Alistair S. Grandison

Chemical composition and coagulating properties of renneted milks from different breeds and species of ruminant

The detailed chemical composition of 25 milks from different breeds of cow, sheep and goat were related to their properties of rennet clotting, coagulum development and syneresis at pH 6·4. Experiments in which concentrations of fat and whey proteins in milk were manipulated, and in which milks were homogenized at different pressures, were also carried out and the effects observed were related to the above processes. The composition of milks varied widely and many relations between concentrations of components could be related to their known modes of secretion from the alveolus or to their structural functions in the colloidal phase of milk. Rennet clotting was related to total Ca concentration and also to the proportions of α s - and β-caseins present. Coagulum strength was strongly related to total casein concentration in milk but was unaffected by total fat content or by casein/fat ratios. Syneresis was inversely related to fat content of milk and little affected by coagulum strength. Homogenization retarded syneresis, probably by altering coagulum structure and modifying entrapment of fat globules within the coagulum.

Effect of forage type and proportion of concentrate in the diet on milk fatty acid composition in cows given sunflower oil and fish oil

Based on the potential benefits of cis-9, trans- 11 conjugated linoleic acid (CLA) for human health there is a need to develop effective strategies for enhancing milk fat CLA concentrations. In this experiment, the effect of forage type and level of concentrate in the diet on milk fatty acid composition was examined in cows given a mixture of fish oil and sunflower oil. Four late lactation Holstein-British Friesian cows were used in a 4 x 4 Latin-square experiment with a 2 x 2 factorial arrangement of treatments and 21-day experimental periods. Treatments consisted of grass (G) or maize (M) silage supplemented with low (L) or high (H) levels of concentrates (65: 35 and 35: 65; forage: concentrate ratio, on a dry matter (DM) basis, respectively) offered as a total mixed ration at a restricted level of intake (20 kg DM per day). Lipid supplements (30 g/kg DM) containing fish oil and sunflower oil (2: 3 w/w) were offered during the last 14 days of each experimental period. Treatments had no effect on total DM intake, milk yield, milk constituent output or milk fat content, but milk protein concentrations were lower (P = C20 (n-3) polyunsaturated fatty acids (PUFA) and lower (P = C20 (n-3) PUFA content, but reduced (P 0.05) on total milk conjugated linoleic acid (CLA) (2.7 and 2.8 g/100 g fatty acids, for M and G, respectively) or cis-9, trans-11 CLA content (2.2 and 2.4 g/100 g fatty acids). Feeding high concentrate diets tended (P<0.10) to decrease total CLA (3.3 and 2.2 g/100 g fatty acids, for L and H, respectively) and cis-9, trans-11 CLA (2.9 and 1/7 g/100 g fatty acids) concentrations and increase milk trans-9, cis-11 CLA and trans-10, cis-12 CLA content. In conclusion, the basal diet is an important determinant of milk fatty acid composition when a supplement of fish oil and sunflower oil is given.

Effect of blending Jersey and Holstein-Friesian milk on Cheddar cheese processing, composition, and quality

The effect of Jersey milk use solely or at different inclusion rates in Holstein-Friesian milk on Cheddar cheese production was investigated. Cheese was produced every month over a year using nonstandardized milk consisting of 0, 25, 50, 75, and 100% Jersey milk in Holstein-Friesian milk in a 100-L vat. Actual, theoretical, and moisture-adjusted yield increased linearly with percentage of Jersey milk. This was also associated with increased fat and protein recoveries and lower yield of whey. The composition of whey was also affected by the percentage of Jersey milk, with lower whey protein and higher whey lactose and solids. Cutting time was lower when Jersey milk was used, but the cutting to milling time was higher because of slower acidity development. Hence, overall cheesemaking time was not affected by the use of Jersey milk. Using Jersey milk increased cheese fat content in autumn, winter, and spring and decreased cheese moisture in spring and summer. Cheese protein, salt, and pH levels were not affected. Cheese was analyzed for texture and color, and it was professionally graded at 3 and 8mo. The effect of Jersey on cheese sensory quality was an increase in cheese yellowness during summer and a higher total grading score at 3mo in winter; no other difference in cheese quality was found. The study indicates that using Jersey milk is a valid method of improving Cheddar cheese yield.

