A comparison study of three heating assisted enzyme inactivation pretreatments on the physicochemical properties and edible quality of highland barley grain and flour

Highlights

• The highland barley grain and flour qualities varied with different heat treatments.

• Roasting treatment had a negative effect on the texture quality of cooked samples.

• Microwave group had the highest gelatinization degree and lowest crystallinity.

• Superheated steam treatment caused the least change in starch pasting properties.

Abstract

Enzyme inactivation is vital to improve the shelf life of highland barley (HB), but the impacts of different heating assisted enzyme inactivation pretreatment (HAEIP) on structure, physicochemical properties and edible quality remain unclear. This study evaluated the effects of three representative HAEIP for microwave, roasting and superheated steam (SS) on the properties of HB grain and flour based on the 50% or 60% inactivation rates for lipase and complete inactivation for peroxidase. Results showed that microwave and SS treatments had the highest and lowest cracking ratio respectively. For the HB edible qualities, the shorter cooking times were observed in R-60 (26 ± 0.84), SS-50 (27 ± 0.09) and SS-60 (27 ± 0.07) groups. Meanwhile, the hardness and chewiness of roasted HB were significantly higher than that in microwaves and SS groups. The HB flour properties varied with different HAEIP. Although all pretreatments showed a substantial reduction in the flour viscosities with treatments intensity increased, the PV values in the two SS groups were the highest. The microwave treatment had the biggest impact on the relative crystallinity and gelatinization degree, while SS pretreatment had the best performance and could be an effective, practical enzyme inactivation methods and enhance HB product quality.

Introduction

Highland barley (HB) (Hordeum vulgare L.var .nudum hook. f .) belongs to the Poaceae family and is a cultivar of hulless barley, which is the most important crop on the Qinghai-Tibetan Plateau of China, and is mainly distributed in alpine areas of 4200–4500 m (Lin et al., 2018). As a major staple food of Tibetans for generations, HB is widely recognized by consumers because of its high nutritional value, such as β-glucan, phenolic acids, flavonoids and anthocyanins (Lin et al., 2018), which are responsible for reducing in the risk of diabetes, heart disease and colorectal cancer (Weng and Yen, 2012). Therefore, it has become an excellent food product and the market demand for HB products as the whole grain and flour has increased recently.

However, the main limitation for HB utilization in the food industry is its short shelf life because lipids depredate easily via enzymatic hydrolysis (i.e., lipase). The release of free fatty acids (FFAs) and peroxidase (POD, EC 1.11.1.7) promotes the oxidation of FFAs which cause the loss of product sensory quality and nutritional properties; for instance, aldehydes and ketone compounds cause harm to the human body (Wang et al., 2020; Wu et al., 2014). Inactivation of enzyme activities is an essential strategy for preventing or slowing lipid degradation and extending the shelf life of HB. Various heating treatments utilize high temperature to destroy the advanced structure of the enzyme, resulting in the decrease or even inactivation of enzyme activity. These methods mainly include microwave, roasting and superheated steam (SS) treatment (Zhao et al., 2020; Wu et al., 2014; Wang et al., 2020). As a traditional enzyme inactivation method, roasting has the advantages of simple operation and low equipment cost, but roasting treatment generally requires a long heating time and seriously destroys the grain structure (Dhua et al., 2021). Zhao et al. (2020) found that synergism of pearling and tempering increased the inactivation of lipase in HB, the residual lipase activity of the samples with 16% pearling and roasting treatment for 16 min was decreased by 81.1% with the moisture content 20%. Microwave pretreatment can provide faster heating rate and effectively shorten the processing time via electromagnetic energy conversion into heat. However, microwave technology still presents the inconvenience of non-uniform heating and generates high internal pressure inside the product, which may cause incomplete inactivation of enzyme in the cold spots or expansion of the product and forms cracks in the grains (Ma et al., 2020). As a new enzyme inactivation method, SS treatment replaces the hot air in the convection drying chamber with SS during heating, which has advantages regarding the high effectiveness of thermal delivery and an oxygen-free environment, as well as minimizing nutrition and quality losses. It has been reported that SS processing can reduce lipase activity by 60% at 160 °C for 10 min which inhibits the lipid rancidity and oxidation and extend shelf life of HB (Wang et al. 2019, 2021). However, studies of these treatments in HB enzyme inactivation are relatively limited, the research results of the effect of heat treatments on enzyme activities of other grains cannot be directly applied to HB due to different nature and activity of enzyme in different cereals. Moreover, to our knowledge, there is a lack of studies on different enzyme activation methods on physicochemical properties and edible quality of HB grain and flour, in particular to achieve the same enzyme inhibition effect.

HB is most often consumed as whole milled food and flour products; therefore, to improve product quality, enzyme inactivation treatments prior to storage are necessary. Additionally, the characteristics of HB kernels and flour will also be affected due to the high temperature in the process of heat-assisted enzyme inactivation pretreatments. Changes in the functional properties of grain and flour vary depending on heat treatment methods and conditions. Some studies have discussed the effects of heat pretreatments on grain quality, and there are more studies on oats, wheat and rice, but fewer on HB. Altan (2014) showed that microwave treatment caused a porous structure, but the effect of microwave puffing on the microstructure and physical properties was strongly influenced by the initial moisture content and pretreatment method. Head et al. (2010) reported that SS could be used to inactivate peroxidase of oat, meanwhile, SS treated samples have significantly (P < 0.05) brighter colour and higher cold paste viscosity compared to that of oat groats processed commercially. Chen et al. (2015) found that peak viscosity was reduced and pasting temperature was increased when wheat flour suffered heat-moisture treatment. Starch occupies a relatively high proportion in cereal crops (60–70%), and the molecular structure and phase transformation of starch upon thermal processing directly affects the quality of grain products (Wu et al., 2014). It is worth noting that most research work on heating-induced enzyme inactivation and grain quality changes have mostly been done alone. Only by clarifying its structure changes can the appropriate treatment method be chosen according to the requirements of different products for raw materials. A cursory survey of the literature revealed that no study has further contrastive analysis of the properties of HB grain and flour when these treatments inactivate lipase by microwave, roasting and SS treatment to same degree.

Therefore, the influence of microwave, roasting and SS on the morphology, microstructure, physicochemical properties, cooking and texture properties of grain, the particle size, pasting and thermal properties of flour were summarized. In addition, the crystalline structure and the short-range and ordered structure of HB starch, and the influence mechanism of HAEIP on HB starch was preliminarily analyzed. The objectives of this study were to provide a reference for the wider application of microwave, roasting and SS in grain processing and broaden the application of HB in the development of nutritious and healthy food.