Viscosity changes during rennet coagulation of Murciano-Granadina goat milk

Rennet-Induced Casein Micelle Aggregation Models: A Review

Two phases are generally recognized in the enzymatic coagulation of milk: hydrolysis and aggregation, although nowadays more and more researchers consider the non-enzymatic phase to actually be a stage of gel formation made up of two sub-stages: micellar aggregation and hardening of the three-dimensional network of para-κ-casein. To evaluate this controversy, the main descriptive models have been reviewed. Most of them can only model micellar aggregation, without modeling the hardening stage. Some are not generalizable enough. However, more recent models have been proposed, applicable to a wide range of conditions, which could differentiate both substages. Manufacturing quality enzymatic cheeses in a cost-effective and consistent manner requires effective control of coagulation, which implies studying the non-enzymatic sub-stages of coagulation separately, as numerous studies require specific measurement methods for each of them. Some authors have recently reviewed the micellar aggregation models, but without differentiating it from hardening. Therefore, a review of the proposed models is necessary, as coagulation cannot be controlled without knowing its mechanisms and the stages that constitute it.

Effect of HTST Thermal Treatments on End-Use Quality Characteristics of Goat Milk

Goat milk samples were pasteurized at high-temperature (72°C, 75°C, and 81°C) and in short-time (15 s and 25 s) combinations. Physical, chemical and microbial qualities of the pasteurized milk samples were evaluated 0, 2, 3, and 4 weeks of storage at 4°C. Despite the different thermal treatments, specific-gravity and viscosity were comparatively stable immediately after pasteurization (IAP). The viscosity of pasteurized milk at 81°C showed significant increase (P < 0.05) from 1.58 ± 0.18 to 2.30 ± 0.15 mPa s during four weeks of storage. Relative lightness “L” value decreased by about 10% during the storage period of 81°C pasteurization samples. Acidity increased with heat treatment irrespective of holding time, but in 81°C pasteurized sample higher acidity was developed at end of the storage. Fat oxidation 2-3 times higher at 81°C than 72°C pasteurized samples. Total protein (TP%) and nonprotein nitrogen contents were stable IAP but TP reduced significantly (P < 0.05) at two weeks storage. The whey protein denaturation increased with pasteurization treatments and storage time. Antioxidant activity of raw goat milk was 34.8 ± 5.01 μmol l⁻¹ and was decreased by 20–43% IAP compared to raw milk samples, but gradually increased during storage. IAP, mesophilic counts were in the range of 980–110 cfu ml⁻¹ (72°C/15–81°C/25 s) and increased from 2236 to 680 cfu ml⁻¹ samples stored at 4 weeks. Results showed that best quality stability of pasteurized goat milk achieved by heat treatments between 72°C/25 s and 75°C/25 s heat treatments up to 3 weeks of storage under 4°C.

Quality parameters of curd

The aim of this work was measurement of the curd firmness prepared by different volume of rennet solution and comparison of differences of curd firmness between these volumes. Further, it was observed the influence of different volumes of rennet up to thevolume of whey release, curd firmness, dry matter of curd and dry matter of whey. The composition of milk was determined according ISO and Czech state standard. Dry matter content (%) was determined by gravimetry, in drying oven at 102 °C to constant weight (ISO 6731:2010), the protein content (%) by Kjeldahl´s method (EN ISO 8968-1:2002), content of fat (%) by Gerber´s acidobutyrometric method (ISO 2446:2008), content of lactose by polarimetry, titratable acidity by titration Soxhlet-Henkel method, pH and calcium content in milk (g.L-1) was determined by complexometric titration with flueroxone as an indicator according to Czech state standart No 57 0530. For coagulation was used microbial rennet CHY-MAXRM 200. There were measured: curd firmness, rennet coagulation time, curd quality, volume of released whey, weight of curd, dry matter of curd and dry matter of whey. Different volumes of rennet solution had influence on curd firmness. Curd firmness (volume of rennet solution) had no significant influence on curd quality (five grade scale). Curd firmness had influence on volume of whey release (mL) out of the curd. With the increasing curd firmness s increased the volume of whey released from the curd. The volume of rennet solution had an influence on weight of curd. Curd dry matter raised with higher volume of rennet solution. Volume of added rennet solution had no statistically significant effect on the change of whey dry matter.