M.B. López

Proteolytic activities of some milk clotting enzymes on ovine casein.

Proteolytic activity of some milk clotting enzymes (calf and lamb rennets, bovine chymosin and pepsin, and proteases from Rhizomucor miehei and Cryphonectria parasitica) on ovine whole casein was determined by urea-PAGE and RP-HPLC. Microbial enzymes were more proteolytic than animal enzymes when acting on ovine whole casein. Lamb rennet and C. parasitica protease showed the lowest and the highest degree of proteolysis, respectively. Urea-PAGE results showed that all enzymes hydrolyzed ovine casein resulting in the formation of αs1-I and β-I as initial breakdown products of αs1-CN and β-CN. In addition to these products, C. parasitica protease produced a series of degradation products with lower mobilities than β-CN. Minor quantitative differences between coagulants from animal origin, but great quantitative and qualitative differences between microbial and animal coagulants (assessed throughout the study of the RP-HPLC peptide profiles), were observed.

Effect of calcium and enzyme in cutting time prediction of coagulating goats’ milk using a light scattering sensor

An on-line fibre optic sensor measuring backscatter at 880 nm was used for predicting the cutting time of goats’ milk. A factorial design was utilised to test the kid rennet and enzyme from Mucor miehei at three levels of added CaCl2. A cutting time prediction equation using the diffuse reflectance parameter, Tmax; predicted the cutting time with a standard error of 0.55 min. Tmax and cutting time decreased with CaCl2 addition. Coagulation was faster for kid rennet. The sensor was found useful for comparing the activity of milk-clotting enzymes. r 2002 Elsevier Science Ltd. All rights reserved.

Hot topic: Microstructure quantification by scanning electron microscopy and image analysis of goat cheese curd

Five microstructural parameters of goat cheese curd (number of pores, their area and perimeter, strand thickness, and porosity) were studied by scanning electron microscopy and image analysis. Image analysis was used to characterize and quantify differences in all parameters and to provide a procedure for the measurement of strand thickness. The micrographs provided visual evidence of differences in the protein matrix and were quantified by image analysis at 3 production times: 34 ± 1 min (cutting), 154 ± 6 min (before molding), and 293 ± 35 min (after pressing). The data showed that this procedure is an adequate tool for quantifying differences in the parameters analyzed in industrial samples despite their natural heterogeneity. The procedure was reproducible and repetitive for the first 2 production times because no significant intragroup differences were observed. Significant differences were found when comparing the values of the microstructure parameters analyzed at 34 ± 1 min and those corresponding to 154 ± 6 min and 293 ± 35 min, but no significant differences between samples analyzed at 154 ± 6 min and 293 ± 35 min were found. All microstructure parameters analyzed were related at a significance level of at least 95%. This procedure enables the characterization of the microstructure of industrial goat cheese curd.

Preliminary evaluation of an optical method for modeling the dilution of fat globules in whey during syneresis of cheese curd

An optical sensor designed to measure whey fat concentration was tested on whey samples from a cheese processing plant. Whey samples were collected to determine syneresis kinetics at different times after gel cutting. Normalized spectral sidescatter intensity was measured by mean of a fiber optic spectrometer (300-1100 nm). A sidescatter waveband ratio (S875/425) was calculated by dividing intensity at 875 by that at 425 nm. Whey fat concentrations were predicted by using the power low type equation previously developed by the above authors, [Fat] = .0 ln (.1 S875/425 + .2). Predicted whey fat concentrations were compared to actual concentrations measured by the Gerber method. The change in whey fat concentration with time after gel cutting was used to estimate the syneresis reaction rate. Results confirmed that fat dilution in whey followed a first order response. The light sidescatter technology for determining syneresis kinetics was considered to have potential but requires additional work to improve measurement accuracy.

Cheese-making capacity of goat's milk during lactation: influence of stage and number of lactations

The effect of the stage and number of lactations on the technological properties and cheese-making capacity was studied in Murciano-Granadina goats milk. Controls were done every month to evaluate the evolution of chemical composition and the values of the clotting properties which define the milk quality with regards to rennet curdling. The influence of the month of kidding was also evaluated. The average composition of the milk in the two kidding periods considered was as follows: fat 5.05% and 5.31%; protein 3.68% and 3.58%; casein 2.80% and 2.66%; lactose 4.71% and 4.68%, and dry matter 14.29% and 14.45%. No significant differences were found in the chemical composition due to the stage and lactation number; however, higher values of the protein fractions were determined in the first kidding period than in the second one. We determined that a higher casein concentration was associated with a lower milk clotting time and higher rate of firmness. We also found a highly significant (P<0.001) correlation between titration acidity values and casein content (0.551). No significant differences, however, were found in cheese composition between kidding periods.

