RELATIONSHIP WITH QUANTITATIVE AND QUALITATIVE PARAMETERS OF MILK PRODUCTION

MORPHOMETRIC CHARACTERISTICS OF SHEEP’S MILK FAT GLOBULES (part II): RELATIONSHIP WITH QUANTITATIVE AND QUALITATIVE PARAMETERS OF MILK PRODUCTION MART...
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MORPHOMETRIC CHARACTERISTICS OF SHEEP’S MILK FAT GLOBULES (part II): RELATIONSHIP WITH QUANTITATIVE AND QUALITATIVE PARAMETERS OF MILK PRODUCTION MARTINI M.1, SALARI F.1, SCOLOZZI C.1, BIANCHI L.2, PAUSELLI M.2, ROSSETTI E.2, VERITA’ P. 1 1 Dipartimento di Produzioni Animali, Facoltà di Medicina Veterinaria, Università degli Studi di Pisa, Viale delle Piagge 2, 56124 Pisa, Italy. Tel.: +39-050 2216897, Fax: +39050 2216901; [email protected] . 2 Dipartimento BVBAZ – Sez. di Scienze Zootecniche, Università degli Studi di Perugia, Italy. KEY WORDS: milk fat globule, quality, sheep. ABSTRACT The relationship between the morphometric characteristics of milk fat globules (MFG) and the quantitative and qualitative parameters of milk production were evaluated on 66 ewes raised in central Italy, belonging to the following genetic types: Sarda, Comisana, Sopravissana, and crossbreed. An amount of 198 milk samples was collected in three moments of lactation and the daily production was recorded. The MFG mean diameter and number, protein, fat and lactose concentration and somatic cell count were assessed. The results showed a correlation between the mean diameter of MFG and milk yield, which was positive and significant (P0.01) for Sarda and crossbred ewes. On the other hand, a negative correlation (P0.01) with the percentage of totals proteins was observed. The data confirmed a negative correlation between MFG mean diameter and number/ml. Moreover, the two specialized dairy breeds (Sarda and Comisana) showed a correlation coefficient of the opposite sign between MFG size and milk fat percentage. In fact, these parameters were probably influenced by different degrees of selection. The results obtained suggest important relationship between milk fat globules and milk quality and support the need for further researches on the subject. INTRODUCTION Milk produced by small ruminants is common in the Mediterranean basin and it accounts for almost one third of the world’s sheep and goat milk production. Sheep breeding is a useful way to exploit marginal lands (Boyazoglu and Morand-Fehr, 2001), which cover vast areas of Italy. This activity is of fundamental importance for both the social and economic development in rural areas, which is something European Community agricultural policies are keen to promote. In central Italy there are various ethnic compositions of sheep flocks that produce milk. They are either specialized dairy breeds or less specialised ones, such as double (and even triple) purpose local breeds and/or crossbreed ewes. In this context, sheep farmers tend to increase the overall milk productivity but they also need to give some added value to products, which they hope to achieve by using local breeds. In fact, sheep cheeses are commonly put on the market as specialities and niche products, and they are generally well accepted by consumers. Nevertheless, in order to justify higher selling prices, the overall

qualitative level of products needs to be improved, and consumers need more detailed information on such important traits. One of the main factors that affects sheep milk quality, from a sensorial, nutritional and technological point of view, is its fat content, and this has been studied in several recent research projects. From some years the authors are investigating on relationships between the morphometric characteristics of milk fat globules and milk quality (Cecchi et al., 2003; Martini et al., 2003; 2004; 2005, 2005a). The aim of the present study is to provide further knowledge on this subject with specific reference to genetic types reared in central Italy. MATERIALS AND METHODS Sixty-six ewes reared in Central Italy and belonging to four different genetic types (20 Sarda, 16 Comisana, 13 Sopravissana and 17 crossbreed sheep) were included in the study. Animals were pluriparous and had lambed in late February – early March. The feeding regime was based on pasture (eight hours a day), mixed hay (ad libitum), and cereal grains (250-1000 g/ewe/day depending on milk production). Only subjects showing no clinical signs of mastitis were included in the experimental groups. After weaning, milk samples were collected three times for each animal, at the beginning, middle and end of the milking period (according to the different lactation lengths of the four genetic types), accounting for a total amount of 198 individual samples. At sampling, ewes were hand-milked, the total production was weighed and an aliquot was taken to the laboratory for further analysis. Milk fat, protein and lactose percentages were determined using a Milkoscan 6000 FT apparatus (Foss Electric, Hilleröd, Denmark), and somatic cell count (SCC) were assessed with a Fossomatic 5000 (Foss Electric) cell counter. The number/ml and diameter (m) of milk fat globules (MFG) were calculated as described by Scolozzi et al (2003). Briefly, fresh milk, diluted 1:100 with distilled water, was added to an Acridine Orange solution (0.1% in phosphate buffer pH 6.8) and placed in a Burker chamber for observation with a fluorescent microscope and a Quantimet 500 image analyzer system (Leica Ortomat Microsystem, S.P.A., Milano, Italy). The frequency distribution of milk fat globules was evaluated according to their size; fat globule diameters were divided into ten size classes of 1 m width (from 9 m). Class width was further utilised to define small (6 m) globules. The differences between different genetic types in terms of milk qualitative parameters were then assessed by using the following linear model: yij= +i+bxij+ij where: yij = parameter considered;  = overall mean;i = fixed effect of the ith genetic type (i = 1,..,4); b= regression coefficient on lambing; (Xij); ij= residual error. The relationships between the morphometric characteristics of milk fat globules and the qualitative parameters were tested using Pearson’s correlation. Statistical analysis was performed using JMP software, Ver.3.1.6.2 for PC (SAS Institute, 1996).

RESULTS AND DISCUSSION Table 1 shows the differences between milk yield, pH, chemical parameters and somatic cell count in Sarda, Comisana, Sopravissana and crossbreed ewes. As expected, both specialized dairy breeds (Sarda and Comisana) had higher (P

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