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Yeast populations in Sardinian feta cheese
International Journal of Food Microbiology 69 Ž2001. 153–156 www.elsevier.comrlocaterijfoodmicro
Short communication
Yeast populations in Sardinian feta cheese M.E. Fadda) , S. Cosentino, M. Deplano, F. Palmas Department of Experimental Biology, Section of Hygiene, UniÕersity of Cagliari, Cittadella UniÕersitaria, S.S. 554, 09042, Monserrato (CA) Italy
Abstract In this study, the yeast populations in feta cheese from two different Sardinian dairies were examined. Samples of good quality feta Ž32. and samples of feta with a slimy surface defect Ž10. were examined from Dairy A. Similar, samples of good quality feta Ž23., feta with slimy surface defects Ž14. and samples with swelling defects Ž6. were examined from Dairy B. KluyÕeromyces lactis was the dominating species in feta from Dairy A Ž95.2% of samples. followed by Debaryomyces hansenii Ž76.2%., Dekkera anomala Ž28.6%. and Dek. bruxellensis Ž19%.. D. hansenii was dominant in samples from Dairy B Ž93%., followed by K. lactis Ž23.3%., Geotrichum candidum Ž23.3%. and Dek. anomala Ž18.6%.. No significant difference was observed between the occurrence of yeast species in feta of good quality and in feta with slimy surface defects, thus confirming that slimy production is not associated with yeast contaminations. The swelling of samples observed in Dairy B seems to be caused by Dek. anomala. In fact, this strong fermenting species was present in all swelled samples in numbers exceeding 10 6 CFU gy1, while it was isolated in very low concentration in only 5.4% of good samples. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Sardinian cheese; Feta cheese; Yeast populations
1. Introduction Feta cheese is the most popular traditional Greek cheese. This soft white cheese is made from sheep’s milk but goat’s and cow’s milk may also be used. In recent years, this type of cheese is also being produced in Sardinia, one of the major ewe’s milk producing regions in Italy. Many studies on yeasts occurrence in feta cheese have been made ŽVivier et al., 1994; Tzanetakis et al., 1996.. Particularly, Kaminarides and Laskos Ž1992. have shown that the dominant yeasts of brine from Greek feta cheese were Saccharomyces cere-
Õisiae, Candida famata, Torulaspora delbrueckii and Pichia membranaefaciens. Westall and Filtenborg Ž1998. examined feta cheese made of pasteurized cow’s milk from three different Danish dairies and found T. delbrueckii, Debaryomyces hansenii, C. sake and KluyÕeromyces marxianus as the dominant yeasts. To our knowledge, the yeast content in Sardinian feta cheese has not been reported yet. This study concerns the identification of yeast flora in Sardinian feta cheese made in two different dairies.
2. Materials and methods ) Corresponding author. Tel.: q39-0706-754-195; fax: q396754-197. E-mail address: [email protected] ŽM.E. Fadda..
Feta cheeses examined in our study were made in two different dairies ŽDairy A and Dairy B. using
0168-1605r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 6 0 5 Ž 0 1 . 0 0 5 8 6 - 4
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
154
similar production processes. The cheeses were made from pasteurized ewe’s milk to which starter culture and animal rennet are added to obtain a coagulation. The curd, drained without pressing, is dry-salted until the cheese contains 3–4% wrv salt. The cheese is then packed in tin cans. Following packaging, 6–8% wrv brine is added to cover the cheese in the containers. The cheese is then kept at 14–16 8C for 10–15 days until a pH of 4–6 and a moisture content less than 56% is reached. At this point, the cheese is transferred to new containers, brine is added and the containers are sealed and stored at 4–5 8C. The cheese may be marketed 2 months after manufacture, but may be stored for well over a year at 2 8C. A total of 42 samples of feta cheese were received from Dairy A: among these, 32 were of good quality and 10 presented slimy surface defects. A total of 43 samples of feta cheese were analyzed from Dairy B: among these, 23 were of good quality, 14 presented slimy surface defects and 6 presented evident swelling after the package. Ten grams of product was taken from the interior of the cheeses, diluted in 90 ml of 0.1% sterile peptone water and homogenized in a Stomacher ŽPBI, Milan Italy. for 30 s at normal speed. For all samples, decimal dilutions were prepared in 0.1% sterile peptone water and numbers of yeasts were determined directly, by surface plating on Yeast Extract Glucose Chloramphenicol Agar ŽYGC. ŽMerck Darmstadt, Germany, pH 6.6. after incubation at 25 8C for 5 days. All samples were prepared and analyzed in duplicate. Results are presented as
