Impact of non-thermal techniques on enzyme modifications for their applications in food

The present review elaborates on the details of the enzyme, its structure, specificity, and the mechanism of action of selected enzymes as well as structural changes and loss or gain of activity after non-thermal treatments for food-based applications. Enzymes are biological catalysts found in various systems such as plants, animals, and microorganisms. Most of the enzymes have their optimum pH, temperature, and substrate or group of substrates. The conformational modification of enzymes either increases or decreases the rate of reaction at different pH, and temperature conditions. Enzymes are modified by different techniques to enhance the activity of enzymes for their commercial applications mainly due to the high cost of enzymes, stability, and difficulties that occur during the use of enzymes in different conditions. On the opposite, enzyme inactivation provides its application to extend the shelf life of fruits and vegetables by denaturation and partial inactivation of enzymes. Hence, the activation and inactivation of enzymes are studied by non-thermal techniques in both the model and the food system. The highly reactive species generated during non-thermal techniques cause chemical and structural modification. The enzyme modifications depend on the type and source of the enzyme, type of technique, and the parameters used.

Critical review on alterations in physiochemical properties and molecular structure of natural polysaccharides upon ultrasonication

Natural polymers, such as polysaccharides, cellulose, and starch, have been widely used in the chemical engineering, medicine, food, and cosmetics industries, which had a great many of biological activities. Natural polysaccharides origin from algae, fungi and plants were components of human diet since antique times. Ultrasonication achieved the breakage the polysaccharides reticulum in an ordered fashion. The factors of temperature, ratio of water/material, sonication frequency, time of exposure, pH of the sonication medium influenced the polysaccharide digestion. Sonication improved the enzyme catalysis over its substrate molecule. Positive health promoting slow digestive starch and resistant starch can be prepared quite easily by the sonication process. The aim of this review is to present the current status and scope of natural polymers as well as some emerging polymers with special characteristic. The physiochemical properties and molecular structure of natural carbohydrates under ultrasonic irradiation were also discussed. Moreover, Polysaccharide based films had industrial applications is formed by ultrasonication. Polysaccharide nanoparticles obtained by sonication had efficient water holding capacity. Sonication is an advanced method to improve the food quality. Hence, this review describes the effects of ultrasonication on physical, chemical, and molecular structure of natural polysaccharides.

Development of different pretreatments and related technologies for efficient biomass conversion of lignocellulose

Lignocellulose, a kind of biological resource widely existing in nature, which can be transformed into value-added biochemical products through saccharification, fermentation or chemical catalysis. Pretreatments are the necessary step to increase the accessibility and digestibility of lignocellulose. This paper comprehensively reviewed different pretreatment progress of lignocellulose in recent year, including mechanical/thermal, biological, inorganic solvent, organic solvent and unconventional physical-chemical pretreatments, focusing on quantifying the influence of pretreatments on subsequent biomass conversion. In addition, related pretreatment techniques such as genetic engineering, reactor configurations, downstream process and visualization technology of pretreatment were discussed. Finally, this review presented the challenge of lignocellulose pretreatment in the future.

Process intensification using immobilized enzymes for the development of white biotechnology

Process intensification of biocatalysed reactions using different techniques such as microwaves, ultrasound, hydrodynamic cavitation, ionic liquids, microreactors and flow chemistry in various industries is critically analysed and future directions provided.

Simultaneous nitrogen fixation and ethanol production by Zymomonas mobilis

This work reports on production of ethanol with simultaneous fixing of nitrogen (N₂) using the anaerobic bacterium Zymomonas mobilis DSM 473. A batch fermentation with an initial glucose concentration of 50 g L^-1, an initial pH of ∼5.5, an inoculum size of 10% by volume and a N₂ feeding rate of 50 mL min^-1 without mechanical agitation was found to provide the highest ethanol productivity (0.401 g L^-1 h^-1). Ethanol yield on glucose exceeded 97% of the theoretical maximum. The nitrogen content of the microbial biomass was 10.4% w/w at 65 h and all of it was derived by fixation of dinitrogen. Repeated-batch fermentations were investigated for ethanol production using simultaneous nitrogen fixation. A 2-cycle repeated-batch fermentation lasting 71 h gave a maximum ethanol yield on glucose of 0.475 g g^-1 and an ethanol productivity of 0.675 g L^-1 h^-1. The yield (0.415 g g^-1) and productivity (0.638 g L^-1 h^-1) were reduced in a 3-cycle repeated batch operation lasting 94 h. The need to fix nitrogen did not reduce the final achievable ethanol concentration, or the ethanol yield on glucose, relative to fermentations provided with fixed nitrogen, but did reduce the ethanol productivity by ∼82% because less cell mass was produced.

