Studies On Differential Uptake Of Organic Acids And Producing Microbial Protein Using Apple Pomace By Candida Utilis

Apple pomace, which contains the abundance of nutrients such as vitamins, organic acids, sugars etc., is the by-product after apple processing. So far, the apple pomace has not been utilized effectively. It can contaminate the environment in some occasions. The development of apple industry has thereafter been affected. Therefore, producing microbial protein using apple pomace fermented by microorganisms is a reasonable and economic way to comprehensively utilize the apple pomace.Apple pomace has excessive organic acids like L-malate, lactate, acetate and citrate. Some apple pomaces are too sour to use as animal feeds directly. This problem can be potentially solved if the organic acids in the apple pomace can be utilized as the source of carbon by some microorganisms during the fermentation. Here, the abilities of Candida utilis to utilize extracellular organic acids as their source of carbon were firstly studied. Then, the effect of some parameters such as inoculum size, ratio of material to water, urea addition, the duration of fermentation and the fermentation temperature, were studied to improve the protein contents in fermented apple pomace products. Based on the preliminary single-factor experiments, four variables (inoculum size, ratio of material to water, urea addition and the duration of fermentation) at four levels have been optimized further by orthogonal experiment to obtain the optimum parameters to produce solid fermented apple pomace microbial proteins. The main research results were as follows:1 Some extracellular organic acids such as L-malate, lactate, succinate and citrate could be utilized as single carbon sources to support the growth of Candida utilis, while some other organic acids such as oxalate, acetate and fumarate could not be utilized.2 Glucose was utilized in priority when it existed with succinate, citrate or L-malate in the medium. No decrease in the contents of these organic acids was observed when the glucose was depleted. This indicated the uptake of these organic acids was inhibited by the existing glucose in the medium.3 When L-malate, lactate, succinate, and citrate simultaneously existed in the medium, lactate was most easily used by Candida utilis as carbon source. However, there was no obvious sequence for l-malate, succinate and citrate to be utilized. 4 Through the single-factor experiments and orthogonal experiments, the optimum fermentation conditions of fermented apple pomace feeds were fixed: ratio of material to water was 1:1.2, fermentation time was 4d, inoculum size was 3%, and urea addition was 3%. Using the optimum conditions, the protein content can be improved to be higher than 14.0% from the original value of 3.0～3.5%.