average values. Yeast colonies grown on YGC agar were sorted on the basis of their morphology Žsmoothness of surface, regularity of border, consistency, colour, etc.., streaked to single colonies on Malt Extract agar ŽMEA. ŽDifco Laboratories, Detroit, USA., incubated for 5 days at 25 8C and checked for purity. Counts for each individual type of colony were made in order to estimate the relative occurrence of the various yeasts present in the samples. Yeast species counts were calculated as number of colony-forming units per gram of sample and are reported as CFU gy1 . The isolates were identified according to the methods and criteria proposed by Lodder Ž1970. and Kreger-Van Rij Ž1984.. The nomenclature of the species was made according to Barnett et al. Ž1990.. Statistical analyses were performed using the x 2 test. 3. Results and discussion Samples from Dairy A showed yeast counts ranging between 10 2 and 10 4 CFU gy1 , whereas in Dairy B the majority of the samples exhibited counts higher than 10 4 CFU gy1 . Nine species were found in samples from Dairy A ŽTable 1., of which K. lactis was the dominant, being present in 95.2% of the samples. Other frequently appearing species were D. hansenii Ž76.2%. and Dekkera anomala Ž28.6%.. All the species recovered were found in low concentrations Ž10 2 to 10 4 CFU gy1 . in the samples examined.
Table 1 Concentration and distribution of yeasts species in feta cheese from Dairy A and Dairy B Yeasts species
K. lactis D. hansenii Dek. anomala Dek. bruxellensis G. candidum K. marxianus R. rubra C. tropicalis C. sake` a
Dairy A samples Ž n s 42. Concentration ŽCFU gy1 .
% of positive sample
10 2 –10 4 10 2 –10 4 10 2 10 2 10 2 10 2 10 2 10 2 10 3
95.2 a 76.2 28.6 19 9.5 9.5 4.8 4.8 2.4
Yeasts species
D. hansenii K. lactis G. candidum Dek. anomala R. rubra Y. lipolytica K. marxianus C. sake Dek. bruxellensis
Numbers indicate the percentage of samples in which the species was present.
Dairy B samples Ž n s 43. Concentration ŽCFU gy1 .
% of positive sample
10 4 –10 8 10 3 –10 4 10 2 –10 3 10 3 – 10 6 10 2 10 2 10 2 10 3 10 3
93 23.3 23.3 18.6 9.3 4.7 4.7 2.3 2.3
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
155
Table 2 Distribution of yeast species isolated from feta of good quality and feta which present slimy surface defect from Dairy A and Dairy B Yeasts species
K. lactis D. hansenii Dek. anomala Dek. bruxellensis G. candidum K. marxianus R. rubra C. tropicalis C. sake` a
Dairy A
Yeasts species
Good samples, n s 32 Ž%.
Slimy samples, n s 10 Ž%.
93.7 a 68.7 31.2 25 12.5 12.5 6.25 0 3.1
100 100 20 0 0 0 0 20 0
Dairy B
D. hansenii K. lactis G. candidum Dek. anomala K. marxianus C. sake Y. lipolytica R. rubra Dek. bruxellensis
Good samples, n s 23 Ž%.
Slimy samples, n s 14 Ž%.
91.3 17.4 17.4 8.7 8.7 8.7 4.3 4.3 4.3
92.9 35.7 28.6 0 7.1 7.1 7.1 0 0
Numbers indicate the percentage of samples in which the species was present.
In feta samples from Dairy B, D. hansenii was the dominant species, being present in 93% of the samples and in very high numbers as well Ž10 4 to 10 8 CFU gy1 .. Other frequently appearing species were K. lactis and Geotrichum candidum. The predominance of D. hansenii Žand the anamorph form C. famata) observed in our study is in agreement with results reported for other dairy products ŽWelthagen and Viljoen, 1997; Deak and Beuchat, 1996.. Kaminarides and Laskos Ž1992. found S. cereÕisiae to be the most dominant species associated with Greek feta brine but D. hansenii was also frequently isolated, Westall and Filtenborg Ž1998. reported T. delbrueckii to be a major part of the yeast flora in Danish feta. S. cereÕisiae and T. delbrueckii were never recovered in our study. This fact suggests that the composition of the yeast flora
is likely to differ according to sites and countries of production. Moreover, the observed differences in yeast flora may be caused by the use of unpasteurized ewe’s milk in Greek cheese and of cow’s milk in Danish cheese. D. hansenii growth in feta cheese is facilitated by its ability to assimilate lactose, lactic and citric acids, to produce lipases and proteases and to resist to high salt concentrations and low pH values. On the basis of these properties it seems promising a potential use of this yeast as a starter culture, as suggested by other authors ŽFatichenti et al., 1983; Deiana et al., 1984; Van Den Tempel and Jakobsen, 1998; Welthagen and Viljoen, 1998.. The yeast species isolated from feta of good quality and feta with a slimy surface defect were very similar, as reported in Table 2. None of the
Table 3 Yeasts concentration Žmean of CFU gy1 . and distribution in swelling and non-swelling feta samples from Dairy B Yeasts species
D. hansenii Dek. anomala G. candidum R. rubra K. lactis a
Swelling samples Ž n s 6.
