Conversion of Food Waste to Lactic Acid

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Introduction Naturally present lactic acid bacteria have been utilized in food preservation for thousands of years, but only recently have experiments demonstrated the potential to use discarded food to produce lactic acid as an industrial commodity (Soomro et al., 2002). Homogenous food waste sources from processing facilities, like wastewater from potato starch extraction or wheat bran from wheat milling, have been the chosen substrate for much of the experimentation to produce lactic acid (Altaf et al., 2006; Huang et al., 2005; Naveena et al., 2005; Ohkouchi & Inoue, 2006; Rojan et al., 2007). Additionally, heterogeneous food sources from hotels and cafeterias have also been used to successfully make food waste.

The potential of achieving significant lactic acid concentrations utilizing heterogeneous food waste sources has been well established in the previous decade. The body of previous work can be characterized in two groups; inoculated digestions and open digestions. Inoculated digestions typically sterilize the substrate then inoculate with specialized bacteria strains. Often the inoculation strain cannot assimilate the array of sugars directly, and amolytic enzymes are a required process input to hydrolyze the complex carbohydrates. Open digestions use naturally present bacterial communities to generate enzymes for hydrolysis internally, reducing process complexity. Theoretically, the microbial ecology of open digestions also provides process stability in the digestion of diverse and varying food waste streams, but this has yet to be verified.

Inoculated Fermentations

The inoculated experiments have generally achieved the highest final concentration and the highest optical purity. Sakai et al. (2003) collected food waste from restaurants, hotels, and hospitals, processed the waste in an autoclave, hydrolyzed with 300-ppm glocomylase, and then inoculated w/ Lactobacillus rhamnosus. Concentrations of 60 g L-1 lactic acid were achieved in 3-4 days (Sakai et al., 2003). Similarly, Sakai and Ezaki hydrolyzed unsterilized model kitchen refuse with glucomylase and inoculated with Bacillus coagulans. High optical purity (97%) was achieved with L(+)-lactic acid concentrations of 86 g L-1 (Sakai & Ezaki, 2006). Other research has sought out lactic acid bacteria strains that express amolytic enzymes, negating the need to add them to them prior to digestion. Wang et al. (2005) isolated amolytic strains of lactic acid bacteria and identified two high yield homolactic Lactobacillus strains; TD175 and TH165. When inoculated with 15% v/w of the strains it was found that each performed well, achieving 29.49 and 28.23 g L-1 lactic acid respectively. However, this was a modest improvement over the control which achieved 24.69 g L-1 (Wang et al., 2005). It is unclear whether the improvement in final concentration was due to the specific bacteria inoculated or a boost in cell count of lactic acid bacteria compared to the control. When the food waste was sterilized prior to inoculation with TD175 and TH165, lactic acid yield was reduced compared to the non-sterilized inoculations. This indicates that a broader microbial ecology, an increase in cell count, or both have a positive impact on the digestion (Wang et al., 2005).

Open Fermentations

Multiple experiments have demonstrated the potential of open digestion using lactic acid bacteria. Under appropriate conditions, naturally occurring lactic acid bacteria will out-compete other flora in the digestion of food waste (Sakai et al., 2000; Wang et al., 2001; Zhang et al., 2008). Sakai et al. (2000) was the first to demonstrate that open digestion of food waste could produce primarily lactic acid. Concentrations of 45 g L-1 were achieved at 37 °C over 120 hours. The substrate used in these experiments was a “model kitchen waste” which was made from a selection of edible foods meant to approximate the consistency of municipal food waste. Sakai also identified the dominant bacterium as Lactobacillus plantarum and L. brevis. Neither bacteria demonstrated the ability to assimilate starch or cellulose directly, indicating that the broader microbial ecology is important in the digestion process (Sakai et al., 2000). Process optimization of food waste digestion to improve optical purity of the resultant acid has thus far achieved concentrations up to 49 g L-1 L(+)-lactic acid. A pH of 8 was found to be optimal (Zhang et al., 2008). However, a pH of 8 is impractical for the economic production of lactic acid because food waste typically has a pH between 4.9 and 6, and thus would require a 100-1000x increase in the natural hydroxide concentration to achieve a pH of 8 for digestion (Komemoto et al., 2009; Kwon & Lee, 2004; Omar et al., 2009). Synergistic effects were found when cafeteria food waste was co-digested with activated sludge. The co-digestion produced higher lactic acid concentrations when compared to digestion of each waste source separately (Chen et al., 2013). Table 2 4 summarizes previous work in the lactic acid digestions of heterogeneous food waste sources.