Cheese Thought

Les Agents de L’Affinage: Les enzymes naturelles du lait

Natural Milk Enzymes…

All information is derived from Le Fromage by Andre Eck andJean-Claude Gillis and from Cheese: Chemistry, Physics, and Microbiology.

Les enzymes, agents de l’affinage, ont trois origines: les enzymes naturelles du lait, les enzymes coagulantes, les enzymes des micro-organismes qui peuplent les pates.

Les enzymes naturelles du lait

Le lait contient a l’etat natif un tres grand nombre d’enzymes, plus de 60 ont ete isolees ou indentifiees et l’activite de nombreux composants a ete detectee.

Les deux enzymes qui sont plus particulierement important: La plasmine et La lipase

La plasmine

La plasmine, une enzyme presente dans le serum sanguin, responsable de l’hydrolyse des caillots de fibrine.

Le <<systeme plasmine>>: la plasmine, le plasminogene, des inhibiteurs de la plasmine, des activateurs du plasminogene, et des inhibiteurs des activateurs du plasminogene.  Il y a 0,1 mg/l du lait.  Les activateurs du plasminogene presents dans le lait sont des proteases a serine de type urokinase….serait relativement limitee dans le lait cru, meme incube a 37 degres C.

La plasmine augmente en fin de lactation et est plus elevee dans le lait des vaches agees.  Les mammites augmententes la permeabilite de la glande mammaire vis-a-vis des proteines du sang et entrainent une augmentation des niveaux de plasmine et de plasminogene, laugmentation de plasmine pourrait etre dans ce cas egalement due a une activation du plasminogene par des proteases des cellules somatiques.

La plasmine est inactivitee par les inhibiteurs specifiques ed ces enzymes tels que le diisopropyl fluorophosphate…La plasmine possede une bonne thermostabilite…Apres la pasteurisation a 72 degres C, la quantite de plasmine augmente, augmentation qui a ete attribuee a l’inactivation thermique des inhibiteurs des activateurs du plasminogene..  L’enzyme resiste partiellement a un traitement UHT de 142 degres C pendant 3 s et necessite un traitement de 16 s pour etre totalement inactivee.  Selon certains auteurs elle pourrait jouer un role dans la gelification des lait UHT.

La plasmine possede une specificite etroite de type trypsine.  Elle hydrolyse les liaisons des caseines. Il y a les caseines specifiques qui sont plus augmentees que les autres.

L’action de l’enzymes au cours de l’affinage est appreciable et elle a ete bien mise en evidence, notamment dans les pates pressees a affinage lent.

La lipoproteines lipase

Compte tenu ses caracteres, on peut admettre que la LPL est capable d’intervenir de maniere signative dans les pates pressees non cuites mais seulement dans les fromages de lait cru.  Le degre de lipolyse dont ellse pourrait etre responsable reste cependant a preciser pour la plupart des pates fromageres.

MOD RIPENED CHEESES: Lipolysis during mold cheese ripening is caused primarily  by microbial lipases because the native lipase in milk is lairgely inactivated by pasteurization (except in cheeses in which rennet paste or pregastric esteraseis used).  As a result of lipolysis, the concentration of free fatty acids in cheese is usually 1-5 g/kg.  There is a close link between the content of free volatile fatty acids in a number of cheese varieties and their flavor.  Most of the free fatty acids arise from lipolysis.

2 lipases: acid and alkaline.  Alkaline lipases has a piquant or soapy taste.

SWISS CHEESES: Lipolysis is unwanted because of the atypical rancid taste it produces.  However, it is sometimes supposed that a slight amount of fat hydrolysis in milk contributes to the development of the characteristic aroma of raw milk Swiss-type cheese.

ITALIAN CHEESES: Extensive lipolysis occurs in hard Italian varieties (Romano, Parmesan, and Provolone) and in blue-veined cheeses.  In blue veined, lipolysis is due mainly to the action of lipases screted by P. Roqueforti, although indigenous milk lipase may also contribute, especially in raw milk cheese.  The hard Italian cheeses are probably unique in that an exogenous lipase is a normal additive.  They have a sharp, peppery flavor primarily due to relatively high levels of short-chain fatty acids, especially butyric.  The desirable ‘piccante’ flavor is due to the action of pregastric esterase (PGE) activity in rennet pastes, which are still used in Italy as the source of both coagulant (proteinase) and lipolytic agent.    PGE exists in the salivary glands and is secreted by at least 13 species.  Its role is to aid in the digestion of fat by young animals with limited pancreatic lipase…it is carried into the stomach with the ingested milk…The development of the desired flavor was related to the concentrations of both butyric acid and glutamic acid in Provolone cheese but only to butyric acid levels in Romano cheese.  Mucor meihi also secretes a lipase that is reported to give satisfactory Fontina and Romano cheeses although this enzyme must be used at five times the lipolytic activity of PGE to obtain satisfactory Romano cheese….it is commmercially available as  ‘Piccantase.’

DOMIATI AND FETA TYPES DURING PICKLING: High concentration of volatile fatty acids.  In Bulgarian Feta, 89% of volatile fatty acids is acetic acid.  Fat breakdown increases with an increase in storage temperature or a decrease in brine strength; however, changes in cheese stored at 3-5 degrees C were slow and independent of brine concentration.  The sharp flavor of Feta is attributed to rennet paste.  Ewe’s milk yields higher levels of short chain fatty acids, which yield that piccante flavor.  Lipolysis happens during pickling.  This is due to milk lipase, bacteria lipase, PGE, milk clottinng enzyme prepartions.  The metabolic origin os acetic acid is still to be determined.  It is likely that acetic acid rises primarily from lactate fermentation.   During the later stages of ripening, acetic acid as well as other short chain volatile fatty acids arises also from oxidative deamination of amino acids by lactic acid bacteria.

One conclusion about lipase is that its sourcing in milk content is found in raw, uncooked, homogenized milk, where the fatty acids and the proteins are well distributed for maximum lipolytic activity.

Les autres enzymes du lait

LA PHOSPHATASE ACIDE: de pH optimal voisin de 4,6-4,8, est tres thermoresistante et elle possede une activite important sur les caseines, mais elle est presente dans le lait  a faible concentration et seule une petite fraction, associee a la membrane du globule gras, doit se retrouver dans le fromage.

LA PHOSPHATASE ALCALINE: en grande partie localisee dans la membrane du globule gras, presente un pH optimal d’action importante sur les caseines isolees de 6,8.  Elle n’a pas d’activite sur la caseine native du lait mais les caseines isolees peuvent etre dephosphorylees sous son action.  Son activite au cours de l’affinage serait donc a preciser mais sa faible thermoresistance rend celle-ci peu probable dans les fromages de lait pasteurise ou thermise.

LA PROTEASE ACIDE: de pH optimum d’action voisin de 4 et presentant une stabilite thermique assez elevee, et sans doute en mesure d’agir sur les caseines au cours de l’affinage…Mais son action, s’apparentant par sa specificite a celle de la chymosine, est difficile a mettre en evidence et elle ne semble pas jusqu’ici avoir retenu l’attention des chercheurs.

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