Journal of Dairy Research (1992) 59 563-560

Printed in Great Britain


Gredos goats' milk cheese: microbiological and chemical changes throughout ripening BY MARGARITA MEDINA, PILAR GAYA AND MANUEL NUNEZ Departamento de Production y Tecnologia de Alimentos, CIT — INIA, Ctra. La Coruna Km 7, Madrid 28040, Espana (Received 28 November 1991 and accepted for publication 24 March 1992)

Gredos cheese is a farm-made variety manufactured from raw goats' milk in central Spain. Milk at 25-30 °C is filtered through a cheese cloth and liquid animal rennet is added. No lactic cultures are inoculated into the milk. Coagulation takes place in 1-5-2-5 h. The curd is cut to rice grain or even lower size, left to settle in the bottom of the vat and the whey is removed 10 min later. Curd is scooped into cleaned moulds made out ofCeltis australis bark and pressed by hand in the moulds. Cheeses ~ 150 mm diam. and 60 mm high are obtained. They are dry salted after 2 and 6 h, the excess salt being washed out after 24 h. After ripening for 15 d at 8-10 °C and 80-90% relative humidity, cheeses are immersed in olive oil and held for at least 45 d at 8-10 °C. In the present work we have investigated the changes in the main microbial groups and in the chemical characteristics during ripening of Gredos cheese. MATERIALS AND METHODS

Six batches of Gredos cheese made at three different dairies following the procedure described above were studied. Samples were transported to the laboratory under refrigeration at 4 °C. Microbiological analyses Decimal dilutions and curd or cheese homogenates were prepared on receipt of samples as indicated by Fernandez del Pozo et al. (1988). Total viable counts were determined on plates of plate count agar incubated at 30 °C for 72 h. MRS agar (Difco Laboratories, Detroit, MI 48232, USA) acidified to pH 5-4 with sterile acetic acid and incubated at 30 °C for 72 h in an anaerobic jar (H2 + CO2 BBL GasPak, Becton Dickinson Microbiology Systems, Cockeysville, MD 21030, USA) was used to estimate levels of lactobacilli. Enterococci were determined on Kanamycin aesculin azide agar (Oxoid Ltd, Basingstoke RG24 0PVV UK) plates incubated at 37 °C for 24 h. Enterobacteriaceae were determined according to Mossel et al. (1962) and coliforms on plates of Violet red bile agar (Oxoid), incubated at 37 °C for 24 h. Ten Enterobacteriaceae colonies per plate were purified and tested for gas production in Brilliant green bile broth (Difco) after incubation at 45 °C for 48 h to estimate faecal coliforms. Staphylococci were estimated on Baird-Parker agar (Difco) plates incubated at 37 °C for 48 h. Ten staphylococci isolates per plate were checked for coagulase production according to Baird-Parker (1979). Presence of Brucella in milk was checked by streaking 0-1 ml milk on the surface of Brucella medium (Oxoid) and



incubating at 37 °C for 10 d in an anaerobic jar (CO2, BBL GasPak). Presence of Listeria in 25 g samples was examined in duplicate using the Listeria-Tek ELISA Test System (Organon Teknika Corporation, Durham, NC 27704-0969, USA). Chemical analyses Moisture, pH and NaCl were determined as indicated by Fernandez del Pozo et al. (1988). Curd and cheese homogenates were prepared from representative 50 g samples, and soluble N fractions in milk, curd and cheese were obtained and analysed as described by Gaya et al. (1990). PAGE was performed according to Andrews (1983). Residual caseins were quantified densitometrically as described by Gaya et al. (1990). Total N content of samples was taken into account for expression of results. Statistical treatment of results Analysis of variance was performed using program BMD 08V (Department of Biomathematics, UCLA, Los Angeles, CA, USA) and regression analysis by means of programs Curve and Regress (Sigstat, Provo, UT, USA). Tukey's test was used for comparison of means. RESULTS AND DISCUSSION

Microbiological characteristics Total viable counts, staphylococci and Enterobacteriaceae levels in Gredos cheese milk (Table 1) were considerably lower than those reported for goats' milk used in the manufacture of Valdeteja and Majorero cheeses (Gutierrez et al. 1988; Fontecha et al. 1990). Brucella and Listeria were absent from Gredos cheese milk. Owing to bacterial multiplication and retention of microorganisms in the curd during whey draining, most microbial groups increased ~ 1 log unit from milk to curd (Table 1). The microbial population in 1 d Gredos cheese was low, with 6-95 as mean log total viable count. Consequently, pH barely declined from milk to 1 d cheese (Table 2). After 3 d, mean log total viable count was still only 7-96, and the pH6-27. Multiplication of Enterobacteriaceae, coliforms and faecal coliforms continued at these high pH values, their maximum levels being attained in 4 d cheese. Staphylococci increased their numbers by 1 log unit from curd to 4 d cheese, but minimal changes were recorded for coagulase-positive staphylococci. Mean log total viable count reached its maximum level on day 15. At that time the pH had declined to 5-09, with decreases in levels of Enterobacteriaceae, coliforms and faecal coliforms. From day 15 to the end of ripening cheeses were immersed in olive oil. Some reduction in the levels of most microbial groups was observed during this period. Death rate of coliforms in Gredos cheese, calculated from linear regression (P < 0-0001) of log cfu/g on cheese age and expressed as the mean decrease in log cfu/g per week of ripening, was 0'615. This value was higher than those calculated for Majorero (0-374) and Ibores (0-302) cheese (Fontecha et al. 1990, Mas Mayoral et al. 1991), but considerably lower than the 1-088 obtained for Valdeteja cheese (Gutierrez et al. 1988). Death rate of staphylococci, calculated from linear regression (P < 0-001), was also higher for Gredos cheese (0-359) than for Ibores (0-103) and Majorero (0-322) cheese, but lower than for Valdeteja (0-429) cheese. Listeria was not detected in curd or in fresh or ripe Gredos cheese. Staphylococci were the only microbial group on which the particular dairy had a significant (P < 0-01) effect, according to the analyses of variance. Differences

pH H2O, % XaCl in moisture, %

Gredos goats' milk cheese: microbiological and chemical changes throughout ripening.

Journal of Dairy Research (1992) 59 563-560 Printed in Great Britain 563 Gredos goats' milk cheese: microbiological and chemical changes throughout...
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