Yeasts species
Concentration mean ŽCFU gy1 .
%
10 3 10 6 10 2 10 2 10 4
100a 100 33.3 33.3 16.7
D. hansenii Dek. anomala G. candidum R. rubra K. lactis
Numbers indicate the percentage of samples in which the species was present.
Non-swelling samples Ž n s 37. Concentration mean ŽCFU gy1 .
%
10 6 10 3 10 2 10 3 10 4
91.9 5.4, p - 0.001 21.6 5.4 24.3
156
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
species identified was proved to be associated with slime production. This defect has been more frequently related to the presence of slime-producing species as lactic acid bacteria ŽNakajima et al., 1990.. Comparison between distribution and concentration of yeast species recovered in swelling samples and the same species present in not-swelling samples are shown in Table 3. Swelling of feta samples was observed only in Dairy B Ž14% of the samples.. Of the five yeast species found in swelling samples both K. lactis and Dek. anomala are known as strongly fermenting yeasts but a significant difference Ž p 0.001. was demonstrated only for Dek. anomala that was present in all swelled samples in counts of 10 6 CFU gy1 , while it was isolated at low numbers in only 5.4% of good samples. K. lactis is known to be one of the most frequent spoilage yeasts of milk and dairy products, because of its capacity to ferment lactose ŽValderrama et al., 1999.. Vivier et al. Ž1994. found C. sphaerica Žanamorph of K. lactis . to be associated with swelling of cans containing feta cheese, whereas in the study of Westall and Filtenborg Ž1998. no samples of Danish feta cheese containing K. lactis showed signs of swelling. The fact that no fermentation problems associated with this lactose-fermenting specie was observed in this study may be due to the low concentration Žabout 10 4 CFU gy1 . of K. lactis and to its association with high concentrations of other yeasts, in particular D. hansenii. In contrast, a clear correlation between the presence in high concentrations of Dek. anomala and the swelling of feta samples was observed. To our knowledge this yeast has not been reported as cause of gas formation in packaged feta even though it is a species characterized by the capacity to ferment and assimilate lactose. The isolation of Dek. anomala in these cheeses may be related to its presence in the environment of the dairy plants, probably on the wooden tables used for dry salting the cheese blocks, since wood is an ecological niche for this species.
References Barnett, J.A., Payne, R.W., Yarrow, D., 1990. Yeasts: Characteristics and Identification. 2nd edn. Cambridge Univ. Press, Cambridge. Deak, T., Beuchat, L.R., 1996. Handbook of Food Spoilage. CRC Press, New York. Deiana, P., Fatichenti, F., Farris, G.A., Mocquot, G., Lodi, R., Todesco, R., Cecchi, L., 1984. Metabolization of lactic and acetic acids in Pecorino Romano cheese made with a combined starter of lactic acid bacteria and yeast. Lait 64, 380–394. Fatichenti, F., Bergere, J.L., Deiana, P., Farris, G.A., 1983. Antagonistic activity of Debaryomyces hansenii towards Clostridium tyrobutyricum and Cl. butyricum. J. Dairy Res. 50, 449–457. Kaminarides, S.E., Laskos, N.S., 1992. Yeasts in factory brine of feta cheese. Aust. J. Dairy Technol. 