Nitrogen matter changes during ripening of semihard cheese based on milk protein coaggregates

Sirevi proizvedeni na osnovi koagregata proteina mlijeka razlikuju se od sireva proizvedenih na tradicionalan način, kako po tehnološkom procesu proizvodnje, tako i po senzorskim karakteristikama a, prije svega, po teksturi sirnog tijesta i po okusu. U okviru ovih istraživanja pošlo se od pretpostavke, da će primijenjeni način toplinske obrade mlijeka, kao i način obrade gruša, imati odgovarajući utjecaj na karkateristike proteina kao supstrata, koji podliježe hidrolizi tijekom zrenja, zbog prisustva serum proteina koji utječu na smanjenje udjela kazeina u ukupnim proteinima sira. Tijekom zrenja pokusnih sireva, koje je trajalo 4 mjeseca, praćene su promjene dušičnih tvari. Uočene su značajne promjene na proteinima mlijeka, koje su vidljive u povećanju udjela topljivih dušičnih tvari, sadržaja primarnih i sekundarnih produkata razgradnje proteina, kao i dušika topljivog u 12 % trikloroctenoj kiselini (TCA) i 5 % fosfovolframskoj kiselini (PTA). Udjel topljivog dušika nakon proizvodnje, 15., 30., 60. i 120-og dana zrenja iznosio je prosječno: 135,48 mg %, 358,72 mg %, 473,52 mg %, 672,32 mg % i 845,13 mg %. S porastom udjela topljivog dušika, povećavao se i koeficijent zrelosti sira, koji je u istom razdoblju zrenja iznosio u prosjeku: 4,42 %, 10,14 %, 12,95 %, 18,21 % i 23,60 %. Udjel dušika primarnih i sekundarnih produkata razgradnje proteina tijekom zrenja izrazito je rastao od prvog do 120-og dana zrenja. Na kraju ispitivanog razdoblja zrenja, udjel primarnih produkata razgradnje proteina bio je 4,90 puta veći u odnosu na prvi dan zrenja, dok je udjel dušika sekundarnih produkata zrenja bio veći za 13,32 puta. Do najintenzivnijeg stvaranja primarnih produkata došlo je u prvih 15 dana zrenja. U usporedbi s literaturnim podacima pokusni sirevi su se malo razlikovali po «dubini» i «širini» zrenja u odnosu na tradicionalno proizvedene sireve tipa Edamskog, pa čak i u odnosu na sireve koje karakterizira duže razdoblje zrenja, kao što je Čedar. Spojevi topljivi u 12 % TCA i 5 % PTA mogu biti značajan pokazatelj dinamike proteolitičkih procesa tijekom zrenja sireva i mogu vrlo slikovito opisati «širinu», a osobito «dubinu» zrenja sireva. Udjel dušika topljivog u 12 % TCA povećavao se tijekom zrenja. Nakon 120 dana zrenja udjel dušika topljivog u 12 % TCA bio je 19,58 puta veći nego prvog dana zrenja. Dušične tvari topljive u 5 % PTA, kojima su molekularne mase manje od 600, tijekom zrenja stalno su rasle. Udjel dušika topljivog u 5 % PTA u vrijeme zrenja od 1. do 120-og dana imao je prosječne vrijednosti:12,98 mg %, 33,23 mg %, 53,87 mg %, 92,28 mg % i 148,32 mg %. Na kraju ispitivanog razdoblja zrenja udjel dušika topljivog u 5 % PTA bio je 11,43 puta veći u odnosu na prvi dan zrenja. Generalno, može se zaključiti da su promjene na proteinima bile vrlo intenzivne tijekom ispitivanog vremena zrenja, a najintenzivnije su bile u prvih 15 dana.Cheeses made on milk protein coaggregate basis are different than traditionally made cheeses, in technological production process and sensory characteristics, especially texture and taste. In this research it was assumed that applied milk thermal treatment, as well as curd processing, will have appropriate influence on proteins as substratum. During ripening, due to a presence of whey proteins, which influence decrease of casein content in total cheese proteins, substratum is hydrolyzed. In traditionally made cheeses, casein is the basis of protein matrix. In comparison to whey proteins, casein is substantially faster changed during ripening, while whey proteins incorporated in the curd give so called «unspecific» ripening. Besides, application of high temperatures influences decrease of plasmin activity in cheese, regardless of its significant thermal stability. During 4 months of experimental cheeses ripening, changes of nitrogen matter were investigated. Significant changes of milk proteins, such as increase of soluble nitrogen matter content, the primary and secondary nitrogen products of protein breakdown during cheese ripening, as well as non-protein nitrogen (12 % TCA) and phospho-tungstic-soluble nitrogen (5 % PTA) were observed. The average content of soluble nitrogen after production after 15, 30, 60 and 120 days of ripening were: 135.48 mg %, 358.72 mg %, 473.52 mg %, 672.32 mg % and 845.13 mg %, respectively. According to soluble nitrogen content increase, coefficient of ripening also increased and for the same ripening period was: 4.42 %, 10.14 %, 12.95 %, 18.21 % and 23.60 %, respectively. Content of primary and secondary products of protein breakdown during cheese ripening had significant rising trend from the first day of production to 120th day of ripening. At the end of investigated ripening period, content of primary products of protein decomposition was 4.90 times higher compared to the first day of ripening, while content of secondary products of protein decomposition was 13.32 times higher, for the same ripening period. The most intensive primary products formation occurred in the first 15 days of ripening. Compared to the literature, experimental cheeses were similar to traditionally made cheeses in the Edam type, and even to cheeses with longer ripening period such as Cheddar, regarding ripening «depth» and «width». Compounds soluble in 12 % TCA and 5 % PTA can be significant parameters of proteolytic processes dynamics during cheese ripening. Also, they can describe ripening «width», and especially «depth» of ripening. Nitrogen content soluble in 12 % TCA increased during ripening. After 120 days of ripening, nitrogen content soluble in 12 % TCA was 19.58 times higher compared to the first day of ripening. Nitrogen matter soluble in 5 % PTA, with molecule masses less than 600, also had permanent rising trend. Average values of nitrogen content soluble in 5% PTA during ripening period from 1 to 120 days were: 12.98 mg %, 33.23 mg %, 53.87 mg %, 92.28 mg % and 148.32 mg %. At the end of ripening period nitrogen content soluble in 5 % PTA was 11.43 times higher compared to the first day of ripening. Generally, it could be concluded that protein changes were very intensive during investigated ripening period. The most intensive changes occurred during the first 15 days of ripening