The influence of storage time and temperature on the distribution of individual milk proteins between the micellar and soluble phases has been examined. Storage at 4 or 7 °C is accompanied by a dissociation of micellar caseins, particularly /?-casein, into the soluble phase during the first 48 h, but on further storage there is a partial reversal of this process. At higher storage temperatures (10-20 °C) the contents of all the individual caseins in the soluble phase decrease throughout storage. During cheese-making, losses of fat and curd fines in whey were greater with increased soluble phase casein and clotting times were prolonged. Curd structure was weaker, curds were more moist and slightly lower cheese yields were obtained in stored milks with elevated soluble-phase casein. When milks were stored for up to 3 d at 4 °C cheese gradings were virtually unaffected by storage, but higher temperatures (10-15 °C) led to cheeses being downgraded, largely for body and texture defects but also for flavour after 3 d.
The effects of increased somatic cell count, whether caused by infection or by experimental infusion of bacterial endotoxin, on the distribution in milk of caseins between the micellar and soluble forms were investigated. The relationship of somatic cell count to some cheese-making parameters was also studied. With quite modestly elevated cell counts (2-3 x 10 6 /ml) increases of up to 37 % in total casein in the soluble phase were observed, most of which was contributed by /^-casein, while K-and a sl -caseins increased only slightly. With storage at 4 °C, the concentrations of all the caseins, Ca and phosphate in the soluble phase increased substantially during the first 48 h, but this was followed by a slight decline on further storage. Rennet clotting time, losses of fat in whey, curd moisture, and losses in curd yield and rigidity were all greater the higher the somatic count. Clear differences were detectable in these parameters between milks of very low cell count (e.g. 5 x 10 4 cells/ml) and milks with counts more typical of those found in bulk supplies (e.g. about 5 x 10 5 cells/ml). If these findings can be reproduced in commercial practice even a modest reduction in bulk milk somatic cell counts might be expected to bring definite benefits.Udder infections modify milk composition by altering membrane permeability within the mammary gland and by reducing synthesis of milk components. Mastitisinduced changes in the gross composition of milk as well as changes in the relative proportions for the individual milk proteins and minerals are well established (O'Donovan
In assessing the performance of milk for cheese-making, the influence of a number of factors in addition to cold storage time and temperature of the milk on the equilibria between the soluble and micellar phases and on cheese-making parameters has now been examined. An important factor was pH and although not apparent in fresh unstored milk, after storage at 4 °C there was a minimum in the soluble casein concentration at about pH 6-6, largely due to variations in /?-casein concentration. As expected, addition of Ca reduced soluble casein concentrations while treatment with EDTA caused increases, and in most cases the cyclic pattern of casein dissociation from the micellar phase followed by a partial reassociation was again observed during storage at 4 °C over a 3-d period. Addition of low levels of urea (up to 75 mg/100 ml milk) led to some dissociation of all casein components, Ca and phosphate into the soluble phase while treatment of milk with urease caused small decreases. Additions of purified casein components to milk were accompanied by only small alterations to the composition of the soluble phase and added casein was largely incorporated into the micellar phase. In spite of this, most added casein was not recovered in curds, and cheese yields were not greatly enhanced by quite considerable additions of caseins to the milk. The effects of pH adjustment, addition of Ca, EDTA, urea, cyanate, purified a sl -, /?-or /c-casein and urease treatment on rennet clotting time, curd strength, curd moisture and cheese yield are presented.
BackgroundHuman brucellosis is a preventable zoonoses that may become persistent, causing, if left untreated, severe localized disease. Occupational exposure to infected animals or animal products and consumption of fresh contaminated dairy are main risk factors.MethodsOne hundred farmworkers employed at two cattle farms one in Khartoum North and one in Omdurman were screened for the presence of specific antibodies and seropositive workers were invited to donate a blood sample for blood culture. Molecular typing was used to characterize Brucella isolates.ResultsTen percent of farmworkers tested seropositive and while Brucella melitensis biovar 1 was isolated from the blood of three individuals, an isolate identical to the B. abortus S19 vaccine strain was isolated from a fourth person. All four bacteremic individuals were employed as milkers and did not have obvious disease.ConclusionsThe isolation of the highly infectious pathogen B. melitensis from seropositive workers is consistent with the notion that the pathogen may persist in the blood without causing overt disease. While vaccination with strain S19 is essential for the control of bovine brucellosis the vaccine strain may be transmitted to the human population and protective measures remain important to prevent exposure also in view of the presence of B. melitensis. To create awareness for this potentially severe disease more information on the prevalence of the pathogen in different risk groups and in livestock in the Sudan is needed.
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