47 Ž1., 68–71. Kreger-Van Rij, N.J.W., 1984. The Yeasts, a Taxonomic Study. In: Kreger-Van Rij, N.J.W. ŽEd.., 3rd. edn. Elsevier, Amsterdam, Netherlands. Lodder, J., 1970. The Yeasts. A Taxonomic Study. In: Lodder, J. ŽEd.., 2nd. edn. North-Holland, Amsterdam, Netherlands. Nakajima, H., Toyoda, S., Toba, T. et al., 1990. A novel phosphopolysaccharide from slime-forming Lactococcus lactis subspecies cremoris SBT 0495. J. Dairy Sci. 73 Ž6., 1472–1477. Tzanetakis, N., Hatzikamari, H., Litopoulon-Tzanetaki, E., 1996. Yeasts of the surface microflora of feta, a traditional Greek cheese. Abstract from the FIL IDF Symposium on Yeasts in the Dairy Industry. Positive and Negative Aspects. Copenhagen 2–3 September. Valderrama, M.J., De Siloniz, M.I., Gonzalo, P., Peinado, J.M., 1999. A differential medium for the isolation of KluyÕeromyces marxianus and KluyÕeromyces lactis from dairy products. J. Food Prot. 62, 189–193. Van den Tempel, T., Jakobsen, M., 1998. Yeasts associated with Danablu. Int. Dairy J. 8 Ž1., 25–31. Vivier, D., Rivemale, M., Reverbel, J.P., Ratomahenina, R., Galzy, P., 1994. Study of the growth of yeasts from feta cheese. Int. J. Food Microbiol. 22, 207–215. Welthagen, J.J., Viljoen, B.C., 1997. The value of certain chemotaxonomic methods in the identification of food related yeasts. Food Microbiol. 14, 231–245. Welthagen, J.J., Viljoen, B.C., 1998. Yeast profile in Gouda cheese during processing and ripening. Int. J. Food Microbiol. 41, 185–194. Westall, S., Filtenborg, O., 1998. Yeast occurrence in Danish feta cheese. Food Microbiol. 15, 215–222.
Short communication
Yeast populations in Sardinian feta cheese M.E. Fadda) , S. Cosentino, M. Deplano, F. Palmas Department of Experimental Biology, Section of Hygiene, UniÕersity of Cagliari, Cittadella UniÕersitaria, S.S. 554, 09042, Monserrato (CA) Italy
Abstract In this study, the yeast populations in feta cheese from two different Sardinian dairies were examined. Samples of good quality feta Ž32. and samples of feta with a slimy surface defect Ž10. were examined from Dairy A. Similar, samples of good quality feta Ž23., feta with slimy surface defects Ž14. and samples with swelling defects Ž6. were examined from Dairy B. KluyÕeromyces lactis was the dominating species in feta from Dairy A Ž95.2% of samples. followed by Debaryomyces hansenii Ž76.2%., Dekkera anomala Ž28.6%. and Dek. bruxellensis Ž19%.. D. hansenii was dominant in samples from Dairy B Ž93%., followed by K. lactis Ž23.3%., Geotrichum candidum Ž23.3%. and Dek. anomala Ž18.6%.. No significant difference was observed between the occurrence of yeast species in feta of good quality and in feta with slimy surface defects, thus confirming that slimy production is not associated with yeast contaminations. The swelling of samples observed in Dairy B seems to be caused by Dek. anomala. In fact, this strong fermenting species was present in all swelled samples in numbers exceeding 10 6 CFU gy1, while it was isolated in very low concentration in only 5.4% of good samples. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Sardinian cheese; Feta cheese; Yeast populations
1. Introduction Feta cheese is the most popular traditional Greek cheese. This soft white cheese is made from sheep’s milk but goat’s and cow’s milk may also be used. In recent years, this type of cheese is also being produced in Sardinia, one of the major ewe’s milk producing regions in Italy. Many studies on yeasts occurrence in feta cheese have been made ŽVivier et al., 1994; Tzanetakis et al., 1996.. Particularly, Kaminarides and Laskos Ž1992. have shown that the dominant yeasts of brine from Greek feta cheese were Saccharomyces cere-
Õisiae, Candida famata, Torulaspora delbrueckii and Pichia membranaefaciens. Westall and Filtenborg Ž1998. examined feta cheese made of pasteurized cow’s milk from three different Danish dairies and found T. delbrueckii, Debaryomyces hansenii, C. sake and KluyÕeromyces marxianus as the dominant yeasts. To our knowledge, the yeast content in Sardinian feta cheese has not been reported yet. This study concerns the identification of yeast flora in Sardinian feta cheese made in two different dairies.
2. Materials and methods ) Corresponding author. Tel.: q39-0706-754-195; fax: q396754-197. E-mail address: [email protected] ŽM.E. Fadda..
