Yogurt Starter Bacteria Research Articles

Use of a selected multi-strain potential probiotic culture for the manufacture of set-type yogurt from caprine milk

Set-type low-fat yogurt from goat's milk, prepared by using yogurt starter bacteria and potential probiotic lactobacilli, was studied for physicochemical, microbiological and sensory characteristics. Selected isolates of Streptococcus thermophilus (2 strains), Lactobacillus delbrueckii subsp. bulgaricus (2) from goat's milk yogurt and Lactobacillus paracasei subsp. paracasei (1) and Lactobacillus acidophilus (1) from new born infant faeces were tested for their ability to grow and metabolize in goat milk and for their in vitro resistance to the specific conditions of the human gastrointestinal tract. Results revealed that L. paracasei subsp. paracasei and L. acidophilus strains were distinguished for their potential probiotic properties (resistance to low pH, tolerance to 0.15% bile salts, cholesterol removal from the medium, resistance to 100ppm lysozyme), as well as for high diacetyl production. The synergistic growth of selected strains was shown by growing six single-strain cultures and one multi-strain culture in goat milk, so the later one was used to produce a set-type yogurt from goat milk. The product was characterized by high numbers (∼108cfu/g) of potential probiotic lactobacilli; it contained carbonyl compounds and had a satisfactory consistency with good organoleptic characteristics. This study depicted the ability to successfully produce set-type yogurt from caprine milk by implementing beneficial lactobacilli strains.

The effects of inoculant variables on the physicochemical and organoleptic properties of Doogh

The combined effects of probiotic sequential inoculation (before or after fermentation), type of native Bifidobacterium strain (Bifidobacterium animalis spp. lactis PTCC 1631 and Bifidobacterium bifidum PTCC 1644) and final pH of fermentation (4.5 or 4.2) on qualitative aspects of typical Iranian fermented milk drink (Doogh) were investigated. The blends inoculated with yoghurt bacteria only and with final fermentation pH of 4.5 had the shortest fermentation time. The highest viability of bifidobacteria in all treatments at the end of fermentation and throughout the refrigerated storage was observed for BlY‐40‐4.5 (B. lactis co‐cultured with yoghurt starter bacteria and incubated at 40 °C until final pH of 4.5) and then for BbY‐40‐4.5 (containing B. bifidum). The most acceptable organoleptic properties were observed for treatments BlY‐40‐4.2 and BbY‐40‐4.2, and then for BbY‐40‐4.5.

Selective and differential enumerations of Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus, Lactobacillus acidophilus, Lactobacillus casei and Bifidobacterium spp. in yoghurt — A review

Yoghurt is increasingly being used as a carrier of probiotic bacteria for their potential health benefits. To meet with a recommended level of ≥10(6) viable cells/g of a product, assessment of viability of probiotic bacteria in market preparations is crucial. This requires a working method for selective enumeration of these probiotic bacteria and lactic acid bacteria in yoghurt such as Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lb. acidophilus, Lb. casei and Bifidobacterium. This chapter presents an overview of media that could be used for differential and selective enumerations of yoghurt bacteria. De Man Rogosa Sharpe agar containing fructose (MRSF), MRS agar pH 5.2 (MRS 5.2), reinforced clostridial prussian blue agar at pH 5.0 (RCPB 5.0) or reinforced clostridial agar at pH 5.3 (RCA 5.3) are suitable for enumeration of Lb. delbrueckii subsp. bulgaricus when the incubation is carried out at 45°C for 72h. S. thermophilus (ST) agar and M17 are recommended for selective enumeration of S. thermophilus. Selective enumeration of Lb. acidophilus in mixed culture could be made in Rogosa agar added with 5-bromo-4-chloro-3-indolyl-β-d-glucopyranoside (X-Glu) or MRS containing maltose (MRSM) and incubation in a 20% CO2 atmosphere. Lb. casei could be selectively enumerated on specially formulated Lb. casei (LC) agar from products containing yoghurt starter bacteria (S. thermophilus and Lb. delbrueckii subsp. bulgaricus), Lb. acidophilus, Bifidobacterium spp. and Lb. casei. Bifidobacterium could be enumerated on MRS agar supplemented with nalidixic acid, paromomycin, neomycin sulphate and lithium chloride (MRS-NPNL) under anaerobic incubation at 37°C for 72h.

The role of resistance to bile salts and acid tolerance of exopolysaccharides (EPSS) produced by yogurt starter bacteria

The aim of this study was to investigate a possible relation between EPS production and resistance to bile salts and tolerance to low pH. Eight strains which produced the highest and lowest amount of EPS (16- 211mg/l) were selected among 54 bacteria isolated from yogurt. Additionally, they were tested for resistance to bile salts (0.15, 0.3 %) and tolerance to low pH (2.0-3.0). After treatment with bile salts and acid, viable bacteria (log cfu ml-1) were determined by surface plating. The high EPS producing strains (B3, G12, W22) showed a significant (P<0.05) protective effect against low pH (pH 2.0). All Streptococcus thermophilus strains showed a higher tolerance to bile salts than the Lactobacillus delbrueckii subsp. bulgaricus strains. The high EPS-producing S. thermophilus (W22, T12) and L. bulgaricus (B3, G2) strains showed a significant (P<0.01) protective effect against bile salts (0.3 %).