Contribution of the lactoperoxidase system to the keeping quality of pasteurized milk

The lactoperoxidase system is a naturally occurring antimicrobial system found in milk, with lactoperoxidase, thiocyanate and hydrogen peroxide as its components. The keeping quality of milk pasteurized at 72 degrees C for 15 s was found to be better than that of milk heated at 80 degrees C for 15 s. This agrees with previous findings and is usually attributed to heat shocking of spores. However, complete deactivation of lactoperoxidase occurred at 80 degrees C-15 s, whereas at 72 degrees C-15 s residual lactoperoxidase activity was approximately 70%, which may provide an alternative explanation. Higher levels of hypothiocyanite (the major antimicrobial agent produced by the lactoperoxidase system) were also detected in milk processed at 72 than at 80 degrees C, which supports the theory that the lactoperoxidase system has a role in the keeping quality of pasteurized milk. Of all the methods evaluated, titratable acidity and alcohol stability gave the most consistent estimates of keeping quality, while dissolved oxygen was a good indication of the onset of spoilage. Lactoperoxidase activity decreased with temperature more rapidly between 70 and 80 degrees C than is usual for an enzyme over a 10 deg C range.

Effects of variations in somatic cell count on the rennet coagulation properties of milk and on the yield, composition and quality of Cheddar cheese

Cheddar cheeses were prepared from low somatic cell count (SCC) milks and from milks containing 10, 20 and 30% added high SCC milk to give modestly elevated SCC. Increased SCO in the milk were associated with a reduction in the coagulum strength and an increase in moisture content of the cheeses. Cheese texture was affected significantly as judged both by the findings of an assessment panel and by instrumental physical measurement. The panel scores for flavour were also slightly altered. It was concluded that even a small increase in SCC is detrimental to cheese composition and quality, although no significant effects on yield were detected. Relationships between cheese yield, composition and properties, coagulation properties and composition of the milk were also studied. Yield was related to the total solids and fat in the milk but not to the coagulation properties or the flavour or texture of the cheeses. Coagulum strength, cheese composition and cheese texture were strongly interrelated.

Secondary (Non-enzymatic) Phase of Rennet Coagulation and Post-coagulation Phenomena

Cheesemakers often state that a good curd is required to make a quality cheese, one which is highly acceptable in both flavour and texture. This implies that the initial stages of casein aggregation to form a network influence the characteristics of the cheese. The mechanisms involved in network formation have been probed over the last 15 years, microscopic methods and rheological techniques being especially useful. It has been shown that the structure of the curd determines its properties and, thus, the retention of fat and moisture on which cheese yield and composition depend. The curd structure is also a direct precursor of the cheese structure, which must be the basis of its texture.

Challenge testing of the lactoperoxidase system in pasteurized milk

Aims: To determine the role of lactoperoxidase (LP) in inhibiting the growth of micro‐organisms in pasteurised milk.

Selenium supplementation of lactating dairy cows: effects on milk production and total selenium content and speciation in blood, milk and cheese