Feta cheeses examined in our study were made in two different dairies ŽDairy A and Dairy B. using
0168-1605r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 6 0 5 Ž 0 1 . 0 0 5 8 6 - 4
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
154
similar production processes. The cheeses were made from pasteurized ewe’s milk to which starter culture and animal rennet are added to obtain a coagulation. The curd, drained without pressing, is dry-salted until the cheese contains 3–4% wrv salt. The cheese is then packed in tin cans. Following packaging, 6–8% wrv brine is added to cover the cheese in the containers. The cheese is then kept at 14–16 8C for 10–15 days until a pH of 4–6 and a moisture content less than 56% is reached. At this point, the cheese is transferred to new containers, brine is added and the containers are sealed and stored at 4–5 8C. The cheese may be marketed 2 months after manufacture, but may be stored for well over a year at 2 8C. A total of 42 samples of feta cheese were received from Dairy A: among these, 32 were of good quality and 10 presented slimy surface defects. A total of 43 samples of feta cheese were analyzed from Dairy B: among these, 23 were of good quality, 14 presented slimy surface defects and 6 presented evident swelling after the package. Ten grams of product was taken from the interior of the cheeses, diluted in 90 ml of 0.1% sterile peptone water and homogenized in a Stomacher ŽPBI, Milan Italy. for 30 s at normal speed. For all samples, decimal dilutions were prepared in 0.1% sterile peptone water and numbers of yeasts were determined directly, by surface plating on Yeast Extract Glucose Chloramphenicol Agar ŽYGC. ŽMerck Darmstadt, Germany, pH 6.6. after incubation at 25 8C for 5 days. All samples were prepared and analyzed in duplicate. Results are presented as
average values. Yeast colonies grown on YGC agar were sorted on the basis of their morphology Žsmoothness of surface, regularity of border, consistency, colour, etc.., streaked to single colonies on Malt Extract agar ŽMEA. ŽDifco Laboratories, Detroit, USA., incubated for 5 days at 25 8C and checked for purity. Counts for each individual type of colony were made in order to estimate the relative occurrence of the various yeasts present in the samples. Yeast species counts were calculated as number of colony-forming units per gram of sample and are reported as CFU gy1 . The isolates were identified according to the methods and criteria proposed by Lodder Ž1970. and Kreger-Van Rij Ž1984.. The nomenclature of the species was made according to Barnett et al. Ž1990.. Statistical analyses were performed using the x 2 test. 3. Results and discussion Samples from Dairy A showed yeast counts ranging between 10 2 and 10 4 CFU gy1 , whereas in Dairy B the majority of the samples exhibited counts higher than 10 4 CFU gy1 . Nine species were found in samples from Dairy A ŽTable 1., of which K. lactis was the dominant, being present in 95.2% of the samples. Other frequently appearing species were D. hansenii Ž76.2%. and Dekkera anomala Ž28.6%.. All the species recovered were found in low concentrations Ž10 2 to 10 4 CFU gy1 . in the samples examined.
Table 1 Concentration and distribution of yeasts species in feta cheese from Dairy A and Dairy B Yeasts species
K. lactis D. hansenii Dek. anomala Dek. bruxellensis G. candidum K. marxianus R. rubra C. tropicalis C. sake` a
Dairy A samples Ž n s 42. Concentration ŽCFU gy1 .
% of positive sample
10 2 –10 4 10 2 –10 4 10 2 10 2 10 2 10 2 10 2 10 2 10 3
95.2 a 76.2 28.6 19 9.5 9.5 4.8 4.8 2.4
Yeasts species
D. hansenii K. lactis G. candidum Dek. anomala R. rubra Y. lipolytica K. marxianus C. sake Dek. bruxellensis
Numbers indicate the percentage of samples in which the species was present.
Dairy B samples Ž n s 43. Concentration ŽCFU gy1 .
% of positive sample
10 4 –10 8 10 3 –10 4 10 2 –10 3 10 3 – 10 6 10 2 10 2 10 2 10 3 10 3
93 23.3 23.3 18.6 9.3 4.7 4.7 2.3 2.3
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
155
Table 2 Distribution of yeast species isolated from feta of good quality and feta which present slimy surface defect from Dairy A and Dairy B Yeasts species
K. lactis D. hansenii Dek. anomala Dek. bruxellensis G. candidum K. marxianus R. rubra C. tropicalis C. sake` a
Dairy A
Yeasts species
Good samples, n s 32 Ž%.
Slimy samples, n s 10 Ž%.
93.7 a 68.7 31.2 25 12.5 12.5 6.25 0 3.1
100 100 20 0 0 0 0 20 0
Dairy B
D. hansenii K. lactis G. candidum Dek. anomala K. marxianus C. sake Y. lipolytica R. rubra Dek. bruxellensis
Good samples, n s 23 Ž%.
