No abstract

C. ANDRIGHETTO, E. PSOMAS, N. TZANETAKIS, G. SUZZI and A. LOMBARDI.2000In the present work randomly amplified polymorphic DNA–polymerase chain reaction (RAPD–PCR) with primers M13 and RF2 was applied to the identification at species level of yeast strains isolated from cheeses. RAPD–PCR analysis of the type strains of different yeast species gave distinctive band profiles that allowed a clear differentiation of all the considered species. Forty‐two of the 48 dairy associated yeasts were clearly assigned to the species Saccharomyces cerevisiae, Kluyveromyces marxianus (anamorph Candida kefyr), Kluyveromyces lactis (anamorph Candida sphaerica), Debaryomyces hansenii (anamorph Candida famata), Yarrowia lipolytica and Torulaspora delbrueckii (anamorph Candida colliculosa). The method, which is rapid and easy to perform, could be a useful tool for the identification of yeasts present in dairy products.

Eight yeast strains isolated from infant feces and the traditional Greek Feta cheese, selected for their probiotic properties, were tested along with a commercially available strain of Saccharomyces boulardii for their ability to remove cholesterol from a growth medium (yeast extract glucose peptone broth) supplemented with 0.3% Oxgall. The amount of cholesterol removed during 72 h of growth at 37 degrees C revealed significant variations among the yeast strains examined. Two isolates from infant feces, namely Saccharomyces cerevisiae KK1 and Isaatchenkia orientalis KK5.Y.1 and one isolate from Feta cheese, namely S. cerevisiae 832, along with the commercial strain S. boulardii, were able to remove cholesterol from the growth medium after 48 h of incubation at 37 degrees C. However, Saccharomyces strains proved to be able to remove cholesterol even after 24 h of growth at 37 degrees C. The cholesterol removed from the growth medium was not metabolically degraded but was rather assimilated into the yeast cells. The ability to assimilate cholesterol in vitro and to tolerate low pH levels, gastric juice, and bile indicate that S. cerevisiae 832, and especially S. cerevisiae KK1 and I. orientalis KK5.Y.1 (being more bile and gastric juice tolerant because of their human origin) may be promising candidate strains for use as probiotics.

The present study was carried out to assess the detection sensitivity of four microbial inhibition assays (MIAs) in comparison with the results obtained by the High Performance Liquid Chromatography with Diode-Array Detection (HPLC-DAD) method for antibiotics of the β-lactam group and chloramphenicol in fortified raw milk samples. MIAs presented fairly good results when detecting β-lactams, whereas none were able to detect chloramphenicol at or above the permissible limits. HPLC analysis revealed high recoveries of examined compounds, whereas all detection limits observed were lower than their respective maximum residue limits (MRL) values. The extraction and clean-up procedure of antibiotics was performed by a modified matrix solid phase dispersion procedure using a mixture of Plexa by Agilent and QuEChERS as a sorbent. The HPLC method developed was validated, determining the accuracy, precision, linearity, decision limit, and detection capability. Both methods were used to monitor raw milk samples of several cows and sheep, obtained from producers in different regions of Greece, for the presence of examined antibiotic residues. Results obtained showed that MIAs could be used effectively and routinely to detect antibiotic residues in several milk types. However, in some cases, spoilage of milk samples revealed that the kits’ sensitivity could be strongly affected, whereas this fact does not affect the effectiveness of HPLC-DAD analysis.

The flow behavior, pH and the main microbial populations of kefir, produced from commercial pasteurized and homogenized bovine milk with fat contents of 0, 1.5 or 3.5% (w/w) and the addition of 1, 3 or 7% (w/w) of kefir grains, were examined after storage for 0, 7 and 14 days at 4C. Increasing the kefir grain inoculum or the storage time of the final product resulted in pH reduction. The viscosity of kefir increased as the fat content of the milk increased, whereas it was reduced during storage for 14 days. Samples prepared with 7% kefir grains exhibited greater viscosity values when compared with samples prepared with 1 and 3%. The increase of kefir grain content resulted in a reduction of lactococci and an increase of yeasts, whereas increasing storage time of kefir resulted in population reduction for lactococci and lactobacilli and to an increase of yeasts. PRACTICAL APPLICATIONS Kefir is a dairy product with noticeable nutritional and health‐related attributes increasing its market potential in today's health‐oriented food market. The quality of the product, its standardization and the parameters affecting it is of scientific and commercial interest. The present study investigates the effect of milk fat content, kefir grain inoculum and storage time on the microbial populations, the pH and especially the rheological properties of the product. A prototype pneumatic tube rheometer was used for the study of the rheological properties of kefir and their development during fermentation and storage.