Incorporation of citrus fibers in fermented milk containing probiotic bacteria

The incorporation of Lactobacillus acidophilus CECT 903, Lactobacillus casei CECT 475 and Bifidobacterium bifidum CECT 870 together with lemon (LF) and orange (OF) fibers obtained from juice by-products were tested in (i) a model system: fiber enriched with de Man Rogosa Sharp (MRS) broth cultured with each probiotic bacteria and (ii) evaluation of populations of probiotic bacteria in fermented milks formulated with citrus fibers. Citrus fibers enhanced L. acidophilus CECT 903, and L. casei CECT 475 survival in MRS during refrigerated storage, whereas erratic results were obtained for B. bifidum CECT 870, OF enhanced its growth and LF had inhibitory effect. Populations of probiotic bacteria decreased with storage time in MRS broth. The presence of yogurt starter bacteria in probiotic fermented milks favored the growth and survival of L. acidophilus and B. bifidum. Citrus fiber presence in fermented milks also enhanced bacterial growth and survival of the tested probiotic bacteria. This study indicates that citrus fiber enriched fermented milk have good acceptability and are good vehicles for a variety of commercial probiotics but survival of B. bifidum will need to be improved.

Isolation and characterization ofLactobacillus delbrueckiissp.bulgaricusandStreptococcus thermophilusfrom plants in Bulgaria

One of the traditional ways of preparation of yogurt starter in Bulgaria is placing a branch of a particular plant species into boiled sheep's milk maintained at about 45 degrees C, which is further incubated until a dense coagulum is obtained. To investigate the possible origin of the yogurt starter bacteria, Lactobacillus delbrueckii ssp. bulgaricus (L. bulgaricus) and Streptococcus thermophilus (S. thermophilus), the traditional way of yogurt-starter preparation was followed. Hundreds of plant samples were collected from four regions in Bulgaria and incubated in sterile skim milk. The two target bacteria at low frequencies from the plant samples collected were successfully isolated. Phenotypic and genotypic characteristics of these bacterial isolates revealed that they were identified as L. bulgaricus and S. thermophilus. Twenty isolates of L. bulgaricus and S. thermophilus, respectively, were selected from the isolated strains and further characterized with regard to their performance in yogurt production. Organoleptic and physical properties of yogurt prepared using the isolated strains from plants were not significantly different from those prepared using commercial yogurt-starter strains. It was therefore suggested that L. bulgaricus and S. thermophilus strains widely used for commercial yogurt production could have originated from plants in Bulgaria. To our knowledge, this is the first report on the isolation and characterization of L. bulgaricus and S. thermophilus strains from plants.

Incorporation of microbial transglutaminase into non-fat yogurt production

This study investigated the physical, chemical and sensory characteristics of non-fat yogurts treated with microbial transglutaminase (MTGase) at varying concentrations from 0 to 0.5 g L −1. Also, the effect of enzyme inactivation prior to fermentation on the selected properties of the yogurts was studied. Acid development rate was reduced with increasing MTGase doses. Cross-linking of milk proteins by MTGase had a growth-slowing effect on yogurt starter bacteria, which was more pronounced at higher concentrations. Physical properties of the yogurts were improved by MTGase throughout 21-day storage; on the contrary, the production of acetaldehyde was slowed down by increasing MTGase concentrations during the same period. Principal component analysis (PCA) and hierarchial cluster analysis (HCA) clearly differentiated the samples with added MTGase at lower (⩽0.3 g L −1) and higher (0.4–0.5 g L −1) concentrations regarding the physical and sensory properties. The physical and sensory properties of non-fat set yogurt could be improved by incorporating MTGase up to a level of 0.3 g L −1.

Effect of Inulin and Lactulose on Survival of Lactobacillus AcidophilusLA-5 and Bifidobacterium Bifidum BB-02 in Acidophilus-Bifidus Yoghurt

The effect of supplementation of lactulose and inulin as prebiotics on the growth of L. acidophilusLA-5 and B. bifidumBB-02 in Acidophilus-Bifidus (AB) yoghurt and some quality parameters of the resulting products were studied. Yoghurts produced from milks with added inulin at levels of 0.5% and 1.0%, or with lactulose at levels of 0.25% and 2.5% were compared with classical yoghurt and AB yoghurt (without added prebiotics). The results showed that inulin and lactulose did not affect the growth of yoghurt starter bacteria, but stimulated the growth of B. bifidumBB-02 to a great extent. Lactulose was found to be more effective on the growth of both probiotic strains than inulin. Inulin did not stimulate the growth of L. acidophilusLA-5. The cell counts of B. bifidumBB-02 and L. acidophilusLA-5 were dependent upon concentrations of lactulose and inulin used. No significant (p< 0.05) difference between the samples were observed with regard to the pH and lactic acid values. However, lactulose or inulin added samples had lower acetaldehyde and tyrosine contents than the untreated samples.

Improvement of carotenoid-synthesizing yeast Rhodotorula rubra by chemical mutagenesis.