Slimy samples, n s 14 Ž%.
91.3 17.4 17.4 8.7 8.7 8.7 4.3 4.3 4.3
92.9 35.7 28.6 0 7.1 7.1 7.1 0 0
Numbers indicate the percentage of samples in which the species was present.
In feta samples from Dairy B, D. hansenii was the dominant species, being present in 93% of the samples and in very high numbers as well Ž10 4 to 10 8 CFU gy1 .. Other frequently appearing species were K. lactis and Geotrichum candidum. The predominance of D. hansenii Žand the anamorph form C. famata) observed in our study is in agreement with results reported for other dairy products ŽWelthagen and Viljoen, 1997; Deak and Beuchat, 1996.. Kaminarides and Laskos Ž1992. found S. cereÕisiae to be the most dominant species associated with Greek feta brine but D. hansenii was also frequently isolated, Westall and Filtenborg Ž1998. reported T. delbrueckii to be a major part of the yeast flora in Danish feta. S. cereÕisiae and T. delbrueckii were never recovered in our study. This fact suggests that the composition of the yeast flora
is likely to differ according to sites and countries of production. Moreover, the observed differences in yeast flora may be caused by the use of unpasteurized ewe’s milk in Greek cheese and of cow’s milk in Danish cheese. D. hansenii growth in feta cheese is facilitated by its ability to assimilate lactose, lactic and citric acids, to produce lipases and proteases and to resist to high salt concentrations and low pH values. On the basis of these properties it seems promising a potential use of this yeast as a starter culture, as suggested by other authors ŽFatichenti et al., 1983; Deiana et al., 1984; Van Den Tempel and Jakobsen, 1998; Welthagen and Viljoen, 1998.. The yeast species isolated from feta of good quality and feta with a slimy surface defect were very similar, as reported in Table 2. None of the
Table 3 Yeasts concentration Žmean of CFU gy1 . and distribution in swelling and non-swelling feta samples from Dairy B Yeasts species
D. hansenii Dek. anomala G. candidum R. rubra K. lactis a
Swelling samples Ž n s 6.
Yeasts species
Concentration mean ŽCFU gy1 .
%
10 3 10 6 10 2 10 2 10 4
100a 100 33.3 33.3 16.7
D. hansenii Dek. anomala G. candidum R. rubra K. lactis
Numbers indicate the percentage of samples in which the species was present.
Non-swelling samples Ž n s 37. Concentration mean ŽCFU gy1 .
%
10 6 10 3 10 2 10 3 10 4
91.9 5.4, p - 0.001 21.6 5.4 24.3
156
M.E. Fadda et al.r International Journal of Food Microbiology 69 (2001) 153–156
species identified was proved to be associated with slime production. This defect has been more frequently related to the presence of slime-producing species as lactic acid bacteria ŽNakajima et al., 1990.. Comparison between distribution and concentration of yeast species recovered in swelling samples and the same species present in not-swelling samples are shown in Table 3. Swelling of feta samples was observed only in Dairy B Ž14% of the samples.. Of the five yeast species found in swelling samples both K. lactis and Dek. anomala are known as strongly fermenting yeasts but a significant difference Ž p 0.001. was demonstrated only for Dek. anomala that was present in all swelled samples in counts of 10 6 CFU gy1 , while it was isolated at low numbers in only 5.4% of good samples. K. lactis is known to be one of the most frequent spoilage yeasts of milk and dairy products, because of its capacity to ferment lactose ŽValderrama et al., 1999.. Vivier et al. Ž1994. found C. sphaerica Žanamorph of K. lactis . to be associated with swelling of cans containing feta cheese, whereas in the study of Westall and Filtenborg Ž1998. no samples of Danish feta cheese containing K. lactis showed signs of swelling. The fact that no fermentation problems associated with this lactose-fermenting specie was observed in this study may be due to the low concentration Žabout 10 4 CFU gy1 . of K. lactis and to its association with high concentrations of other yeasts, in particular D. hansenii. In contrast, a clear correlation between the presence in high concentrations of Dek. anomala and the swelling of feta samples was observed. To our knowledge this yeast has not been reported as cause of gas formation in packaged feta even though it is a species characterized by the capacity to ferment and assimilate lactose. The isolation of Dek. anomala in these cheeses may be related to its presence in the environment of the dairy plants, probably on the wooden tables used for dry salting the cheese blocks, since wood is an ecological niche for this species.
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