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CHLADEK, G., CEJNA, V., FALTA, D., MACHAL, L.: Eff ect of season and herd on rennet coagulation time and other parameters of milk technological quality in Holstein dairy cows. Acta univ. agric. et silvic. Mendel. Brun., 2011, LIX, No. 5, pp. 113–118 In fi ve herds of purebred Holstein dairy cows, altogether 220 bulk milk samples were collected with the following average parameters: rennet coagulation time (RCT) 265.2 sec., titratable acidity 6.40 °SH, active acidity 6.72 pH, specifi c density 1.0281 kg.l−1, fat content 3.81 g.100g−1, protein content 3.28 g.100g−1, lactose content 4.72 g.100g−1, and content of solids non-fat content 8.89 g.100g−1; average values of milk batch volume per herd and diurnal air temperature were 6,072 kg and 8.02 °C, respectively. It was found out that the variable “season“ (spring, summer, autumn or winter) signifi cantly infl uenced nearly all parameters under study (the only exceptions were values of titratable acidity and milk batch volume). As compared with all other seasons, the signifi cantly shortest RCT was recorded in summer. Further it was also found out that the variable “herd” infl uenced all parameters with the exception of average diurnal air temperature. Regardless to diff erences existing among individual herds, the shortest average RCT was recorded always in summer. dairy cow, milk, composition, coagulation time, season, Holstein breed, herd According to Janů et al. (2007) and Hanus et al. (2010), the technological quality of milk is infl uenced not only by fat, protein, lactose and solids non-fat but also by such parameters as active acidity and titratable acidity. Besides, rennet coagulation time (RCT), fi rmness of rennet curd, and volume of whey separated out during the process of enzymatic coagulation of milk are also very important (Hanus et al., 2007). RCT of milk infl uences signifi cantly the process of cheesemaking and also cheese yield, and its quality (Johnson et al., 2001). It is also an important parameter when evaluating milk suitability for processing and cheesemaking (Cassandro et al., 2008). Ikonen and Ruottinen (1999), as well, mentioned that their observations indicated that an improvement in coagulation properties of herd bulk milk could improve the effi ciency of cheesemaking. However, also those authors who did not primarily study problems of technological quality of milk concluded that the coagulation properties of milk (i.e. its rennetability) should be involved among eff ects analysed in experiments with dairy cows (Marchesini et al., 2009; Masoero et al., 2009). As mentioned by Daviau et al. (2000), it is well known that reducing the milk active acidity from 6.7 to 5.8 pH resulted in a faster rennet coagulation and that also reducing concentrations of casein led to a shortening of the coagulation time. Joudu et al. (2008) concluded that an increase in milk protein, casein, casein fractions, and the casein number decreased the rennet coagulation time of milk. Najera et al. (2003) observed in their study that the rennet coagulation time decreased in dependence on increasing coagulation temperature; on the other hand, however, an increase in pH resulted in their experiments in a prolongation of rennet coagulation time. The existence of phenotypic and genotypic 114 G. Chladek, V. Cejna, D. Falta, L. Machal correlations between milk coagulation time and protein content (0.02 and 0.22, respectively) was mentioned by Ikonen et al. (2004); according to Comin et al. (2005) the values of these correlations were −0.041 and 0.128, respectively. As far as the eff ect of breed on composition and technological quality of milk is concerned, it can be also considered as a relatively important factor. A lower technological quality of milk produced by Holstein dairy cows was mentioned by several authors who compared them with other breeds (Malacarne et al., 2005; De Marchi et al., 2007; Hanus et al., 2010). In spite of this negative feature, the Holstein breed is widely used and preferred by many cattle breeders, above all because of its high milk performance. However, it should be also mentioned in this context that diff erences in milk technological quality can be found out even within one single breed. Matějicek et al. (2008) studied the eff ect of kappa-casein (CSN3) and betalactoglobulin (LGB) genotypes on milk quality and coagulation properties in the Czech Fleckvieh breed. The above authors found out that in this breed the maximum value of RCT (135 s.) occurred in the genotypic variant BBBB while the shortest RCT (90 s.) existed in the variant BBAA. The content of milk protein, however, showed an opposite trend in these genotypes. Marked environmental eff ects on individual parameters of milk technological quality also cannot be neglected. These eff ects are usually involved into such models as eff ects of herd, year or season. So, for example De Marci et al. (2007) mentioned that in their experiments, milk produced by dairy cows of Holstein-Friesian breed showed the worst coagulation properties (including RCT) among all other breeds under study. In September and October, however, values of RCT were slightly better. Besides an important eff ect of breed, these authors mentioned also a highly signifi cant withinbreed eff ect of herd. In the Czech Republic, Hanus et al. (2011) observed that herd, year, and season showed a signifi cant eff ect on milk composition and its technological properties (including RCT) of milk produced by dairy cows of the Czech Fleckvieh breed. The aim of this study was to analyse eff ects of season and herd on the rennet coagulation time and other parameters of technological quality of milk produced by dairy cows of Holstein breed. MATERIAL AND METHODS Within one year, altogether 220 bulk milk samples were collected in fi ve herds (A, B, C, D, E) in the course of all four seasons: spring, summer, autumn and winter. All herds under study consisted only of purebred Holstein dairy cows. These bulk samples were a mixture of morning and evening milk. Milk samples were collected in all herds simultaneously (i.e. within one day) in one-week intervals for a period of 44 weeks and their analysis was performed immediately a er the delivery of produced milk into the dairy factory. The following parameters of milk technological quality were analysed: Rennet coagulation time (RCT), which was measured by means of a turbidimetric sensor of milk coagulation process (Skýpala and Chladek, 2005). Active acidity (AA) was measured with the pHmeter WTW pH 197. Titratable acidity (TA) was measured by milk titration (100 ml) using alkaline solution up to a light pink colour of the mixture (in ml of the 0.25 mol.l−1 naOH.100ml−1). Diurnal air temperature (T) was calculated as an arithmetic mean of temperatures measured at 7:00, 14:00 and 21:00 (twice) hours on the day preceding the day of milk analysis. Specifi c density (SD) was measured by lactometer. Milk composition involved fat content (F; g.100g−1), crude protein (P; g.100g−1), lactose (L – lactose monohydrate; g.100g−1) and solids non-fat (SNF; g.100g−1); these values were estimated using the NIR analyser MilkoScan S50. Milk batch volume (Q) was estimated in milking parlours before bulk milk transportation from dairy farms. Eff ects of season and herd were analysed using the GLM method of the programme STATISTICA (version 9.0) in which season, herd and week were used as constant eff ects. RESULTS AND DISCUSSION Eff ects of individual seasons on milk parameters under study are presented in Tab. I. As one can see, numbers of samples evaluated within the framework of an individual season ranged from 45 to 65 in summer and in the spring, respectively. In individual seasons, the following average diurnal temperatures were ascertained: 8.30 °C in the spring, 17.39 °C in summer, 6.48 °C in the autumn and 1.10 °C in winter. Although the spring temperatures were similar to those measured in the autumn, the diff erences among all seasonal temperatures were signifi cant. Excepting titratable acidity and milk batch volume, the eff ect of season on all parameters of milk technological quality was signifi cant. As compared with spring, autumn and winter, the signifi cantly shortest rennet coagulation time was observed in summer (winter values of RTC were the longest).

Effect of season and herd on rennet coagulation time and other parameters of milk technological quality in Holstein dairy cows