INFLUENCE OF CURD PARTICLES DRYING TEMPERATURE ON THE COMPOSITION OF CURD MADE OF MILK IN WHICH COAGGREGATES WERE FORMED

The influence of different drying temperatures of curd particles in order to obtain curd particles with chemical composition characteristic for Dutchtype semi-hard cheeses was investigated. The experiments were performed with heattreated milk at 87oC for 10 minutes to induce chemical interaction between casein and serum proteins and to form the co-called coaggregates. The curd particles were dried at 42oC and 45oC for 60 minutes and two types of curd, named curd A and curd B were formed afterwards. The results of the experiments showed that the used temperatures had no significant influence on chemical composition of the curd. The average total solids of curd A was 49.09%, moisture in fat free basis (MFFB) 68.78%, protein content 22.23%, lactose content 1.55%, ash 1.98%, titratable acidity 71.70T and pH 6.22. On the other side, the average values for curd B were 49.40%, 68.58%, 22.74%, 1.47%, 1.94%, 75.92T and 6.07, respectively. The use of different drying temperatures, as well as the other treatments (pressing, salting and ripening) provide conditions for semihard cheese producing, with the chemical composition similar to the Dutch-type semihard cheese prepared according to the traditional method.


I n t r o d u c t i o n
The curd processing treatments essentially determine the production of cheese with defined physical properties and chemical composition specific for the different type of cheese.The curd processing includes two different phases, the phase of curd particles formation and the whey phase.The whey draining as well as molding and pressing cause their separation completely.These steps define chemical composition, macro-and microstructure of the cheese, the way of ripening and the cheese sensory characteristics.
The curd processing includes several treatments such as curd cutting, stirring or agitation, scalding, removing part of the whey and drying.The purpose of the drying treatment is the formation of the curd particles with specific chemical composition and rheological properties.The aim of the curd processing is to obtain optimal conditions for the starter culture growth, which is very important for the aroma formation during ripening ( Sweet casein gel formed from the milk with heat induced chemical complex of casein and serum protein (coaggregates) has properties different from the heat induced gel prepared according to the traditional procedures.Higher heat treatments that induce coaggregate formation, produce less firmness and more cross-linked curd.This Protein matrix of the curd formed from the milk heat treated at 85ºC for 10 minutes is composed of short, branchy chains (fine crosslinking).These curds are characterized by the lower contraction.Also, they have lower whey separation ability (K a l a b , 1995).Due to the curd particles contraction, the increase of temperature accelerates sineresis directly.Unsuitable heating can cause the formation of curd particles crust and the decrease of the rate of whey releasing from the interior of a curd particles.This will produce unequalized cheese structure.Curd particles prepared at high temperatures are characterized by low acidity and higher content of calcium.Cheese prepared from these curd particles had higher elasticity (P e j i ć , 1959).
M a ć e j and J o v a n o v i ć (1998) showed that heating at 87ºC, 90ºC and 95ºC for 10 minutes induced the same level of coagreggate formation.Furthermore, according to these authors the changes of milk sugar caused at 87º C were the smallest.
For these experiments milk was treated at 87 o C for 10 minutes.These conditions gave several advantages including better utilization of milk protein and fat, higher yield and higher biological value of cheese.

Material and Method
Milk treated at 87ºC for 10 minutes was used.These conditions induced chemical interaction between casein and serum proteins (coaggregates formation).Coagulation of milk was performed with rennet at 35ºC for 20 minutes.The curd was processed according to modified traditional Dutch semi-hard cheese procedure.Drying temperature was modified.The curd particles were dried at 42ºC and 45ºC for 60 minutes and formed two types of curd, named curd A and curd B. We registered the effect of drying temperatures on curd chemical composition.
Each experiment was performed in 12 replicates (n=12) The following analyses were performed: -Determination of total solids by standard drying method at 102±2ºC (AOAC 16.Statistical analysis was performed.All data for the investigated parameters are shown as mean values.Also, analyses of variance for all data were performed (standard deviation and coefficient of variation).

Results and Discussion
The most important quality parameters of milk treated at 87ºC for 10 minutes are presented in table 1 According to these parameters, total solids were 11.45%-12.41%; the average value was 11.82%.Total solids in non-fat matter were 8.25%-9.21%(the average value 8.57%).Due to the standardization performed before heating, milk fat content was lower relative to the raw milk.The average milk fat content was 3.25%.Nitrogen content was 0.4470%-0.5044%(the average value 0.4834%), while protein content was 2.85%-3.22%.
The average content of lactose and ash was 4.63% and 0.75%, respectively.Titratable acidity was 6.99 0 SH and pH value was 6.52.

