Cheese whey valorization to obtain single-cell oils of industrial interest: An overview

Abstract

Cheese whey is a by-product of cheese manufacture. Even though it is used in many activities, large amounts of cheese whey are discarded without proper treatment, affecting the environment in cheese-producing regions due to its biological and chemical oxygen demand. Cheese whey represents a potential substrate in biotechnological processes due to its composition of nutrients. Several studies have focused on the valorization of cheese whey to obtain bio compounds of industrial interest while mitigating the environmental impact that this by-product can cause. In this work, emphasis is placed on the production of microbial lipids, also known as single-cell oils, and information is compiled on the main microorganisms that can synthesize and store them in large quantities. The mechanisms and components for substrate assimilation and accumulation of lipids, fermentation conditions, strategies to improve lipid extraction and its applications are discussed.

Introduction

One of the most relevant activities in the dairy industry is the production of cheese, which generates a large amount of an important liquid by-product known as cheese whey (CW). Its composition depends on the method used for milk curdled and is divided into acid or sweet whey, generally, they have a large amount of lactose, proteins, and minerals (Addai et al., 2020; Lagoa-Costa et al., 2020; Pires et al., 2021). Although CW is used in activities in various industries, the increase in cheese production due to the growing population limits the use of this by-product, causing more than half of the CW to be discarded without pretreatment, which represents an environmental problem in cheese-producing regions due to its high organic load (Chan et al., 2018). For this reason, solutions have been sought to take advantage of the nutrients present in cheese whey while reducing its environmental impact. One of the strategies is the incorporation in biotechnological processes where CW is used as a low-cost substrate in fermentations for microbial growth and synthesis of value-added compounds (Lappa et al., 2019; Zotta et al., 2020). Among the microorganisms that have been evaluated, the so-called oleaginous ones stand out, thus known for their great capacity to accumulate single-cell oils (SCO) intracellularly. For these microorganisms to produce lipids there must be an imbalance in the medium, an excess of the carbon source, and the limitation of another element (such as nitrogen), along with other important factors that must be carefully selected during fermentation (Mhlongo et al., 2021; Pires et al., 2021). Oleaginous bacteria, filamentous fungi, microalgae, and yeasts have demonstrated the ability to grow and synthesize lipids using CW as a substrate, and the fatty acid profile is usually similar to those obtained in the traditional way in the oleochemical industry (Carota et al., 2018; Herrero & Alvarez, 2016; Russo et al., 2021; Vyas & Chhabra, 2019). Saturated fatty acids (SFA) and monounsaturated fatty acids (MUFA) are used as raw material in the production of biodiesel and are obtained from vegetable crops, which has triggered the dispute over arable lands (Alvarez & Steinbüchel, 2019, pp. 299–332; Mhlongo et al., 2021; Zhang et al., 2021). On the other hand, polyunsaturated fatty acids (PUFA) have a great nutraceutical interest in the prevention and treatment of diseases. PUFA are mainly obtained from marine species, which has caused overfishing that affects the ecosystem and the quality of the fatty acids obtained (Fazili et al., 2022; Morales et al., 2021; Saini & Keum, 2018). The production of microbial lipids does not depend on the season, accumulation of lipids in short cultivation periods, and the process can be scaled, representing an excellent alternative in the oleochemical industry to traditional vegetable oil and animal fats production (Fayyaz et al., 2020; Kothri et al., 2020; Shields-Menard et al., 2018). Since microbial lipids are found intracellularly, a few treatments have been evaluated for cell wall rupture before lipid extraction. Furthermore, the search for alternatives for extraction that are environmentally friendly has been carried out because toxic solvents are used in classic methods (Saini et al., 2021; Yousuf et al., 2017). In the present work, the use of cheese whey for the production of SCO by oleaginous microorganisms, the effect of culture conditions on lipid synthesis, and the challenges to overcome the technical and environmental problems of the bioprocess are discussed.