Some aspects of the production and quality of fermented milks using bifidobacteria

Consumption of fermented milk products containing "probiotic" organisms is expanding in most countries around the world. The present study comprised two approaches; firstly, the production of set-type fermented milks using single strains of bifidobacteria and secondly the production of set-type fermented milks using commercial mixed starter cultures containing bifidobacteria. The former approach was unsuccessful because the products lacked the typical characteristics of a good fermented milk (i.e. they exhibited excessive whey leakage and weak gel formation). Consequently, attention was focused on evaluation of the quality of eight set-type fermented milks of which seven were made using commercial mixed starter cultures containing bifidobacteria and one using yoghurt starter culture. The viable cell counts of micro-organisms in the eight starter cultures were evaluated by the plate count method. Morphological studies using confocal microscopy were performed on all of the micro-organisms in the starter cultures. Carbohydrate utilisation and enzymatic tests, using miniature reaction strips were also performed on all of the micro-organisms. The products were made from reconstituted skimmed milk powder (SMP), milk protein concentrate (MPC), and anhydrous milk fat (AMF), and were inoculated with the eight different starter cultures, dispensed into cups and incubated at 40 °C until the pH was ~4.60. The fermented milks were then transferred to a cold store and evaluated when fresh and after storage for 20 days. The recovery of viable counts of micro-organisms in the eight starter cultures agreed with the suppliers' specifications. The morphological studies using confocal microscopy allowed identification of the micro-organisms at generic level. Thus, it was confirmed that the single strain isolates were typical of bifidobacteria, lactobacilli and streptococci. The enzymatic activity tests generally, confirmed the identity of the isolates of bifidobacteria in the starter preparation. The isolates from starter cultures MSK 2, AC/BL and DV B-100 were identified as Bif. longum, Bif. infantis and Bif. bifidum respectively. Isolates of Bifidobacterium from starter cultures AB, ABT-1, ABT-3 and MSK B2 were not identified. However, isolates of Lb. delbrueckii ssp. bulgaricus from starter culture MY 087 were identified as Lb. delbrueckii ssp. lactis which was subsequently confirmed by the manufacturer. The tests confirmed the identity of all the Lb. acidophilus and Str. thermophilus isolates. The gross chemical composition of the fermented milks produced using the mixed starter cultures was similar, but the organic acids profile was different. The fermented milks made using AB and AC/BL starter cultures (i.e. containing bifidobacteria and Lb. acidophilus) had a higher content of acetic acid compared to the rest which contained the same organisms in addition to Str. thermophilus. The growth of bifidobacteria was inhibited by the presence of Str. thermophilus in starter cultures. However with the exception of MSK 2 and ABT-1 the number of viable cells remained at acceptable levels (i.e. >10 6 cfu g-1) after the storage period. A large number of viablecells of bifidobacteria and Lb. acidophilus in the products AB and AC/BL was observed, suggesting a possible associative growth between them. The viable cell counts of Lb. acidophilus were within acceptable levels (i.e. >10 6 cfug-1) after the storage period, but the product made with starter culture MSK B2 was the exception. The count of Lb. delbrueckii ssp. bulgaricus from starter culture MY 087 fell to the borderline (~10 6 cfu g-1) after the storage period.However, the viable cell counts of Str. themophilus were very high for all the products after the storage period. The rheological properties (firmness and syneresis) followed similar patterns for all the fermented milks. Syneresis decreased while the firmness increased with time. Sensory evaluation clearly differentiated the three products made with starter cultures AB, AC/BL and MSK 2 from the rest of the fermented milks. The first two were noted for their acid and vinegar character and MSK 2 for being the least acid of the products.