Influence of ripening on proteolysis and lipolysis of Murcia al Vino cheese

The aim of this study was to evaluate the influence of five different manufacturers and two ripening periods on the proteolysis and lipolysis patterns of Murcia al Vino goat cheese. The manufacturers significantly affected the water activity (aw), pH, dry matter and fat content, several nitrogen fractions: water soluble nitrogen (WSN), trichloroacetic acid (12% w/v) soluble nitrogen (TCASN) and phosphotungstic acid (5% w/v) soluble nitrogen (PTASN); also the free amino acid (FAA) and free fatty acid (FFA) contents, with the exception of C4:0, C16:0 and C18:0. Different ripening periods significantly affected the dry matter content, WSN and PTASN and all FAA, except serine.

Influence of lamb rennet paste on the lipolytic and sensory profile of Murcia al Vino cheese

The influence of lamb rennet paste (71.1% chymosin, 177 international milk-clotting units/mL, 4.57U/g of lipase activity) during the ripening of Murcia al Vino goat cheese was studied. The aim of this study was to improve the knowledge of the effect of lamb rennet paste on the lipolytic patterns in this type of cheese by reference to the evolution of total and free fatty acids. A sensory analysis was carried out to compare cheeses made with commercial and paste rennet. The rennet paste showed higher lipolytic activity, enhancing the production of short-chain free fatty acids. In addition, the cheese produced with lamb rennet paste had a slightly more bitter and piquant taste, making it an attractive commercial alternative that can be used to develop new varieties of goat cheese.

OPTICAL MONITORING OF MILK COAGULATION AND INLINE CUTTING TIME PREDICTION IN MURCIAN AL VINO CHEESE

A fiber optic sensor measuring light backscatter (880 nm) was installed in an industrial manufacturing vat for Murcian al Vino cheese to monitor milk coagulation and predict cutting time. The coagulation of 20 manufacturing batches was analysed. Light backscatter profile responded consistently to typical coagulation rate changes normally encountered during processing. The light backscatter parameter, t2min, was used with the equation tcut = .0 t2min (1 + . [protein], %) to predict cutting time with an R2 of 0.8 and standard prediction error of 1.1 min. The constant . corrects the cutting time prediction for the effect of protein concentration. The regression coefficient .0 is the only parameter needed in plant calibration.

Effect of feeding goats with distilled and non-distilled thyme leaves (Thymus zygis subp. gracilis) on milk and cheese properties.

The aim of this study was to evaluate the effect of feeding goats with distilled and non-distilled thyme leaves (Thymus zygis subsp. gracilis) on the physicochemical composition and technological properties of pasteurised goat milk, and on the physicochemical composition, phenolic content, oxidative stability, microbiology, sensory and texture profile of Murcia al Vino goat cheese. One group of goats was fed the basal diet (control), the second and third groups were fed with different levels of distilled (10 and 20%) or non-distilled (3·75 and 7·5%) thyme leaves. Goat milk physicochemical composition was significantly affected by the substitution of 7·5% of basal goat diet with non-distilled thyme leaves (increase in fat, protein, dry matter and PUFA content), while goat milk clotting time was increased significantly by the introduction of 20% distilled thyme leaves, which reduces its technological suitability. Microbiology, sensory and texture profiles were not affected by the introduction of distilled thyme leaves. The introduction of distilled and non-distilled thyme leaves as an alternative feed to diet can lead to an inhibition of lipids oxidation. The introduction of distilled and non-distilled thyme leaves into goats diet can be successfully adopted as a strategy to reduce feeding costs and to take advantage of the waste from the production of essential oils, minimising waste removing costs and the environmental impact.

Technological Characterization of Experimental Natural Rennets Pastes

Four types of lamb rennet pastes were characterized according to the sort of abomasa and the treatment it received: full fresh abomasa (FFA), full dried abomasa (FDA), empty dried abomasa (EDA), and empty frozen abomasa (EFA). These rennet pastes were studied by means of different technological parameters (milk clotting time, chymosin content, lipase activity, and aptitude against milk coagulation).The highest level of clotting activity (391.20 IMCU/g) corresponded to the rennet EDA whereas the lowest (172.87IMCU/g) was found in EFA. The chymosin content of the pastes classified them as rennet extracts, the highest chymosin level (80.46%) being found in the paste from FDA and the lowest (71.10%) being obtained from FFA.The highest level of lipase activity (10.57 U/g) was found in EDA whereas the lowest (1.46 U/g) was in paste FDA. Milk coagulation aptitude was studied through the use of a near infra-red radiation dispersion sensor. The highest value of Rmax (0.049 min-1) found in the rennet paste made with EDA indicated a higher level of casein hydrolysis and greater aggregation speed. For the same rennet at tmax value of 6.1 min was established lower than for other rennet preparations which reflected the fact that this rennet hydrolyzed 80% of the K-casein in a shorter period of time. It is concluded that the best natural lamb rennet paste of this work was FFA due to it combines appropriate proteolytic and lipolytic activities although it does not reach the maximum values for each technological parameters analyzed.