Recycling ethanol fermentation stillage from food waste could help to resolve the pollution problems, but it would lead to the accumulation of lactic acid (LA), which had a negative effect on ethanol fermentation. In this study, bipolar membrane electrodialysis was used to recover LA as well as other fatty acids in the stillage, and the corresponding parameters that affected this process were investigated. Pretreatment via centrifugation and ultrafiltration reduced the turbidity successfully. Experiments on simulated and real stillage showed that organic acids such as formic acid and acetic acid had a slight effect on the separation of LA, but the existence of chloride ions could form strong anion competition with lactate ions. Simulation experiments with sodium chloride reduced the recovery of LA from 98% to 92%. The recovery rate of LA from real stillage could reach 71.2%. After electrodialysis, the stillage can be reused in ethanol fermentation, the fermentation time was shortened to 24 h, and the ethanol yield increased.

Introduction

For the sustainable development of society, the recycling of waste has become more and more important. Food waste (FW) accounts for a large proportion of municipal solid waste (15%–63%) [1]. It has high organic, volatile solid (85%–95%), and moisture content (75%–85%) and is easily degraded and deteriorated. There are many ways of reusing the waste, such as composting and production of pig feed, biomethane, biodiesel, ethanol, and lactic acid (LA) [2]. The use of FW to produce ethanol can take full advantage of the rich nutrients in FW and obtain fuel. However, a large quantity of stillage will be produced; one way for dealing it is stillage recycle fermentation [3]. This method can reduce pollution and utilize proteins, amino acids, and various metal ions in stillage [4]. In our previous study, LA was determined to be the major accumulated by-product along with stillage recyling. A significant inhibition phenomenon was observed after 5 times recycling. Although the addition of calcium carbonate could alleviate the influence of LA on ethanol fermentation, if we can separate the accumulated LA in the stillage, it will provide better economic benefits. LA is an important natural organic acid. It has received increasing attention as one of the most important building blocks for the production of polylactic acid [5,6]. At present, the main methods of LA production are microbial fermentation and chemical synthesis. Biological fermentation is inexpensive, utilizes a wide range of raw materials, and produces optically pure L- or D-lactic acid, which make it superior to chemical synthesis. More attention has also been paid to the utilization of renewable carbohydrates as a carbon source for LA fermentation [7,8]. However, the major cost of LA production is due to its downstream separation and purification steps [9]. As LA exists in the form of lactate salt in the fermentation broth, acidification and removal of impurities are required for future separation [10]. The extraction methods of LA include precipitation, solution extraction, adsorption, distillation, and nanofiltration [11]. Electrodialysis has the advantages of diversity, sophisticated functions, and technological compatibility [12,13]. Danner et al. used monopolar electrodialysis to extract LA from grass silages and obtained press extraction yields of 31–96 g of lactate per kilogram of silage dry matter (g LA/kg DM) [14]. Chen used conventional electrodialysis (CED) to remove 80% lactate from acid whey [15]. Lopez-Garzon used bipolar membrane electrodialysis (BMED) to recycle LA and other carboxylic acids in the fermentation broth [16].

Production of lactic acid (LA) from lignocellulosic materials has become increasingly prominent in the market. However, for industrial scale to be reached, several challenges in second-generation lactic acid production must be overcome. Besides obstacles such as the hydrolysis process, one of the major challenges of the lactic acid industry is the separation and purification process. This study evaluates the separation of lactic acid produced from hemicellulose hydrolysate from sugarcane bagasse using a Hybrid Short Path Evaporation (HSPE) system. Results showed that a lactic acid concentration of 3.1 times the feed concentration (27.85 g/L) is achievable. Three operational parameters were studied: evaporator temperature, internal condenser temperature and feed flow rate. Maximum lactic acid concentration was 86.69 g/L with an evaporator temperature of 120 °C, condenser temperature of 13 °C, and feed flow rate of 8.27 mL/min. Separation of LA from hemicellulosic sugars using HSPE was a more difficult process than separation of LA from 6-carbon sugars.

