Study On The Selected Strains Of Lactic Acid Bacteria And DVS Commercial Starter Cultures

large number of strins for yoghurt sPecies (LactObacUha buwtns and SmptoCOCasthermOPhilus) are selected and cultUred, at the same time, the proliferation and lyOphiliZationhgh activity Lactic Acid BaCteria GAB) Starters, the mechanism of multi-strains combinaionand technique conditions are stUdied Systematically in order to hoprove the quality of yOghurtand level of production in our counny and shorten the differnce in directedded(DVS or Dvi)N compaha with the developed countries.1. The separation and purification & selection and cultivation of LAB strainsThe Commercial Starter Cultures (BS-Bulk Startr & DVS) from differen sources arepurified in sPecial medium, tWenty ofcocci and tWeIve ofbacilli are obnined. The stains havebeen cultivated for continuous five generations using fabfree milk after actiboo4 the aciddegree, pH vaue, sense value, viscosity and time of curd are comPretheively studied; thensix stheins ofbarilli are selected, including 350L, 380L, TM-l l lL, 95 L and B3L, and the fivestalns of cocci are TMl l1S, 900S, 96S, YC380 and SH9. Affer elemetw ndficatbos ofmorphologn physiology and biochemiWh the results of preliminary determinatons are thathe cocci are S thermoPhiIus (S.t)and the bacilli are L. btwus (L.b).The selected cocci and bacilli are mixed in the ratio of 1:l to ferment the fatffee milk.According to the acid degree, pH value and viscosity the combinaion of oPtimal fermenhooncharacteristics are SCreened, and they are TM-lllS & TMlllL, 90()S & 95L, 900S &TM-l l lL, 900S & VlL, and TM-l l1S & B3.2. The U'V mutation and domestication of the PreffrendsUy selected L bu~The UV mutations of L. buIgaheus B3. TM-lllL is canied out the lethai rate is80%~90o/o, the radioartive condition is as following the UV ligh of 20W is chose, thedistance between the lgh and cultUre susPension is 30cm, and the ndiation tiIne is 2minutes. Four bons of arid sensitivbe are screened; but only one bon shows its acidsensitivty affer the fiot inoculation, and al1 the mutated bons loss the acid senstw dsthe second inoculation.'3. Cultivating coudthens of LAB with high activtylThe selectal PrOliferation medium are, according tO the gh of screened L.b and S.t inthe classic mediam,4% fat-free milk and 5% eope hydrolyZed wtw 0he hydrolyalng timeis 60 min, and whey POwdeme2O is l:l). The oPtiInai formulalHriPtion is detennined bysingle factor testS of proliferation factors and the quadratic disign.The oPtimai fOrmula of S2:5% tOmato juice, 0.7% yeast extration, 0.5% maltose; 2% larese, 0.5% serine, l% calciumcarbonate. The oPimal formula of L.b: 5% tomato juice, 0.7% yean eXbetion, 0.5% malose;2% lectose, 70mg/kg methanoic acid.In addition, the supplement of l0% Gq can improve the activity of the cocci.The control of arid degree and pH value has grea effeCt on acidTroducing ability of LAB;the suhable growth conditions ofcocci are at higher pH (6. l ) and the temperature of 4l 'C, andthe proper condihons of bacilli are at lower pH (5.7) and the temPeratUr of 4l 'C. The bufferability of balance saltS can't satisfy the need in the vigorous growth phase, and so it is not fitfor the Prolifhaion of LAB which always PtOduce a lot of acid. It can't improve theproduction of baCteria body to infiate N2 and CO2.The D-oPiInai wton test design of quadratic three fawt (3llA) is aPPlied, and therecursive equaion of the S.t cultivallng conditions is found:po --83.07+2.40xl+ l0. l5X2so.99x,so.03x,xpo.02x,x,so.ro.04x,'The maxhoum contain of lactic acid is 59o/, obtained under the condition oftemp.=4l .8C,pHed. l, and cultivating time of 4.6 hrs.The L.b cultivating conditions is found:y--ed7.5ffel .79xl+9.57x2ro.82X3~.02xlxpo.0l x,x,--0.02x,X3ro.02x,to.88X2' --0.04x,'The maximum value of acidity is 55.lty obtuined under the condition of temp.=42.5'C,pH=5.9, and cultivating time of 4.8 hrs.The acid-producing activity of L.b declines ther 4-hours Prolifetaling culhVation in atfermentaion tank, while the pH and temperature are contrOlled. During the prolifndion thechewededcs of the bacteria is observed, it is found that the L.b is short...