3.4 Coproduction of D-tagatose, bioethanol, and microbial protein from WP
WP is a by-product of the dairy industry during the cheese production process. We analyzed the chemical composition of five different commercial sources of WP and found that the lactose content was 70%–80%, and the soluble protein content was 0.2%–0.7% (Table S8).β -Galactosidase hydrolyzed lactose into glucose and galactose. Galactose is used to synthesize tagatose, while glucose and other nutrients can be used for yeast growth and ethanol production. Therefore, a scheme workflow was designed to illustrate the process for producing tagatose, bioethanol, and microbial protein from WP (Figure 4).
The WP solution contained 100-g/L lactose, which was hydrolyzed byβ -galactosidase to yield 52.4-g/L glucose and galactose. Then, galactose in the enzymatic hydrolysate was converted into 32.9 g/L tagatose based on Pathway II–condition 3 (Figure 5A, Table 4); however, the glucose in the enzymatic hydrolysate was not consumed. Subsequently, 0.8 g S. cerevisiae was added into the enzymatic hydrolysate, and the glucose was entirely consumed in 12 h, whereas there was no obvious change in D-tagatose concentration (Figure 5B). First, the yeast cell was separated using solid/liquid separation, and 26.6 g/L yeast cells (dry weight, including 38% protein) was obtained. Second, the ethanol was distilled from the broth using reduced pressure distillation, and 45.8 g/L ethanol under anaerobic conditions was produced. Third, tagatose was purified through concentration and crystallization.