investigations led to discovery of transcarboxylase, a biotin-containing enzyme, and were the result of studies showing that the formation of propionate does not involve the expected turnover of CO2. This enzyme appears to be unique to propionibacteria; it could be speculated that it is the first discovered of a group of transcarboxylases. One of the breakthroughs relative to the mechanism of biotin fixation reactions was the discovery that biotin can be carboxylated enzymatically.

This paper, the third of four, discusses the ripening and flavor production processes of Swiss cheese. First, general ripening changes are reviewed; then, a discussion of eye formation and body and texture changes follows. The important flavor compounds found in Swiss cheese are examined in detail beginning with consideration of the analytical methods used to isolate and measure fatty acids. The different volatile and nonvolatile fatty acids and their importance in Swiss cheese are considered, as well as production of propionic and acetic acids from lactate, production of keto acids, and lipolysis. Then, analyses for and significance of carbonyl compounds, alcohols, esters, lactones, hydrocarbons, and diacetyl are presented. Possibilities for production of these compounds by microorganisms are given. Subsequent sections deal with nitrogeneous compounds, degradation of caseins to peptides and amino acids and their importance to flavor, and end with a review of sulfur compounds present.

The fourth and last paper of this series deals mainly with bacteriological defects in Swiss cheese. These include: lack of eye formation, crystal formation, split defect, early and late fermentation, bitterness, and other flavor and color defects. Ways to counteract development of these defects are discussed.

Major events that led to acceptance of 30 to 300 as the most suitable number of colonies on plates for counting were reviewed. Three new sets of data were collected, involving triplicate plates of fifteen 1: 1.4 serial dilutions of 65 samples of raw milk. Statistical methods were developed to analyze bias (variability introduced primarily by crowding and analyst counting errors) and variance (sampling and dilution errors). Bias and variance were combined as mean-squared error, which was expresed as a function of the number of colonies per plate, The counting range that minimized the mean squared error could then be determined for selected dilution series. For two-fold, five-fold and ten-fold dilution series, respectively, the most suitable limits on plates for counting were 70 to 140, 40 to 200 and 25 to 250 colonies/plate. A range of 25 to 250 colonies/plate was suggested for the analysis of dairy products. Limitations in application of the data to other systems are discussed.