The increasing global population will require sustainable means to sustain life and growth. The continuous depletion and increasing wastage of the energy resources will pose a challenge for the survival of the increasing population in the coming years. The bioconversion of waste generated at different stages of the food value chain to ethanol can provide a sustainable solution to the depleting energy resources and a sustainable way to address the growing food waste issue globally. The high carbohydrate and nitrogen content in the food waste can make it an ideal alternative substrate for developing a decentralized bioprocess. Optimizing the process can address the bottleneck issues viz. substrate collection and transport, pretreatment, fermentative organism, and product separation, which is required to make the process economic. The current review focuses on the opportunities and challenges for using the food loss and waste at different stages of the food value chain, its pretreatment, the fermentation process to produce bioethanol, and potential ways to improve the process economics. The impact of substrate, fermentative organisms’ process development, downstream processing, and by-product stream to make the bioethanol production from the waste in the food value chain a commercial success are also discussed.

The progressive rise in energy crisis followed by green house gas (GHG) emissions is serving as the driving force for bioethanol production from renewable resources. Current bioethanol research focuses on lignocellulosic feedstocks as these are abundantly available, renewable, sustainable and exhibit no competition between the crops for food and fuel. However, the technologies in use have some drawbacks including incapability of pentose fermentation, reduced tolerance to products formed, costly processes, etc. Therefore, the present study was carried out with the objective of isolating hexose and pentose fermenting thermophilic/ thermotolerant ethanologens with acceptable product yield. Two thermotolerant isolates, NIRE-K1 and NIRE-K3 were screened for fermenting both glucose and xylose and identified as Kluyveromyces marxianus NIRE-K1 and K. marxianus NIRE-K3. After optimization using FCCD (Face-centered Central Composite Design), the growth parameters like temperature and pH were found to be 45.17 oC and 5.49, respectively for K. marxianus NIRE-K1 and 45.41 oC and 5.24, respectively for K. marxianus NIRE-K3. Further, batch fermentations were carried out under optimized conditions, where K. marxianus NIRE-K3 was found to be superior over K. marxianus NIRE-K1. Ethanol yield (Yx/s), sugar to ethanol conversion rate (%), microbial biomass concentration (X) and volumetric product productivity (Qp) obtained by K. marxianus NIRE-K3 were found to be 9.3%, 9.55%, 14.63% and 31.94% higher than that of K. marxianus NIRE-K1, respectively. This study revealed the promising potential of both the screened thermotolerant isolates for bioethanol production.

Lignocellulosic biomass is an attractive, inexpensive source of potentially fermentable sugars. However, hydrolysis of lignocellulose results in a complex mixture containing microbial inhibitors at variable composition. A single microbial species is unable to detoxify or even tolerate these non-sugar components while converting the sugar mixtures effectively to a product of interest. Often multiple substrates are metabolized sequentially because of microbial regulatory mechanisms. To overcome these problems, we engineered strains of Acinetobacter baylyi ADP1 to comprise a consortium able to degrade benzoate and 4-hydroxybenzoate simultaneously under batch and continuous conditions in the presence of sugars. We furthermore used a thermotolerant yeast, Kluyveromyces marxianus, to convert the glucose remaining after detoxification to ethanol. The two engineered strains, one unable to metabolize benzoate and another unable to metabolize 4-hydroxybenzoate, when grown together removed these two inhibitors simultaneously under batch conditions. Under continuous conditions, a single strain with a deletion in the gcd gene metabolized both inhibitors in the presence of sugars. After this batch detoxification using ADP1-derived mutants, K. marxianus generated 36.6 g/L ethanol. We demonstrated approaches for the simultaneous removal of two aromatic inhibitors from a simulated lignocellulosic hydrolysate. A two-stage batch process converted the residual sugar into a non-growth-associated product, ethanol. Such a two-stage process with bacteria (A. baylyi) and yeast (K. marxianus) is advantageous, because the yeast fermentation occurs at a higher temperature which prevents growth and ethanol consumption of A. baylyi. Conceptually, the process can be extended to other inhibitors or sugars found in real hydrolysates. That is, additional strains which degrade components of lignocellulosic hydrolysates could be made substrate-selective and targeted for use with specific complex mixtures found in a hydrolysate.

The adverse climatic conditions due to continuous use of fossil-derived fuels are the driving factors for the development of renewable sources of energy. Current biofuel research focuses mainly on lignocellulosic biomass (LCB) such as agricultural, industrial and municipal solid wastes due to their abundance and renewability. Although many mesophilic cellulolytic microorganisms have been reported, efficient and economical bioconversion to simple sugars is still a challenge. Thermostable cellulolytic enzymes play an indispensible role in degradation of the complex polymeric structure of LCB into fermentable sugar stream due to their higher flexibility with respect to process configurations and better specific activity than the mesophilic enzymes. In some anaerobic thermophilic/thermotolerant microorganisms, few cellulases are organized as unique multifunctional enzyme complex, called the cellulosome. The use of cellulosomal multienzyme complexes for saccharification seems to be a promising and cost-effective alternative for complete breakdown of cellulosic biomass. This paper aims to explore and review the important findings in cellulosomics and forward the path for new cutting-edge opportunities in the success of biorefineries. Herein, we summarize the protein structure, regulatory mechanisms and their expression in the host cells. Furthermore, we discuss the recent advances in specific strategies used to design new multifunctional cellulosomal enzymes, which can function as lignocellulosic biocatalysts and evaluate the roadblocks in the yield and stability of such designer thermozymes with overall progress in lignocellulose-based biorefinery.