Recombinant lamb chymosin (RLC) was prepared and tested for its potential use in cheese production. The milk clotting activity and proteolytic activity of RLC were evaluated in comparison with commercial recombinant calf chymosin (RCC), cow rennet (CR), and microbial coagulant (MC). RLC, RCC, and MC showed similar responses to pH, with a sharp increase of the coagulation time at pH 6.6 to 6.8 and decrease of curd firmness at the pH 6.5 to 6.6. In the case of CR, we observed two clear increases in the coagulation time and decreases in the curd firmness, at pH 6.4 to 6.5 and 6.6 to 6.8. Optimal clotting activity was obtained for RLC at 40 degrees C, for both CR and RCC at 45 degrees C, and for MC at 60 degrees C. The temperature instability of RLC at temperatures above 45 degrees C could constitute a benefit in making hard cheese varieties. The additon of CaCl2 to milk resulted in enhanced clotting activity of all coagulants, most prominently for CR. The proteolytic activity of RLC was significantly lower from that of CR but not significantly different from the activity of RCC. The lower proteolytic activity in the cheese made with RLC did not have negative effect on organoleptic properties. The overall quality of the cheese made with RLC was at least comparable to that of the cheese made with RCC, and both cheeses were better scored than the cheese made with CR.
The fate of ivermectin (IVM) residues was studied throughout the processing of daily bulk milk from 30 ewes (taken up to 33 d following subcutaneous administration of 0 . 2 mg IVM/kg b.w.) in the following milk products: yoghurt made from raw and pasteurized milk; cheese after pressing ; 30-and 60-day ripened cheese; and whey, secondary whey and whey proteins obtained after cheese-making (albumin cheese). The concentration of the H 2 B 1a component of IVM was analysed in these dairy products using an HPLC method with fluorescence detection. The mean recovery of the method was, depending on the matrix, between 87 and 100%. Limits of detection in the order of only 0 . 1 mg H 2 B 1a /kg of product were achieved. Maximum concentrations of IVM were detected mostly at 2 d after drug administration to the ewes. The highest concentration of IVM was found on day 2 in 60-day ripened cheese (96 mg H 2 B 1a /kg cheese). Secondary whey was the matrix with the lowest concentration of IVM (< 0 . 6 mg H 2 B 1a / kg). Residue levels fell below the limits of detection between day 5 (for secondary whey) and day 25 (for all cheese samples). In the matrices investigated, linear correlations between daily concentrations of IVM, milk fat and solid content were evident. During yoghurt production, fermentation and thermal stability of IVM was observed. During cheese production, approximately 35 % of the IVM, present in the raw (bulk) milk samples, was lost. From the results it was concluded that the processing of ewes' milk did not eliminate the drug residues under investigation. The consequences of IVM in the human diet were discussed. Milk from treated animals should be excluded from production of fat products like cheese for longer after treatment with IVM than for lower fat products.
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