Advances in the biochemistry and microbiology of Swiss-typecheeses

Microbial and biochemical characterization of the Swiss-typecheeses Em~ental, Comt and Beaufort, and factors influencing the ripening and .qu~]!ty ~t.~yese~. we~e assessed. During the ripening of Comt, the numbers of thermophilic lactic acid bacteria decreased continuously, with simultaneous growth of non-starter bacteria. Facultatively heterofermentative lactobacilli (FHL) and propionic acid bacteria (PAB) increased from 1Q3-1Q4to 1Q8cfu:g-1 , rapidly reching a plateau at the end of the pre-ripening period in the case of FHL, and afte~thewarrn room in the case of PAB. Enterococci remained at levels of 10 3 _10 4 cfu-g " throughout ripening. Lactobacillus helveticus, which originated only from the starter, died off rapidly during ripening. L. delbrueckii ssp. dominated the milk and cheese. Several genotypes of L. delbrueckii ssp. lactis were identified. Among FHL, L. paracasei ssp. paracasei dominated; L. plantarum and L. rhamnosus were also isolated. Streptococcus thermophilus was present in ail milk sampi es and remained in the cheese for 1 month. Genotypic diversity was higher for starter and non-starter lactobacilli than for S. thermophilus. In model mini-Comt, 68 % of PAB were identified as Propionibacterium freudenreichii. Variability in the evolution of different species and subspecies of PAB and strong interactions between LAB and growth of PAB were observed during ripening. Overall casein hydrolysis of mature cheeses and parti cularly the level of small peptides and free ami no acids (FAA) increased in the following order: Beau-fort> Comt> Emmental. At the end of ripening, Emmental had the highest level of native caseins and the lowest level of y-CN. RP-HPLC mapping of the water soluble fraction showed significant differences between the three cheese varieties. Pasteurization (Pa) or microfiltration (MF) of milk reduced the number of non-starter microorganisms in the initial stages of ripening but similar levels of FHL and PAB to those in the raw (Ra) milk cheeses were reached at the end of cheese maturation. Pa increased the activity of plasmin (PLM) with concomitant hydrolysis of I3-CN. Ra cheeses had significantly higher levels ofFAA and a more intense and diversified aroma than Pa or MF cheeses. Cooking the curd/whey mixture enhanced the activation of plasminogen into PLM. Increasing the cooking temperature decreased chymosin activity and increased both the concentration and activity of PLM in cheese. These findings explain the higher level of I3-CN breakdown in Comt and Beaufort com-Oral communication at the 2nd Symposium on Propionibacteria, Cork, Ire1and, June 25-27, 1998.