-Microbial and biochemical characterization of the Swiss-typecheeses Em~ental, Comté and Beaufort, and factors influencing the ripening and .qu~]!ty~t.ç~yese~. we~e assessed. During the ripening of Comté, 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 reâching 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-Comté, 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: Beaufort> Comté> 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 Comté and Beaufort comOral communication at the 2nd Symposium on Propionibacteria, Cork, Ire1and, June 25-27, 1998.
Reçu le 25 mai 1994 ; accepté le 12 octobre 1994) Résumé -Afin de mettre en évidence le rôle de la flore indigène du lait dans la protéolyse et la flaveur du fromage, des fabrications ont été réalisées en parallèle à partir de lait cru et de lait microfiltré. La protéolyse des fromages a été appréciée à l'aide de plusieurs techniques: mesure des fractions azotées classiques par la méthode de Kjeldahl et des groupements aminés libres par l'acide 2,4,6 trinitrobenzène sulfonique (TNBS), étude par électrophorèse sur gel de polyacrylamide-agarose des caséines et de la fraction azotée soluble par chromatographie liquide haute performance (CLHP). Après 4 mois d'affinage, la composition physico-chimique et la qualité sensorielle des 2 types de fromages sont différentes. En effet, les teneurs en acide acétique et propionique sont plus faibles dans les fromages fabriqués à partir de lait microfiltré, ce qui laisse supposer une élimination de la flore propionique par microfiltration. Ces fromages, en comparaison avec ceux fabriqués au lait cru renferment moins d'acides aminés et petits peptides. Ce sont des produits peu typés et plus amers mais ils sont en revanche moins acides et moins piquants. L'étude montre que les fromages fabriqués au lait cru donnent une meilleure qualité organoleptique globale que les fromages fabriqués au lait microfiltré ; la flore indigène du lait semble donc avoir une influence significative sur la formation de la flaveur. flore indigène 1 microfiltration 1 protéolyse 1 flaveur 1 minifromage Summary -Comparison of the final quality of a Swiss-type cheese made from raw or microfiltered milk. The main objectives of this work were to study the influence of the indigenous microflora on the physico-chemical and sensory characteristics of Comté cheese. Experimental mini-cheeses were made from raw and microfiltered milk. To estima te the proteolysis, several analyses were carried out: nitrogen fractions by Kjeldahl method, free amino acids by 2,4,6-trinitrobenzenesulfonic acid (TNBS), agarose-polyacrylamide gel electrophoresis of caseins, high performance liquid chromatography of water soluble extract (HPLC). After 4 months of ripening, the physico-chemical composition and sensory quality of both types of cheeses were different. The ace tic and propionic acid levels were lower in y Bouton, R Grappin cheeses made from raw microfiltered milk suggesting that propionic flora was eliminated by microfiltration. These cheeses held less amino acids and small peptides than cheeses made from raw milk.They were also bitter and developed a poor flavor but on the other hand, they were less acid and pungent. The study showed that cheeses made from raw milk gave a better organoleptic quality than cheeses made from microfiltered milk; the indigenous microflora influenced the flavor formation.indigenous flora / microfi/tration / proteolysis / flavor / minicheese
(Reçu le 13 avril 1993; accepté le 8 septembre 1993) Résumé -Cette étude a permis de vérifier en site industriel, l'aptitude protéolytique de souches de lactobacilles thermophiles isolées de levains et de Comté et sélectionnées à partir de tests d'activités sur cultures (dosages des groupements -NH 2 par l'acide 2,4,6 trinitrobenzène sulfonique et de l'activité leucine aminopeptidasique). L'importance et la nature de la protéolyse observées sur fromages n'ont pas de relation étroite avec l'activité protéolytique des souches mesurée en laboratoire. En association avec des streptocoques thermophiles, les espèces' L delbrueckii subsp lactis produisent davantage d'acides aminés et de petits peptides que les associations avec L helveticus. La comparaison de profils peptidiques obtenus par CLHP pour les 2 espèces montre que la surface cumulée des pics en fin de chromatogramme, plus importante chez les fromages fabriqués avec les souches de L helveticus L79 et L80, est corrélée avec la note d'amertume.
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