산양유 kefir에서 분리한 Str. thermophilus LFG를 이용한 산양유 요구르트의 특성에 관한 연구 = Studies on the characteristics of goat milk yogurt manufactured with Str. thermophilus LFG isolated from goat milk kefir

국문 초록 (Abstract)

영문초록 : This study was carried out to develop a new type of goat milk yogurt, which could be formed to high-slime with lactic acid bacteria. Str. thermophilus LFG strain was isolated from goat milk kefir and investigated for biochemical, physiological and polysaccharide productive characteristics. To evaluate qualities of the goat and cow milk yogurt produced by Str. thermophilus LFG, its physicochemical properties, sensory characteristics and reduction of cholesterol were compared with those produced by other strains(Str. thermophilus TH3, Str. thermophilus Body-1, commercial strain and Str. thermophilus LFG and L. bulgaricus LB12, mixed culture). The results obtained were summarized as follows : Two kind of pure lactic acid bacteria capable of forming high slime were isolated from the goat milk kefir made by Korean. The isolated strains observed by morphological, physiological and 16S rDNA partial sequence were identified as Str. thermophilus(LFG-1) and Lc. lactis(LFG-2) with over 99% homology. Optimum temperature of Str. thermophilus LFG was 40-45℃, and generation time was 40.6 minutes. Final pH of yogurt made by Str. thermophilus LFG and commercial strain Str. thermophilus Body-1 after 24 hrs fermentation at 37℃ were 4.30 and 4.55, respectively. The coagulative activity of Str. thermophilus LFG was almost as strong as that of commercial strain Str. thermophilus Body-1. But LFG-2 strain showed lower coagulative activity than Str. thermophilus Body-1. All lactic acid bacteria were survived about 22-29% in 0.3% bile extract. At pH 1.0 all of the bacteria were killed, also most of lactic acid bacteria became extinct against pH 3.0. However, all lactic acid bacteria survived well at pH 4.5 The crude EPS(CEPS), which consists of carbohydrate(37%,w/w) and protein(63%,w/w), was extracted from goat milk yogurt produced by Str. thermophilus LFG. The CEPS was consisted of monosaccharides, such as fucose 0.27%(w/w), galactose 42.35%(w/w), glucose 56.45%(w/w), galactosamine 1.37%(w/w) and glucosamine 1.09%(w/w), and all kinds of amino acid, mainly proline, leucine, valine, glutamic acid, glutamine, alanine, isoleucine and threonine. The yields of CEPS from Str. thermophilus LFG at high temperature(40-45℃) were shown higher than yields at low temperature(30-35℃). Goat milk yogurt with Str. thermophilus LFG was shown lower values in viscosity than cow milk yogurt. Also, the yields of CEPS from Str. thermophilus LFG were shown higher for 3% glucose yogurt(14-21%) and 3% sucrose yogurt(4-16%) than for control yogurt. Antioxidants activity of supernatant and CEPS extracted from yogurt produced by Str. thermophilus LFG were higher in goat milk yogurt supernatant(21%) and CEPS(28%) than cow milk yogurt supernatant(11%) and CEPS(24%). Changes of pH and acidity were slower at yogurt produced by Str. thermophilus LFG than at yogurt produced by non-slime forming strain( Str. thermophilus TH3), whereas those were fast at yogurt produced by the mixed culture with Str. thermophilus LFG and L. bulgaricus LB12. The initial count of lactic acid bacteria in yogurt produced by Str. thermophilus LFG(slime forming strain) and Str. thermophilus TH3(non-slime forming strain) were 2.1×109 CFU/mL and 8.7×108 CFU/mL, after 25 days they were 8.1×108 CFU/mL, 5.7×107 CFU/mL, respectively. The content of lactic acid of goat milk yogurt produced by the mixed culture with Str. thermophilus LFG and L. bulgaricus LB12 was higher than yogurt produced by single culture with Str. thermophilus LFG, Str. thermophilus TH3 and all cow milk yogurt. All cultures exhibited weak anti-microbial activities against E. coli and S. typhimurium, however, these showed strong anti-microbial activities against Staph. aureus. Among others, mixed culture with Str. thermophilus LFG and L. bulgaricus LB12 exhibited high degree anti-mcrobial activity. Flavor compounds identified from goat milk were acetone, ethylbutanoate, ethyl-3-methylbutyrate, ethyl-2-butenoate and ethylhexanoate. Flavor compounds from cow milk were ethylbutanoate, acetone, 2-heptanone and acetoin. Flavor compounds from goat milk and cow milk yogurts were acids such as acetic acid and butanoic acid, alcohols such as butanol and diethylcarbinol, carbonyl compounds such as acetone, diacetyl, decane, 2-methyl-3-pentanone, hexanal, 2-heptanone, acetoin, benzaldehyde, and sulfur compounds such as dimethyldisulfide, dimethyltrisulfide. Concentrations of total short-chain free fatty acids(SFFA) of goat milk was 22.61 ppm(C4:0 2.37 ppm, C6:0 4.60 ppm, C8:0 6.07 ppm, C10:0 9.57 ppm). This value was higher than 14.16 ppm of cow milk(C4:0 3.86 ppm, C6:0 3.66 ppm, C8:0 2.38 ppm, C10:0 4.26 ppm). Total SFFA contents of goat milk yogurts were somewhat lower than those in the raw goat milk, whereas cow milk yogurts showed slightly higher than cow milk, irregularly. The syneresis of cow milk and goat milk yogurts produced by non-slime forming culture, Str. thermophilus TH3 showed the range of 17.9-21.3%, however, yogurt produced by slime forming culture, which consisted of Str. thermophilus LFG and mixed culture with Str. thermophilus LFG and L. bulgaricus LB12 didn't show any syneresis. These showed 9.6-16.1% syneresis after storage at 4℃ for 10 days. The sensory evaluation of goat milk and cow milk slime yogurts was higher than non-slime yogurt, especially. The yogurt produced by the mixed culture with Str. thermophilus LFG and L. bulgaricus LB12 resulted in the highest score at overall acceptability. The cholesterol was reduced to 29.0-32.8% in the all yogurts produced by lactic acid bacteria. Both of goat milk and cow milk yogurts showed the similar trend in the reduction of cholesterol. The effects of goat milk yogurt on CEPS hydrolyzates produced by Bifidobacterium were reduced fermentation time(20%), and increased average Bifidobacteria counts(25%), acetic acid(14.5%) and lactic acid(16.6%). In conclusion, goat milk yogurt with Str. thermophilus LFG was higher yields of CEPS than cow milk yogurt, and was improved quality such as viable cells of lactic acid bacteria and sensory evaluation. Also, high-CEPS yogurt could be used as a functional food for improving colon environment