Low-frequency Ultrasound with Short Application Time Improves Cellulase Activity and Reducing Sugars Release

In this study, we investigated the effect of ultrasound (US) on the activity of commercial cellulase (Celluclast® 1.5 L) in the absence and in the presence of a cellulosic substrate (Avicel®, 2% w.v^-1) using a central composite rotatable design. Sonication time (10 to 330 s), US intensity (120.6 to 263.7 W cm^-2), and reaction temperature (25 to 50 °C) were varied using a horn-type ultrasound reactor, and endoglucanase (CMCase) and total cellulase (FPase) activities were determined. US intensity had a positive effect on enzyme activity. Under optimal conditions (170 s, 180.8 W cm^-2, and 25 °C), CMCase activity was 13% higher than that of the control. In the presence of substrate, CMCase activity increased by 33.87% and K_M reduced by 23% in relation to that of the control. The theoretical yield of cellulose was 42.08%. Cellulase activity can be improved by US treatment to maximize productivity gains and reduce costs in second-generation ethanol production, by the action of a low-frequency ultrasound with a short ultrasonication time of application.

A reactor designed for the ultrasonic stimulation of enzymatic esterification

A laboratory scale ultrasonic flow reactor capable of enhancing enzymatic reactions has been built and characterized using as a model reaction the enzymatic synthesis of isoamyl acetate using Lipozyme 435 immobilized on a macroporous anion exchange resin. The efficiency of the reactor was determined in relation to ultrasonic power density (measured by 4-nitrophenol dosimetry), position of ultrasonic horn and temperature. The results show that ultrasound can enhance the process efficiency and also reduce the reaction time.

Mechanistic investigations in ultrasound-assisted xylitol fermentation

This study has investigated ultrasound-assisted xylitol production through fermentation of dilute acid (pentose-rich) hydrolysate of sugarcane bagasse using free cells of Candida tropicalis. Sonication of fermentation mixture at optimum conditions was carried out in ultrasound bath (37 kHz and 10% duty cycle). Time profiles of substrate and product in control (mechanical shaking) and test (mechanical shaking + sonication) fermentations were fitted to kinetic model using Genetic Algorithm (GA) optimization. Max. xylitol yield of 0.56 g/g and 0.61 g/g of xylose was achieved in control and test fermentations, respectively. The biomass yield also increased marginally (∼17%) with sonication. However, kinetics of fermentation increased drastically (2.5×) with sonication with 2× rise in xylose uptake and utilization by the cells. With comparative analysis of kinetic parameters in control and test experiments, this result was attributed to enhanced permeability of cell membrane that allowed faster diffusion of nutrients, substrates and products across cell membrane, higher enzyme-substrate affinity, dilution of toxic components and reduced inhibition of intracellular enzymes by substrate.

Ultrasound promotes enzymatic reactions by acting on different targets: Enzymes, substrates and enzymatic reaction systems

With the extensive application of enzyme-catalyzed reactions in numerous fields, improving enzymatic efficiency has attracted wide attention for reducing operating costs and increasing output. There are three targets throughout enzymatic reactions: the enzyme, substrate, and mixed reaction system. Ultrasound has been known to accelerate enzymatic reactions by acting on different targets. It can modify both enzyme and substrate macromolecules, which is helpful for enhancing enzyme activity and product yields. The synergistic effect of ultrasound and enzymes is widely reported to increase catalytic rates. The present review discusses the positive effect induced by ultrasound throughout the enzymatic process, including ultrasonic modification of enzymes, ultrasound assisted immobilization, ultrasonic pretreatment of substrates, and ultrasound assisted enzymatic reactions.

Ultrasound assisted intensification of enzyme activity and its properties: a mini-review

Over the last decade, ultrasound technique has emerged as the potential technology which shows large applications in food and biotechnology processes. Earlier, ultrasound has been employed as a method of enzyme inactivation but recently, it has been found that ultrasound does not inactivate all enzymes, particularly, under mild conditions. It has been shown that the use of ultrasonic treatment at appropriate frequencies and intensity levels can lead to enhanced enzyme activity due to favourable conformational changes in protein molecules without altering its structural integrity. The present review article gives an overview of influence of ultrasound irradiation parameters (intensity, duty cycle and frequency) and enzyme related factors (enzyme concentration, temperature and pH) on the catalytic activity of enzyme during ultrasound treatment. Also, it includes the effect of ultrasound on thermal kinetic parameters and Michaelis-Menten kinetic parameters (k_m and V_max) of enzymes. Further, in this review, the physical and chemical effects of ultrasound on enzyme have been correlated with thermodynamic parameters (enthalpy and entropy). Various techniques used for investigating the conformation changes in enzyme after sonication have been highlighted. At the end, different techniques of immobilization for ultrasound treated enzyme have been summarized.

