Effect of cold and hot enzyme deactivation on the structural and functional properties of rice dreg protein hydrolysates

Highlights

• Enzyme deactivation effects on the rice dreg protein hydrolysate were investigated.

• Hot deactivation hydrolysates with the low degree of hydrolysis but high solubility.

• Cold deactivation hydrolysates had more excellent emulsifying stability.

• Hot vs cold deactivation hydrolysate exhibited different antioxidant activities.

Abstract

This study explored the effect of three different enzyme deactivation treatments: 4 °C slow cold deactivation (RDPH-(4 °C)), −18 °C rapid cold deactivation (RDPH-(−18 °C)) and 100 °C water bath (RDPH-(100 °C)), compared to that without enzyme deactivation (RDPH-(control)) on the structural and functional properties of rice dreg protein hydrolysates (RDPHs). The RDPHs from the different enzyme deactivation methods led to significant differences in the degree of hydrolysis, surface hydrophobicity, average particle size, intrinsic fluorescence and emulsion stability. FTIR analysis revealed that the strength of RDPH-(100 °C) spectrum peaks decreased significantly. All samples showed high solubility (>85%) and potent antioxidant capacity: DPPH (~90%), ABTS (~99%), and reducing power (0.86–1.03). Among the hydrolysates evaluated, the RDPH-(100 °C) led to the lowest reducing power and hydroxyl radical scavenging activity. Results reported here will be instrumental for the development of rice protein-based products and in the optimization and scale up of manufacturing process.

Introduction

Plant protein has been drawing significant attention in the food industry because of the sharp increase in the utilization of its functional compounds for future food development. Of these compounds, plant-based protein hydrolysates will potentially play an important role in human health as they are carriers of bioactive peptides (Tkaczewska et al., 2020). These high value compounds can be easily produced from the hydrolysis of plant-based byproducts which further reduces the environmental footprint and provides the potential to offset the cost in the management of the byproducts.

Rice proteins, as one of the valuable plant-based protein sources, have the advantage of providing particular nutrition and hypoallergenic properties in contrast to other cereal-based proteins (Zhao et al., 2012); this has also led to further considerations in the use of rice protein as protein substitutes in milk formula. One challenge for commercial application hitherto is the relatively low protein content in rice when compared to other cereal. One avenue to overcome this limitation is to explore alternative unconventional sources of rice proteins. Rice dreg is a byproduct generated from the rice syrup industry and has a rice protein content above 50%, almost five times higher than that in raw rice (Zhao et al., 2012). However, most of them are currently discarded as an industrial solid waste or underutilized as animal feed or fertilizer. Thus, utilization of rice dreg (such as recoveries of the proteins from it) for subsequent application in human foods play a significant role in the economic viability and the value addition to the rice processing industry. From a processing perspective, a key step to facilitate the utilization of the proteins from rice dreg is to overcome the inherent low solubility of the rice dreg protein in the aqueous phase (Nisov et al., 2020). This can be undertaken by hydrolyzing the protein with enzyme to decrease the molecular size and structure of the protein, allowing it to dissolve in water. In doing so, as reported for other plant-based proteins in addition to rice proteins, the hydrolysates will also exhibit enhanced techno-functional properties such as: better emulsifying properties as observed for peanut proteins (Jamdar et al., 2010); more stable foaming properties for rice proteins (Nisov et al., 2020); higher antioxidant ability in soy proteins (Coscueta et al., 2019).

It is well known that as part of the enzyme hydrolysis process, the enzymatic deactivation step which terminates the hydrolysis process, plays a key role in determining the final properties (Marquez Moreno & Fernandez Cuadrado, 1993). Hydrolysis reactions are normally terminated by thermal treatment or pH alternation (Eijsink et al., 2005). Thermal treatments are typically preferred from a consumer and product development view point as it obviates the need for pH adjustment material in the final product. As a case in point, at the laboratory scale, enzymes are commonly inactivated in a batch manner by immersion of the samples in water baths (10–20 min) at high temperatures (80–100 °C) (Le Maux et al., 2018); with variations in the temperature-time combination depending on the characteristics of the enzyme. The heat treatment itself, however, may result in structural rearrangements within the hydrolysates (Foegeding et al., 2002) leading to physicochemical and functional differences between hydrolysates (Ozorio et al., 2019). It is noteworthy, however, that the conventional thermal processing technologies for enzyme deactivation always lead to quality deterioration and the loss of nutritional components (Thirumdas & Annapure, 2020). This can be further exacerbated if heat-resistant enzymes are used which necessitates more intense or longer heat treatment (Rawson et al., 2011). Thus, there is a need for alternative technologies to terminate the enzyme activity without significant detrimental modification to the food product. At the other end of the thermal deactivation spectrum is cold inactivation, which suppresses the protein and enzyme activity without denaturing the protein or enzyme. Compared to the commonly used thermal processing, the effect of cold treatment on the food quality is very mild. While there are reports in the literature focusing on understanding the effectiveness of the hydrolysis on protein hydrolysates using single step enzyme inactivation (Firmansyah and Abduh, 2019, Pihlajaniemi et al., 2020), to the best of our knowledge, there are currently no reports systematically comparing the cold deactivation and the high temperature deactivation approaches with the same protein-enzyme system. A question remains: which method is more suitable for such plant-based protein hydrolysis? As alluded earlier, it is expected that any conclusion drawn to this question will be protein and enzyme specific.

Hence, the aim of this work was to investigate and compare the effectiveness of cold and hot enzyme deactivation on the structural and functional properties of rice dreg protein hydrolysates (RDPHs). The experimental results reported included a slow cooling deactivation (storage in 4 °C cold chamber), rapid cooling deactivation (storage in −18 °C freezer) and the conventional high temperature deactivation (100 °C water bath for 15 min). The outcome from this work, in addition to providing more detailed understanding on the use of enzymatic hydrolysis in the production of rice dreg protein hydrolysates, will also provide more evidence to support further evaluation on the use of alternative enzyme inactivation processes (for other plant-based protein) for the functional food ingredient industry.