No abstract

The objective of this study is to characterize purebred Arab mares' milk through monitoring its physicochemical and microbiological composition during different lactation stages and to test its ability to coagulate. Sixteen purebred Arabian mares were selected among 45 mares with approximately the same foaling dates. Milk samples were collected once a week for each mare during 4 months of lactation and analysed in three replicates for physicochemical composition, nitrogen fractions, somatic cell count and total bacterial count. Coagulation assays were carried out in triplicates on fresh mare's milk using increasing doses of calf rennet, starter (Streptococcus thermophilus) and CaCl 2 . Arab mare's milk is characterized by an alkaline pH, low-fat and protein contents and high lactose content. The somatic cell and bacterial counts are very low, indicating good health status of mares. Fat and protein contents decreased as the stage of lactation progressed. However, as lactation number increased, fat level tended to increase while protein level tended to decrease. Clotting assays proved that it is possible to coagulate Arabian mare's milk using appropriate doses of calf rennet and starter, while respecting the coagulation procedures and parameters.

The aim of this study was to investigate the effect of somatic cell count (SCC) on mozzarella cheese quality. Three categories of raw milk were selected: low (SCC <115 000 cells/ml), medium (SCC=422 000 cells/ml), and high (SCC>987 000 cells/ml). Cheeses were produced on the same day of milk collection and were vacuum packed in plastic bags and analysed weekly for 60 days at cold storage (4 °C). As somatic cell count increases, casein content, casein as a percentage of true protein, lactose content, and titratable acidity decrease signifi cantly in raw milk. No signifi cant differences were observed in moisture, fat, and total protein contents among mozzarella cheese samples from the different SCC categories. However, cheese samples produced from high SCC milk had signifi cantly higher pH (6.83) compared to samples produced with low and medium SCC milk, 5.58 and 5.46, respectively. The extension of proteolysis was not signifi cant for cheese samples made from raw milk with low SCC during the fi rst 30 days of cold storage. Proteolysis levels increased signifi cantly on the 15 th storage day for cheeses made with medium and high SCC, whereas signifi cant increases were only observed on the 45 th storage day for cheeses made with low SCC.

The present study examined the enzymes responsible for proteolysis of casein in milk. Reversed-phase HPLC was used to differentiate the peptide products, of each proteinase, soluble in 12% trichloroacetic acid and in pH 4.6 milk filtrates. Peptides produced by bacterial proteinase were less hydrophobic and eluted early in the RP-HPLC chromatogram, while peptides produced by plasmin and somatic cell proteinase were more hydrophobic and eluted later. β-caseins were preferential substrates for plasmin, whereas α s-and κ-caseins were hydrolysed to a lesser extent. Proteases from bacterial origin predominantly affected κ-casein, while ß-casein and α s-casein were less susceptible. Somatic cell proteinase degraded mainly β-casein, followed by α s-casein and κ-casein. When milk was contaminated by bacterial proteinases, chromatograms of the TCA 12% filtrate showed early peaks, while the pH 4.6 filtrate showed early and late peaks when proteolysis was caused by either plasmin, somatic cell proteinase or bacterial proteinase.

For yoghurts were made from milk with different levels of somatic cell count (SCC) (low 95,000 cells ml(- 1), intermediate 398,000 cells ml(- 1) and high 1,150,000 cells ml(- 1)). Yoghurt samples were analysed for the degree of proteolysis, lipolysis (free fatty acid (FFA) content), acidity, pH and apparent viscosity on days 1, 14 and 28. The SCC had no significant effect (p>0.05) on either the acidity or the pH of the yoghurt after 1 day of cold storage. However, significant effects (p < 0.05) of SCC were observed after 14 and 28 days of storage. Yoghurt samples made from intermediate and high SCC milk showed higher viscosity (p < 0.05) and lower (p < 0.05) casein content on days 14 and 28 of cold storage than yoghurt made from low SCC milk. High FFA concentrations (p < 0.05) were observed only in yoghurt made from high SCC milk. High SCC in milk increased both proteolysis and lipolysis in yoghurt during storage.

No abstract

Hachana, Y . and Krai'em, K. 2009. Proteolysis and storage stability of ultra high temperature processed milk as influenced by somatic cell count and bacterial count. J. Appl. Anim. Res., 36: 65-70.To study keeping quality of UHTprocessed 1.5% fat milk of different somatic cell count (SCC) and bacterial count (BC), that would be sufficient to produce an off-flavor, a 180 days experiment was conducted. Sixty eight per cent ofpanelists detected off-flavor related to proteolysis, when the decrease in casein as a percentage of true protein (CN/ TP) was > 5%. UHT milk produced from low SCC milk (50,000 cells m1-I) and low bacterial count (2xl06 c f i ml-l). It was concluded that initial somatic cell and bacterial counts of UHT milk determined its keeping quality.

The objectives of this study were to compare conventional preservatives used in milk analysis, including potassium dichromate and bronopol with a non-toxic and biodegradable preservative such as methylene blue, to verify their effectiveness and limitations of use. This was achieved by comparing their effects on raw milk composition at different storage temperatures. The best performing preservative doses were 0.1, 0.036 and 0.01% for potassium dichromate, bronopol, and methylene blue respectively. Raw milk composition was significantly affected (p 0.05) during the storage period at 25°C for all tested preservatives. However, at 4°C, potassium dichromate and bronopol preservatives maintained stable milk composition during 10 days of storage, whereas methylene blue maintained stable milk composition for three days. Methylene blue can be considered as an environmentally friendly and safe alternative for short-term preservation of refrigerated raw milk samples.