Introduction

Lactic acid (LA) is a high added-value product that has been gaining market share every year [1]. It stands out in the environmental, ecological, and medical areas, especially as a building-block molecule. The use of LA has been common in the food industry for a long time and new uses become available every year [2]. LA production by fermentation has several advantages when compared to chemical synthesis, such as low cost of substrates, relatively low required temperatures, low energy consumption, better environmental traits, high purity [3] and easiness to achieve products with tailor-made characteristics [2]. About 90% of all industrially-produced LA worldwide comes from bacterial fermentation [4]. LA biorefineries can exploit various types of feedstock for obtaining second-generation (2G) LA, for example sugarcane bagasse [5], with many advantages such as employment of cheap and non-food competitive raw materials as substrates in the fermentation process. Brazil is the biggest sugarcane producer in the world. The production of sugarcane bagasse in Brazil (2015/2016) has reached 166.4 million tons [6], making this a substrate of great importance in the biofuel and biochemicals scenario. Downstream processes are decisive for LA industry development. This is especially because in medical and pharmaceutical applications, for example, a high-purity product is often required [7]. It is also necessary to consider environmental issues, with increasingly stringent legislation governing the use of solvents and waste generation, in favor of a sustainable production chain [2]. The traditional LA production chain involves a series of downstream treatments like precipitation, conventional filtration, acidification, carbon adsorption, evaporation, crystallization and others. LA separation and purification require a simpler and cheaper alternative to conventional precipitation process, still achieving high purity and yield. In this sense, Short Path Evaporation (SPE) can be an attractive alternative. Economic feasibility analyses of HSPE have been carried out in a few studies, and in any case the proposed method had fewer separation steps in comparison to conventional LA downstream, allowing for a reduction in equipment and operational costs. Furthermore, industrial interests in the purification of LA via SPE is made abundantly clear by the many published patents [8].

Lactic acid enantiomers, normally found in fermented food, are absorbed into the blood and interact with plasma carrier protein human serum albumin (HSA). Unveiling the effect on the function and structure of HSA during chiral interaction can give a better understanding of the different distribution activities of the two enantiomers. Multi-spectroscopic methods and molecular modelling techniques are used to study the interactions between lactic acid enantiomers and HSA. Time-resolved and steady-state fluorescence spectra manifest that the fluorescence quenching mechanism is mainly static in type, due to complex formation. Binding interactions, deduced by thermodynamic calculation, agree with the docking prediction. Docking results and kinetic constants represent chiral-recognizing discriminations consistently. The bindings of lactic acid enantiomers lead to some microenvironmental and slight conformational changes of HSA as shown by circular dichroism (CD), synchronous and three-dimensional fluorescence spectra. This investigation may yield useful information about the possible toxicity risk of lactic acid enantiomers to human health.

Introduction

Lactic acid affects the rheological and sensory qualities of milk and gives good storage properties to fermented products. As a nutrient, it provides 3.6 kcal/g or 15.2 kJ/g of energy (Alm, 1982). Lactic acid is chiral, consisting of two optical enantiomers (Fig. 1). One is L-(+)-lactic acid (L-Lac) and the other is D-(−)-lactic acid (D-Lac). Lactic acid enantiomers are characteristic in all fermented dairy products (wines, sake and milk products) which are generated by both homo- and heterofermentative microbes. In fermented milk (Alm, 1982), the amount of total lactic acid is about 0.6–1.2%, and L-Lac is the major enantiomer formed. The amount of D-Lac is about 0–10% of the total lactic acid in acidophilus milk. In yogurt, about 40% of the total lactic acid is D-enantiomer. Physiological experiments in man and animals showed that lactic acid enantiomers were absorbed from the human intestinal tract (Duran, Van Biervliet, Kamerling, & Wadman, 1977). L-Lac can promote calcic absorption. However, the rate of metabolism of the D-enantiomer was considerably lower than that of L-Lac (Flemström, 1971). However, after intake of large quantities of D-Lac, enhanced Ca2+ was secreted in the urine (Alm, 1982). Restricted consumption of products which contain a high concentration of D-Lac is worth advocating. Infant formulae containing D- or DL mixture should be avoided (Organization, 1974). It is known that ligand–protein interactions affect the distribution, free concentration, and metabolism of various small molecules in the bloodstream. When lactic acid enantiomers are absorbed into the blood, they may bind to plasma proteins and subsequently change the structure and function of the protein. Yet, no reports have so far examined the chiral effects of lactic acid enantiomers at the molecular level, and the possible effect on plasma proteins is still poorly understood. Nutrition and safety of foods are concerns around the world, and thus information is needed to fill in this gap. HSA is the major protein component of human blood plasma. The physiological and pharmacological functions of HSA are maintaining osmotic pressure of blood, buffering pH, and serving in the transportation and distribution of a variety of nutrients and medicines. HSA is a monomeric protein which contains 585 residues. It contains 3 structurally similar α-helical domains (I–III), and each domain can be divided into subdomains A (containing 6 α-helices) and B (containing 4 α-helices) (He & Carter, 1992). Drugs or compounds mainly bind to one of the two primary binding sites on the protein, known as Sudlow’s sites I (warfarin binding site) and II (benzodiazepine binding site).