A mutant Rhodotorula rubra with enhanced carotenoid-synthesizing activity for synthesizing total carotenoids and beta-carotene was obtained by N-methyl-N'-nitro-N-nitrosoguanidine mutagenesis. When co-cultivated with yogurt starter bacteria (Lactobacillus bulgaricus + Streptococcus thermophilus) in whey ultrafiltrate it produced 15.7 mg total carotenoids l(-1) culture fluid or 946 microg total carotenoids g(-1) dry cells of which 71% was beta-carotene. Grown as a monoculture in glucose substrate, the mutant shown 1.4 times lower carotenoid-synthesizing activity, and the relative share of beta-carotene in the total carotenoids was lower (63%). The individual pigments torulene and torularhodin were identified, whose mass fractions were (29% and 7%) and (24% and 4%), respectively, for the mutant grown as a monoculture and as a mixed culture with the yogurt bacteria.

Impact of engineered Streptococcus thermophilus trains overexpressing glyA gene on folic acid and acetaldehyde production in fermented milk

The typical yogurt flavor is caused by acetaldehyde produced through many different pathways by the yogurt starter bacteria L. bulgaricus and S. thermophilus. The attention was focused on one specific reaction for acetaldehyde and folic acid formation catalyzed by serine hydroxymethyltransferase (SHMT), encoded by the glyA gene. In S. thermophilus, this enzyme SHMT also plays the typical role of the enzyme threonine aldolase (TA) that is the interconvertion of threonine into glycine and acetaldehyde. The behavior of engineered S. thermophilus strains in milk fermentation is described, folic acid and acetaldehyde production were measured and pH and counts were followed. The engineered S. thermophilus strains StA2305 and StB2305, have the glyA gene (encoding the enzyme serine hydroxymethyltransferase) overexpressed. These engineered strains showed normal growth in milk when it was supplemented with Casitione. When they were used in milk fermentation it was observed an increase in folic acid and in acetaldehyde production by StA2305 and for StB2305 it was noticed a significative increase in folic acid formation.

Microflora of Austrian Natural-Set Yogurt

Randomly selected Austrian natural-set yogurts were examined for storage-induced changes in the populations of viable yogurt starter organisms and microbial contaminants. Typically, numbers of Streptococcus thermophilus and Lactobacillus bulgaricus remained above 108 cfu/g in yogurt stored at 10°C until the sell-by-date (15–20 d after manufacture). Both elevated storage temperature and storage periods markedly reduced the survival of yogurt starter bacteria. Depending on the sample origin, the yeast populations increased from less than 10/g to above 106/g when yogurts were stored at 10°C until the sell-by-date. The isolated yeasts were identified as Candida inconspicua, C. intermedia, C. parapsilosis, C. rugosa, C. tropicalis, C. zeylanoides, Debaryomyces hansenii, Metschnikowia reukauffii, Pichia guilliermondii, Rhodotorula mucilaginosa, Torulaspora delbrueckii, Trichosporon beigelii, and Yarrowia lipolytica. While a few out of 233 isolates were found to assimilate lactose, none were able to ferment lactose.

Effect of Viable Starter Culture Bacteria in Yogurt on Lactose Utilization in Humans

Breath hydrogen production was used as a measure of lactose malabsorption in human test subjects following the consumption of both heated and unheated cultured yogurt. Less hydrogen was produced when the subjects consumed the unheated cultured yogurt than when they consumed the heated product, indicating that lactose hydrolysis was improved in the small intestine of the individuals consuming the unheated cultured yogurt. Lactase activity in yogurt samples was increased in the presence of bile. Yogurt starter bacteria growing in milk normally do not hydrolyze more lactose than needed for their growth. However, the increased lactase activity in the presence of bile indicates that these bacteria could function as a source of lactase to hydrolyze lactose in the small intestine even though the organisms themselves are not expected to grow in that environment.

Susceptibility of cheese and yoghurt starter bacteria to antibiotics.

Eight single-strain lactic streptococci, three commercial cheese starters, and six lactic acid bacteria isolated from yoghurt were examined for their susceptibility to penicillin, cloxacillin, tetracycline-hydrochloride and streptomycin. The ranges of the antibiotics causing 50% inhibition of the bacteria were (mug/ml): penicillin, 0.009 to 0.20; cloxacillin, 0.24 to 2.50; tetracycline, 0.09 to 0.60; and streptomycin, 0.35 to 13.0. The average concentrations required to cause 50 and 100% inhibition of the cheese starters were (mug/ml): penicillin, 0.12 and 0.26; cloxacillin, 1.91 and 3.9; tetracycline-hydrochloride, 0.13 and 0.36; and streptomycin, 0.59 and 2.06. All the cocci were about equally susceptible to tetracycline, and all organisms were more resistant to cloxacillin than penicillin. The yoghurt isolates were more resistant to streptomycin and more susceptible to penicillin than the cheese starters. The 2, 3, 5-triphenyltetrazolium chloride test, using Streptococcus thermophilus BC as assay organism, does not detect low levels of streptomycin in milk. However, it is useful in detecting cloxacillin residues.