Ultrasound-assisted bioalcohol synthesis: review and analysis

The present article highlights the efficacy of ultrasound in the intensification of all the steps of bioalcohol synthesis with a critical analysis of the literature.

Pretreatment of microalgal biomass for enhanced recovery/extraction of reducing sugars and proteins

Microalgae species including Chlamydomonas mexicana, Micractinium reisseri, Scenedesmus obliquus and Tribonema aequale were cultivated in batch cultures, and their biochemical composition was determined. C. mexicana showed the highest carbohydrate content of 52.6% and was selected for further study. Sonication pretreatment under optimum conditions (at 40 kHz, 2.2 Kw, 50 °C for 15 min) released 74 ± 2.7 mg g(-1) of total reducing sugars (TRS) of dry cell weight, while the combined sonication and enzymatic hydrolysis treatment enhanced the TRS yield by fourfold (280.5 ± 4.9 mg g(-1)). The optimal ratio of enzyme [E]:substrate [S] for maximum TRS yield was [1]:[5] at 50 °C and pH 5. Combined sonication and hydrolysis treatment released 7.3% (27.1 ± 0.9 mg g(-1)) soluble protein of dry cell weight, and further fermentation of the dissolved carbohydrate fraction enhanced the soluble protein content up to 56% (228.4 mg g(-1)) of total protein content. Scanning and transmission electron microscopic analyses indicated that microalgae cells were significantly disrupted by the combined sonication and enzyme hydrolysis treatment. This study indicates that pretreatment and subsequent fermentation of the microalgal biomass enhance the recovery of carbohydrates and proteins which can be used as feedstocks for generation of biofuels.

Ultrasound enhanced enzymatic hydrolysis of Parthenium hysterophorus: A mechanistic investigation

This study has attempted to establish the mechanism of the ultrasound-induced enhancement of enzymatic hydrolysis of pretreated and delignified biomass of Parthenium hysterophorus. A dual approach of statistical optimization of hydrolysis followed by application of sonication at optimum conditions has been adopted. The kinetics of hydrolysis shows a marked 6× increase with sonication, while net sugar yield shows marginal rise of ∼ 20%. The statistical experimental design reveals the hydrolysis process to be enzyme limited. Profile of sugar yield in ultrasound-assisted enzymatic hydrolysis has been analyzed using HCH-1 model coupled with Genetic Algorithm optimization. The trends in the kinetic and physiological parameters of HCH-1 model reveal that sonication enhances enzyme/substrate affinity and reaction velocity of hydrolysis. The product inhibition of enzyme in all forms (free, adsorbed, complexed) also reduces with ultrasound. These effects are attributed to intense micro-convection induced by ultrasound and cavitation in the liquid medium.

Combination of ultrasonic irradiation with ionic liquid pretreatment for enzymatic hydrolysis of rice straw

The application of ultrasonic irradiation and ionic liquids (ILs) in the degradation of rice straw under different processes of pretreatment and enzymatic hydrolysis was investigated. Various substrates for enzymatic hydrolysis by cellulase with and without ultrasound were as follows: untreated rice-straw powder (RS); RS treated by ILs of 1-ethyl-3-methylimidazolium ethylsulfate and trihexyl (tetradecyl) phosphonium decanoate with ultrasound at 300 W/(40 kHz, 28 kHz); RS treated by IL of choline hydroxide ([Ch][OH]) with ultrasound at 300 W/40 kHz (CHRS). In ultrasound-mediated enzymatic hydrolysis, the yield of total reducing sugar (TRS) converted from CHRS was up to 96.22% at 240 min and was greater than that from the other substrates; the TRS yield for CHRS with ultrasound was 19.5% greater than that without irradiation. Lignocellulosic biomass pretreated with [Ch][OH] showed the highest efficiency among the tested ILs, and ultrasound can be applied effectively in rice-straw pretreatment and enzymatic hydrolysis.