Variability in True Protein, Casein, Nonprotein Nitrogen, and Proteolysis in High and Low Somatic Cell Milks

Abstract

Monthly variation in milk protein (total nitrogen X 6.38) and milk fat was determined for 115 farms over 2 yr. Yearly average milk protein and fat tests in each year were 3.16 and 3.62%, respectively. The mean regression coefficient for milk protein with respect to milk fat was .47 for the entire period. Twenty-four farms were selected and grouped high or low based on their previous 2-yr somatic cell history. Monthly milk samples for each farm were tested for direct microscopic somatic cell count, total nitrogen, noncasein nitrogen, and nonprotein nitrogen. No differences in monthly nonprotein nitrogen, true protein, and casein were found between groups. Casein as a percent of total nitrogen was significantly higher for the low somatic group for seven of the 12 mo studied but was significantly higher for 9 mo when expressed as a percent of true protein. The average increase in tyrosine value for incubated preserved milk was significantly higher for the high somatic cell milk, indicating higher proteolytic activity in high somatic cell milk. Electrophoretic analysis of high and low somatic cell milk indicated that there was substantial proteolytic breakdown of alpha S-casein and beta-casein by proteases associated with elevated somatic cell counts.

Genetic parameters for�lactation traits of�milking ewes: protein content and�composition, fat, somatic cells and�individual laboratory cheese yield

“…It is commonly assumed that milk from a teat affected by mastitis is richer in nitrogenous matter [3,20,27,41,49]. An increase in the protein fractions was in fact accompanied by an increase in LSCC.…”

“…The somatic cell count is a good indicator of the existence of subclinical mastitis [25], and some studies on dairy cows associated the variation of protein content and composition to a change in somatic cell count [3,5,49]. Nevertheless, little information is available on the genetic parameters of lactation measures for somatic cell count (LSCC) or the relationships between LSCC and yield or the composition of the milk of dairy ewes [7,17].…”

Avaliação do iogurte produzido com leite contendo diferentes níveis de células somáticas.

“…O aumento da CCS associado à deterioração na qualidade da proteína (GRANDISON & FORD, 1986) ocorre devido ao fato de, no leite mastítico, ocorrer aumento na atividade proteolítica em conseqüência à presença de altos níveis de proteases, que podem ter origem na plasmina (Enzyme Commission -3.4.21.7), nas células somáticas (DE RHAM & ANDREWS, 1982;ANDREWS, 1983;VERDI et al, 1987;SAEMAN et al, 1988) ou nas bactérias (GRIEVE & KITCHEN, 1985).…”

“…A atividade proteolítica pode estar associada em grande parte à CCS (SENYK et al, 1985;VERDI et al, 1987;SAEMAN et al, 1988;BALLOU et al, 1995), o que predispõe o leite a uma substancial hidrólise da α S -e β-caseína por proteases, havendo diminuição nos níveis destas frações no leite ANDERSON & ANDREWS, 1977;DE RHAM & ANDREWS, 1982;ANDREWS, 1983;VERDI et al, 1987).…”

“…Porém, a caseína expressa como porcentagem da proteína verdadeira não varia sazonalmente, mas é menor no leite com alta CCS (VERDI et al, 1987).…”