| Crop straw is abundant in China, but the vast majority of crop straw has not been utilized effectively. The result is that not only resources are wasted, but also serious environmental pollution is induced. Using crop straw to produce single cell protein (SCP) feed is an effective way to achieve the recycling of resources. This will be helpful in easing the contradiction between food and feed, solving environmental pollution problems and promoting sustainable development of agriculture and animal husbandry. The purpose of this study is to produce SCP feed via fermentation of the double strains using corn straw as substrate. A series of studies have been done in order to improve the biomass of both strains.The optimal conditions of liquid fermentation of Geotrichum candidum by single factor experiments were determined:3% carbon source addition, glucose and xylose as the mixed carbon source (the ratio is 3:2),0.1% urea,0.15% MgSO4,0.5% KH2PO4,0.5% yeast extract; pH6.5, culture temperature 30℃, shaking speed 180 r/min, inoculum dose 5%, culture time 48 h. Then, the response surface optimization test of three main factors affecting the growth of Geotrichum candidum: glucose-xylose ratio, initial pH, shaking speed was conducted. The optimal culture conditions were glucose:xylose=1.53, initial pH6.59, shaking speed 174.11 r/min. Under these conditions, the biomass of Geotrichum candidum had reached 10.5305 g/L.The optimal conditions of liquid fermentation of Candida utilis by single factor experiments were also determined:2% carbon source addition, glucose and xylose as the mixed carbon source (the ratio is 2:1),1% peptone,0.1% MgSO4,0.5% KH2PO4, 0.5% yeast extract; pH6.5, culture temperature 30℃, shaking speed 160 r/min, inoculum dose 10%, culture time 24 h. And the response surface optimization test of three main factors affecting the growth of Candida utilis:glucose-xylose ratio, initial pH, shaking speed was also conducted. The optimal culture conditions were glucose:xylose=2.25, initial pH6.59, shaking speed 162.37 r/min. Under these conditions, the biomass of Candida utilis had reached 5.6492 g/L,It is an important and unsolved problem to measure biomass precisely and conveniently in solid-state fermentation (SSF). In this work, the feasibility of whether the cell components in Geotrichum candidum and Candida utilis—ergosterol and glucosamine could be used as indicators for biomass estimation in SSF was examined and analyzed. The results indicated that the linear relationships between biomass and two kinds of cell components in each strain did exist respectively. But in Geotrichum candidum and Candida utilis, ergosterol amount changed with the different culture time or medium type, so it was not suitable for biomass estimation. Glucosamine could be a good indicator to estimate the amount of biomass of Geotrichum candidum and Candida utilis during SSF process as it remained constant in both cells with the changes of culture time, culture condition and medium type. And the method was successfully applied in the measurement of biomass of Geotrichum candidum and Candida utilis during SSF process.In flasks the culture media and culture conditions of the SSF for producing single cell protein by Geotrichum candidum and Candida utilis were studied by single factor experiments method. The best culture medium of Geotrichum candidum for SSF of corn straw was that pretreated corn straw:bran:soybean meal= 7:2:1,0.5% (NH4)2SO4,0.1% MgSO4,0.3% KH2PO4,0.05% CaCl2, water addition 170%. After determination of its medium, through a series of tests, the best solid culture conditions were as follows:500mL flask containing 20 g solid medium (medium thickness was about lcm), inoculated with 10% Geotrichum candidum seed broth,32℃ for 3 days, the maximum biomass could reach 0.2813 g/g dry matter. Similarly, the best medium of Candida utilis for SSF of corn straw was that pretreated corn straw:bran:soybean meal=6:3:1,0.7%(NH4)2SO4,0.1% MgSO4,0.3% KH2PO4,0.05% CaCl2, water addition 150%. After fixation of the medium, the best solid culture conditions were also studied.500 mL flask containing 20 g (medium thickness was about lcm), inoculated with 10% Candida utilis seed broth,32 ℃ incubated for 2 days, the biomass could reach 0.1617 g/g dry matter.The yield and adaptability of Geotrichum candidum were much better than those of Candida utilis, so when the two strains grew together, the optimum fermentation medium and culture conditions of Candida utilis were selected. The total inoculum dose was 10%. Firstly, Geotrichum candidum seed broth was inoculated,32℃ for 24h; then Candida utilis seed broth was inoculated, and the two yeasts were co-cultured for 2 days; inoculation ratio was 1:1. Under these conditions, the two strains grew well together, which could produce the highest single-cell protein. |
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