The structure of six Irish kefir samples was studied in the electron microscope, and the microbial composition and fermentation kinetics during growth in 10% reconstituted skim milk at 21 degrees C. The microbial composition of the six samples was similar; at the end of the fermentation the counts of lactococci, leuconostocs, lactobacilli, acetic acid bacteria and yeasts were 10(9), 10(8), 5 x 10(6), 10(5) and 10(6) ml(-1) respectively; the levels of acetic acid bacteria and lactobacilli showed some intersample differences. Lactate was the major metabolite followed in order by ethanol, acetate and acetoin. The final concentrations of L-lactate produced (66-90 mmol kg(-1)) were 10-fold higher than those of D-lactate. Acetate and ethanol concentrations varied from 4 to 14 and 2 to 40 mmol kg(-1) respectively. The rates of citrate utilization and concentration of acetoin produced during growth differed between samples. Scanning electron microscopy showed not only variation between the interior and exterior of the sample but also large variation between different sections of the interiors and exteriors of the same sample. Long and short, and straight and curved rods and yeasts were seen in all samples, the curved rods observed in the interior, but lactococci were seen on the surface of only one sample. There were no gross differences in structure between samples.