Forty multiparous Holstein cows were used in a 16-week continuous design study to determine the effects of either selenium (Se) source, selenised yeast (SY) (derived from a specific strain of Saccharomyces cerevisiae CNCM I-3060) or sodium selenite (SS), or Se inclusion rate in the form of SY in the diets of lactating dairy cows on the Se concentration and speciation in blood, milk and cheese. Cows received ad libitum a total mixed ration (TMR) with a 1 : 1 forage : concentrate ratio on a dry matter (DM) basis. There were four diets (T1 to T4), which differed only in either source or dose of Se additive. Estimated total dietary Se for T1 (no supplement), T2 (SS), T3 (SY) and T4 (SY) was 0.16, 0.30, 0.30 and 0.45 mg/kg DM, respectively. Blood and milk samples were taken at 28-day intervals and at each time point there were positive linear effects of Se in the form of SY on the Se concentration in blood and milk. At day 112, blood and milk Se values for T1 to T4 were 177, 208, 248 and 279 ± 6.6 and 24, 38, 57 and 72 ± 3.7 ng/g fresh material, respectively, and indicate improved uptake and incorporation of Se from SY. In whole blood, selenocysteine (SeCys) was the main selenised amino acid and the concentration of selenomethionine (SeMet) increased with the increasing inclusion rate of SY. In milk, there were no marked treatment effects on the SeCys content, but Se source had a marked effect on the concentration of SeMet. At day 112, replacing SS (T2) with SY (T3) increased the SeMet concentration of milk from 36 to 111 ng Se/g and its concentration increased further to 157 ng Se/g dried sample as the inclusion rate of SY increased further (T4) to provide 0.45 mg Se/kg TMR. Neither Se source nor inclusion rate affected the keeping quality of milk. At day 112, milk from T1, T2 and T3 was made into a hard cheese and Se source had a marked effect on total Se and the concentration of total Se comprised as either SeMet or SeCys. Replacing SS (T2) with SY (T3) increased total Se, SeMet and SeCys content in cheese from 180 to 340 ng Se/g, 57 to 153 ng Se/g and 52 to 92 ng Se/g dried sample, respectively. The use of SY to produce food products with enhanced Se content as a means of meeting the Se requirements is discussed.

Modeling of oligodextran production in an ultrafiltration stirred-cell membrane reactor

The aim of this study was to investigate the types of oligodextrans that could be generated via controlled enzymatic depolymerization of dextran using an endodextranase. An ultrafiltration stirred-cell membrane reactor was employed to provide the means of controlling product molecular size and characteristics. The process was operated in batch mode and the products were recovered as permeates. The effect of enzyme concentration (0.625–62.5 U ml−1 substrate), substrate concentration (2.5–7.5% w/v), and transmembrane pressure (200–400 kPa), each at three levels, on the product characteristics were studied. The experiments were arranged according to a central composite statistical design. Response surface methodology (RSM) was used to assess factor interactions and empirical models regarding four product response variables (i.e., permeate flux, yield, dextrose equivalent, and oligosaccharide content) were fitted. The fit of the models was found to be good. Permeate flux ranged from 0.01–0.11 (ml cm−2 min−1) and was significantly affected by the interactions among the operating variables. Dextrose equivalent values (DE) ranged from 18–38 with significant interaction between the enzyme concentration and substrate concentration. DE increased with increasing enzyme concentration and decreased substrate concentration. Product yields ranged from 25–84% and were principally related to the enzyme concentration. The oligosaccharide content of the products ranged from 27–82% (w/w) and was characterized by significant interactions among the operating variables. The use of RSM for modelling these types of membrane reactors is discussed.

Measurement of ionic calcium, pH, and soluble divalent cations in milk at high temperature.

Dialysis and ultrafiltration were investigated as methods for measuring pH and ionic calcium and partitioning of divalent cations of milk at high temperatures. It was found that ionic calcium, pH, and total soluble divalent cations decreased as temperature increased between 20 and 80 degrees C in both dialysates and ultrafiltration permeates. Between 90 and 110 degrees C, ionic calcium and pH in dialysates continued to decrease as temperature increased, and the relationship between ionic calcium and temperature was linear. The permeabilities of hydrogen and calcium ions through the dialysis tubing were not changed after the tubing was sterilized for 1h at 120 degrees C. There were no significant differences in pH and ionic calcium between dialysates from raw milk and those from a range of heat-treated milks. The effects of calcium chloride addition on pH and ionic calcium were measured in milk at 20 degrees C and in dialysates collected at 110 degrees C. Heat coagulation at 110 degrees C occurred with addition of calcium chloride at 5.4mM, where pH and ionic calcium of the dialysate were 6.00 and 0.43mM, respectively. Corresponding values at 20 degrees C were pH 6.66 and 2.10mM.