title: Proteolytic degradation of ewe milk proteins during fermentation of yoghurts and storage.
abstract: Yoghurts are mostly produced from cow milk and to a very limited extent from ewe milk. The evolution of caseins and whey proteins in ovine milk submitted to different thermal treatments (63 degrees C/30 min; 73 degrees C/15 min; 85 degrees C/10 min or 96 degrees C/5 min) was followed during fermentation of yoghurts and during their storage up to 14 days, using two different sets of starters. One set of starter LAB was a "ropy" culture (YC-191), which is a well-defined mixed strain culture containing Streptococcus thermophilus ST-143 and Lactobacillus delbrueckii subsp. bulgaricus (LB-18 and LB-CH2). The other set of starter bacteria (YC-460) was a standard yoghurt culture("non-ropy") containing mixed strain culture of Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Contents of free amino groups in produced yoghurts increased gradually during the fermentation, up to a maximal value obtained after 4 h fermentation, then they did not change significantly during storage of yoghurt produced with YC-191 starter. In contrary, a large drop in the amount of free amino groups was observed in the first 24 h of storage in the case of yoghurt made with YC-460 indicating that microorganisms continue still to grow in low temperatures. During fermentation and storage of both yoghurt types, alpha-lactalbumin was hydrolyzed to a slightly bigger extent than beta-lactoglobulin. During fermentation, beta-casein was slightly more degraded than alpha(s)-caseins; however, the opposite was observed during storage up to 14 days. Generally, a more intense heat pretreatment led to a higher degradation of whey proteins and caseins during fermentation and storage. Differences in proteolytic activity between the two starters used (whey proteins more degraded by YC-191; caseins more degraded by YC-460) may lead to improvement in production and formulation of yoghurts differing in their physicochemical and rheological properties.
authors.affiliation: Institut National de la Recherche Agronomique, Laboratoire d'Etude des Interactions des Molécules Alimentaires, BP 71627, F-44316 Nantes Cedex 3, France.
authors.forename: Khaled
authors.initials: K
authors.lastname: El-Zahar
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Chobert
authors.forename: Mahmoud
authors.initials: M
authors.lastname: Sitohy
authors.forename: Michèle
authors.initials: M
authors.lastname: Dalgalarrondo
authors.forename: Thomas
authors.initials: T
authors.lastname: Haertlé
mesh_terms: D000596:Amino Acids / Q000032:analysis / Q000378:metabolism; D000818:Animals; D005285:Fermentation; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007778:Lactobacillus / Q000254:growth & development / Q000378:metabolism; D008892:Milk / Q000737:chemistry*; D008894:Milk Proteins / Q000378:metabolism*; D012034:Refrigeration; D012212:Rheology; D012756:Sheep; D013291:Streptococcus / Q000254:growth & development / Q000378:metabolism; D013997:Time Factors; D015014:Yogurt / Q000382:microbiology*
chemical_list: D000596:Amino Acids; D008894:Milk Proteins