A.R. Hill

Effect of milk concentration, pH and temperature on aggregation kinetics and coagulation properties of ultrafiltered (UF) milk

Abstract Aggregation kinetics of renneted ultrafiltered (UF) milk were monitored by measuring the complex viscosity ( μ ∗ ) at various times with a Nametre viscometer. The rate of aggregation increased with an increase in milk concentration (1×−4×) and temperature (28–37°C) and a decrease in pH (6·8-6·0). The coagulation time ( CT ) decreased with a decrease in pH from 6·8 to 6·0 and with an increase in temperature. The CT decreased with an increase in milk concentration when milk pH was unadjusted (7·0), but remained unaffected when pH was adjusted by adding lactic acid (6·8-6·0). The curd firmness index increased with an increase in both milk concentration and temperature, and with a decrease in pH. The aggregation kinetics suitably followed the Scott-Blair and Burnett model ( μ ∗ = μ ∗ ∞ exp ( −λ t ∗ ) ), where μ ∗ ∞ = limiting value of μ ∗ , cP ; λ = time at which μ ∗ = μ ∗ ∞ e , min and t ∗ = time after aggregation , min.

Rheological Properties of Rennet Gels Containing Milk Protein Concentrates

Different milk protein concentrates (MPC), with protein concentrations of 56, 70, and 90%, were dispersed in water under different treatments (hydration, shear, heat, and overnight storage at 4 degrees C), as well as in a combination of all the treatments in a factorial design. The particle size distribution of the dispersions was then measured to determine the optimal conditions for the dispersion. Heating at 60 degrees C for 30 min with 5 min of shear was chosen as the best condition to dissolve MPC powders. The samples were also characterized for composition, presence of protein aggregates, and ratio of calcium to protein. The total calcium present in MPC increased with increasing concentration of protein; however, the total calcium-to-protein ratio was lower in MPC90 than in MPC56 and MPC70. The level of whey protein denaturation, the presence of kappa-casein-whey protein aggregates in the supernatant after centrifugation, and the amount of caseins dissociated from the micelle increased as the protein concentration in the powder increased. The total amount of casein macropeptide released was lower in samples from powders with a higher protein concentration than for MPC56 or the skim milk control. The gelation behavior of reconstituted MPC was tested in systems dispersed in water (5% protein) as well as in systems dispersed in skim milk (6% protein). The gelation time of MPC dispersions was considerably lower and the gel modulus was higher than those of reconstituted skim milk with the same protein concentration. When MPC dispersions were dialyzed against skim milk, a significant decrease in the gelation time and modulus were shown, with a complete loss of gelling functionality in MPC90 dispersed in water. This demonstrated that the ionic equilibrium was key to the functionality of MPC.

Extending the shelf-life of cottage cheese using monolaurin

Abstract Shelf-life problems due to premature spoilage of cottage cheese result in approximately 5% return of product to the manufacturer each year. While preservatives are not permitted in Canada, it may be possible to use naturally occurring compounds to extend the storage life of cottage cheese. The monoglyceride monolaurin has been shown to possess anti-microbial properties as well as being an emulsifying agent. Incorporation of monolaurin into naturally contaminated cottage cheese at levels of 250 and 500 ppm resulted in >90% inhibition of both Pseudomonas spp. and coliforms during 7 days of storage at 6, 15, and 21°C. There was also >90% inhibition of growth of yeasts and moulds under the same storage conditions in the presence of monolaurin. These results suggest that the use of monolaurin as an emulsifying agent in cottage cheese will have beneficial effects by extending microbial shelf-life by approximately 35% at 6°C. Monolaurin also increased the shelf-life of cottage cheese (approximately 5–10 days) when assessed by organoleptic analysis.

Short communication: Determination of inulin in milk using high-performance liquid chromatography with evaporative light scattering detection

The objective of this study was to develop an HPLC method for the determination of inulin in a dairy matrix. Inulin is often added to dairy products to act as a source of dietary fiber as well as to provide technological functionality (e.g., water-holding or fat-replacing functions). The method includes hydrolysis of inulin with inulinase enzyme and determination of released fructose and glucose by HPLC coupled with evaporative light scattering detection, using water as the mobile phase. The effect of the milk background was investigated, preparing standards in milk, whey, or permeate and subjecting them to inulinase treatment. The developed method showed satisfactory linearity (R(2)=0.98 for glucose and 0.99 for fructose), good repeatability (relative standard deviation, RSD, ranging from 2.2 to 7.3% for glucose and from 0.6 to 2.3% for fructose), satisfactory reproducibility (RSD from 8.1 to 12.9% for glucose and from 3.1 to 4.9% for fructose), and good recovery (98.4 to 103.6%).