Bioethanol is generated through the fermenting of simple sugars found in biomass as well as sugars derived from earlier enzymatic hydrolysis treatment of food wastes [25]. Fermentation is then carried out by microorganisms, generally yeasts. However, bacteria such as Zymomonas mobilis [26] have also been utilised. Co-culture of S. cerevisiae and P. stipitis leads to higher ethanol yield of 0.13 ± 0.01 g/g of food waste [9]. Following fermentation, the ethanol produced is recovered from the fermentation medium using either traditional rectification and distillation or more efficient separation techniques such as membrane filtration, pervaporation, or molecular sieves. Figure 1 depicts a schematic of starch-based bioethanol manufacturing.

Food waste comes in a variety of forms. It can either be in raw or in cooked form. Because it is regarded as waste, it necessitates some preprocessing before it can be processed for the production of ethanol [27]. Physical, chemical, and physio-chemical pretreatments have been used in this manner. Pretreatment can be used depending on the nature of the food waste. In most circumstances, extensive pretreatment prior to enzymatic hydrolysis is not required. Various modified hydrolysis and enzymatic hydrolysis are conducted to boost ethanol output. Instead, autoclaving food wastes before fermentation is frequently required to increase the purity and yield of the product, albeit at the expense of increased energy and water usage. It should be mentioned that heat treatment might cause a partial breakdown of sugars and different biological function components and side reactions (e.g., the Maillard reaction) in which the quantity of beneficial amino acid and sugars square measure could be reduced [28].

Furthermore, recent and wet food waste appears to be far more efficient than rewetted dried food waste [1]. This is due mainly to the surface area of the dried substrate, which manifests in the substrate–enzyme reaction efficiency. Consequently, drying food waste is beneficial for high-yielding ethanol with controlled contamination by microorganisms. Contamination by microorganisms can be avoided in acidic conditions without thermal treatment. As a result, acid-tolerant alcohol microbes such as Zymomonas mobilis were used for fermentation [1].

Starch hydrolysis is an essential stage in starch-based food waste processing for bioethanol generation. The primary function of this process is to convert two key starch polymer constituents, branched amylopectin, a α-D-(1-4)-glucan with α-D-(1-6) linkages at the branching, and amylose, a mainly linear α-D-(1-4)-glucan, to simple sugars that can then be turned to alcohol by microorganisms (bacteria and yeast). Acids can be used to perform hydrolysis, an older method that has mostly been abandoned in favour of a more effective enzymatic method. Recently, some researchers have also used bacterial consortia for this purpose [16,22,23,24]. Starch-based bioethanol production has been widely popular for around 30 years; during that time, enormous advances in process cost, enzyme efficiency, time reduction, and increased hydrolysis and bioethanol production have been accomplished [29]. Current discoveries in the development of thermostable α-amylases, which are starch hydrolysing enzymes that catalyse the hydrolysis of internal α-D-(1-4)-glucosidal linkages in starch in a random fashion, and efficient glucoamylases, that are saccharifying starch enzymes that catalyse the hydrolysis of α-D-(1-6)- and α-D-(1-4)-glycosidic bonds in starch to glucose have brought about the commercial establishment of the popular two-step enzymatic cold process. The main benefits of this technique are the consumption of lesser energy and a reduced proportion of non-glucosidal contaminants, making it considerably more suitable for ethanol synthesis. Enzymatic starch hydrolysis is carried out under relatively mild operative conditions: lower temperatures (up to 100 degrees Celsius), normal pressure, and a pH of between 6–8 [30]. The quantity of endogenous enzymes used in starch hydrolysis, and the hydrolysis parameters, including temperature, process time, concentration, pH, etc., are influenced by the type of food waste, its chemical composition, the source and activity of endogenous enzymes, and the presence of native autoamylolytic potential. Additionally, primarily physical treatments, such as cooking and steaming, micronisation, grinding, ultrasound, microwave, and so on, enhance the gelatinisation process and the susceptibility of the food waste substrate to enzymes, and can strongly impact and enhance the influence of hydrolysis and subsequent fermentation of ethanol [29].

Efficient bioethanol production necessitates an accelerated fermentation that results in high ethanol concentrations; consequently, the microbial strain used should possess a good specific growth rate and specific ethanol production rate at high ethanol concentration and high osmotic pressure [31]. A critical problem for efficient ethanol production is optimising the fermentation phase in terms of the following key parameters: pH, temperature, the composition of the medium, aeration, mixing, elimination of infection, etc. [32].The fermentation phase is carried out under temperature range of 28–32 °C, and pH range of 4.8–5.0 [33]. Additionally, anaerobic digestion produces an acidic substrate, which could interfere with the fermentation process [33,34]. It is critical to select and develop an efficient production microorganism. As a result, much research is currently being conducted to develop a microorganism resistant to high concentrations of substrate and ethanol. A yeast strain’s ability to produce a high level of alcohol is significantly dependent on the nutritional conditions and protective activities that specific nutrients can supply [35].

At 14% (v/v), the threshold for ethanol production from starch fermentation is reached [36]. Over this threshold, the growth of the microbes responsible for fermentation is inhibited, and creative approaches are applied to overcome this limitation. The immobilisation of yeast or the fermentation microorganism for bioethanol production has been extensively researched to overcome substrate and product inhibition and enhance ethanol tolerance. Among these approaches, the most studied are yeast immobilisation in/on appropriate matrices like poly-acrylamide-alumina calcium, k-carrageenan gel, alginate, orange peel, PVA gel, wooden chips, etc. [29]. Bai et al. [37] prioritised self-flocculation and simple adsorptive immobilisation techniques because these allow slow developing cells to be removed from the system. The most challenging research on the subject is obtaining a fermentation microorganism with a metabolism that would enable the utilisation of a broader sugar spectrum and thus facilitate complete substrate utilisation [29]. These are the most common applications of technologies of genetic engineering.