Bioconversion of cheese whey into a hetero-exopolysaccharide via a one-step bioprocess and its applications

Highlights

• Lactobacillus plantarum JNULCC001 can bioconvert cheese whey into value-added EPS.

• The produced EPS shows a record biosorption ability toward MB.

• EPS-001 plays important roles in thickening and gelling.

• EPS-001 can serve as a stabilizer in the biosynthesis of SeNPs.

Abstract

Continuing efforts have been made to produce functional exopolysaccharides (EPSs) using cheap substrates for valuable applications. Cheese whey is a lactose-rich byproduct of cheese-producing industries. In this research, a one-step bioprocess for the bioconversion of cheese whey to a hetero-exopolysaccharide using a ropy EPS producing strain Lactobacillus plantarum JNULCC001 with high β-galactosidase activity was performed. The prepared hetero-exopolysaccharide is named EPS-001 and composed of galactose, glucose, mannose, and arabinose with several functional groups, including carboxyl, hydroxyl, and amide groups, endowing its thermo-stable and highly negatively charged properties. The rheological analysis of the fermented whey indicates its high viscous and shear thinning (pseudoplastic non-Newtonian fluid) behavior, demonstrating the potential role of EPS-001 as a thickening and gelling agent. Besides, EPS-001 shows an excellent biosorption ability toward methylene blue (MB) and can serve as a stabilizer in the biosynthesis of selenium nanoparticles (SeNPs). Our research advances an alternative method for management of cheese whey, which may reduce its impact on the environment, and develops a novel value-added EPS for thickening, bioremediation and stabilizing nanomaterials.

Introduction

Cheese whey is a lactose-rich byproduct during cheese-making or coagulation of the milk casein process in the dairy industry [1]. As a major by-product of the dairy industry, cheese whey possesses high Chemical Oxygen Demand (COD) (57−75 g/L) and Biological Oxygen Demand (BOD) (35−40 g/L) that needs to be treated before discharged into the environment [2]. However, cheese whey is actually a nutrient-rich effluent accounting for 90 % of the milk volume consisting mainly of lactose (70∼80 %), soluble proteins (8∼14 %), minerals (12∼15 %), lactic acid (0.8∼12 %) and lipids (1∼7 %), and can be used as a good substrate of microorganisms to produce valuable products [3,4]. Based on the reported data, annual worldwide cheese whey is around 1.15 × 10⁸ ∼ 1.40 × 10⁸ tons, with an estimated annual increase of 1∼2 % [5]. To reutilize the large amounts of cheese whey, several approaches have been investigated. It is commonly used for food supplements, but is limited due to human lactose intolerance [6]. Additional attractive methods include its use as a substrate for microbial or enzymatic process to generate valuable products by converting lactose using β-galactosidase (EC 3.2.1.23). The β-galactosidases catalyze the hydrolysis and transgalactosylation of lactose, and are thus used in dairy industry to reutilize lactose to produce valuable bioproducts, like galacto-oligosaccharides [7], acetic acid, propylene glycol [8], monosaccharides [9], organic acids [10], polyhydroxyalkanoates (PHAs) [11], microbial polysaccharides [3], and so on. As a waste by-product of the dairy industry, cheese whey showed advantages over other fermentation media, such as corn starch [12]. Thus, researches on converting cheese whey to value-added products are still hot-point.

Exopolysaccharides (EPSs) derived from bacteria, fungi and blue-green algae are strain-specific high-molecular-weight polymers composed of homo- or hetero-monosaccharides [13]. EPSs play many important roles in protecting the producers from severe or toxic conditions, improving microbial competition in different environments [14]. Also, EPSs have been effectively applied in many different fields, such as food, pharmaceutical, cosmetics, bioremediation, biomedical, and so forth [[15], [16], [17], [18], [19], [20]]. For example, EPSs have been extensively investigated for their biosorption ability on dyes due to their biocompatibility, biodegradability, renewability, and environmentally friendly properties [[21], [22], [23]]. An alginate-like exopolysaccharide is used for removal of methylene blue (MB) with a maximum dye removal rate of 69 % [24]. The extracellular polysaccharides from Bacillus subtilis are also found to be good adsorbents for Reactive Blue 4 [25]. In addition, EPSs such as succinoglycan, curdlan, levan, pullulan, EPS-605 and other microbial EPSs are also used as reducing agents and stabilizers for the green synthesis of metal nanoparticles (NPs) [21,26]. Due to the structural and functional diversity of EPSs, there is a growing demand for EPSs synthesized by microorganisms. However, bacterial EPS accounts for only a small portion of the current biopolymer market due to their high production cost, which is mostly related to substrate cost and recovery. Therefore, the use of cheaper cheese whey may represent an alternative approach to reduce the production cost of microbial EPS. However, most wild-type EPS producing bacteria are unable to efficiently use lactose as a carbon source due to the low-galactosidase activity [27]. Lactose is chemically or enzymatically converted into glucose or galactose that can be used as carbon sources by bacteria. Genetically engineered bacteria that can use lactose as substrate are also developed. But there are some disadvantages for the above processes like high cost and genetic instability. Thus, it is of interest to find bacteria that can directly use lactose and convert it into the value-added EPS. L. plantarum is one of the most studied LAB species, which is mostly isolated from food products. EPS producing L. plantarum is usually used in the food industry due to its GRAS status as a textural agent or its potential probiotic properties [28].

In the present study, an EPS hyper-production L. plantarum strain JNULCC001 with β-galactosidase activity that can utilize cheese whey as carbon source was screened from a traditional Chinese fermented food, Fuyuan pickles. The ability of L. plantarum JNULCC001 to use cheese whey as the sole substrate for EPS production was demonstrated in fed-batch culture. The prepared EPS was characterized in terms of monosaccharide composition, functional groups, surface charge and thermo-stability. Moreover, the applications of EPS in thickening, bioremediation, and stabilizing selenium nanoparticles (SeNPs) were also evaluated. Collectively, we successfully carried out the one-step bioconversion of cheese whey by using L. plantarum with β-galactosidase activity to produce valuable EPS.