The effect of drying temperature on curd composition
The important stage of curd processing is drying of curd particles.During this stage the whey separation (syneresis) is completed.In contrast, G r e e n and G r a n d i s o n (1993) suggested that syneresis had occurred through the earlier phase of rennin process.The purpose of the drying treatment is to obtain desirable level of total solids and optimal acidity of curd particles.Optimal acidity is an important regulation factor for biochemical processes occurring through molding, pressing, salting and, especially ripening.Drying stage caused a suitable structure of curd particles incorporated in cheese.Thus, this treatment is important for their microstructure (G r e e n and G r a n d i s o n ,1993; P e j i ć , 1956; S c o t t , 1986; W a l s t r a , 1993; W a l s t r a et al.,1993).Two drying methods were used for these experiments, heat treatment at 42ºC and 45ºC for 60 minutes.These values are higher relative to the temperatures used for the traditional semi-hard cheese making method (P e j i ć , 1956; W a l s t r a et al., 1993).This could be explaind by slower whey separation (syneresis), which occurs in soft curd made by coagulation of milk in which coaggregates were formed (J o v a n o v i ć et al., 1996, M a ć e j , 1989, M a ć e j , 1992, M a ć e j , 1994, M a ć e j et al., 1999, P u d j a , 1992, P u d j a et al., 1995).
The composition of curd (t=42ºC) A and curd B (t=45ºC), depending on drying temperatures is reported in table 2 According to our results, different temperatures have no significant influence on chemical composition of curds.The curd B had only 0.31 % higher total solids.Total solids of curd A and B were 46.50%-50.89%(average 49.09%) and 47.46%-51.18%(average 49.40%), respectively.Coefficients of variations were low (2.58% for curd A and 2.33% for curd B).These values indicate that both data series were homogenous.Also, it can be seen that total solids data were similar to those obtained according to the traditional method for semi-hard cheese production (P e j i ć , 1956; W a l s t r a et al, 1993).Milk fat content of curd A was 24.00%-28.00%(average value 25.94%) and represented 48.63%-58.52% of total solids.
Simultaneously, fat content of curd B was 24.00%-29.75% and represented 47.38%-60.84% of total solids.The total solid contents as well as fat contents indicate that cheeses prepared from curd A and B could be classified as full fat cheeses.The protein content of both curds was high.The average protein content of curd A and B was 22.23% and 22.74%, respectively.Total protein represented 45.27% (curd A) and 46.01%(curd B) of total solids.According to these results, the decrease of fat content in milk (for example to achieve the level of 45% in total solids in the cheese) will result in a protein ratio increase.Furthermore, comparing our results those obtained by traditional method, it can be concluded that we obtained higher degree of protein utilization.
The average lactose content of curd A was 1.55%.Curd B had lower lactose content (1.47%).The same effect was registered for ash content.These values were 1.98% and 1.94%, respectively.
According to the aforementioned, it can be concluded that the different drying temperatures without removing part of the whey had no significant influence on chemical composition of the curd.

C o n c l u s i o n
According to the results of these investigations, it can be concluded that: Heat treatments at 42 o C and 45 o C had no significant influence on curd composition; Total solids content of curd A and B was 46.50%-50.89%,47.46%-51.18%,respectively; Moisture in fat-free basis (MFFB) of curd A and Curd B was 68.78% and 68.58%, respectively.This moisture in fat-free basis content by applying other treatments (pressing, salting and ripening), will no doubt provide conditions for classifying these cheeses as semi-hard Dutch-type cheeses.
C o g a n et al., 1990, C o g a n et al., 1993; E n g e l s et al., 1997; F o x et al., 1990; K o n i n g s et al., 2000; M i l a n o v i ć et al., 2000; P u d j a et al., 1994).
curd expresses low ability to contract and to release the whey (G r e e n and G r a n d i s o n , 1993; P e a r s e and M a c k i n l a y , 1989; P e a r s e et al., 1985; P e a r s e et al., 1986; W a l s t r a , 1993; J o v a n o v i ć et al, 2002).