Introduction
Walnut (Juglans regia L. ) belongs to a tree nut due to consisting of a hard nutshell protecting the kernel. It is widelyc oncerned and popular for its high nutritional value. Walnut is distinguished by high oil content (43.3-78.48%) of unsaturated fatty acids (about 70% total fatty acids) [1]. Besides, walnut also an excellent source of protein, carbohydrate and polyphenolic compounds [2,3]. Especially, the defatted walnut meal is discarded as a by-product of oil extraction. It consists of the abundant protein (about 55 d.w%) and carbohydrate (about 28 d.w%), which can act as the high nutritional components in food products [4]. Food and Drug Administration and European Food Safety Authority declares that the walnut kernel can decrease the risk of heart disease, cardiovascular disease, cancer, hypertension and all-cause [5]. In short, the walnut kernel has the characteristics of improving human health [5]. The references reported that thermal treatment had significant influence on polyphenols content, vitamin E content and fat acid profile [6], and also improved the utilization and availability of protein and reduced the anti-nutritional factors [7]. In addition, the studies proved that the quality of walnut meal flour was not damaged under mild thermal treatment procedures [6], and applied as a functional ingredient in food industry [3].
However, the utilization of walnut in the food system is often limited partially because of the digestibility and allergy of walnut protein [8]. The thermal processing can conquer these shortcomings, which was proved that it could be used to process beans and cereals, such as soybean, common bean [9,10]. In general, the thermal processing method is a common processing, which includes wet-heat and dry-heat treatment. The wet-heat method transfers heat to food through steam or water, such as boiling, steaming and cooking in simmering liquid; while the dry-heat transfers to food by hot air, such roasting and air frying [9].
Roasting method is a kind of typical food hot processing, which can improve the digestibility, eating qualities and bioavailability of food products by modifying the physicochemical properties and structure of food components [11]. The reason is due to biochemical changes in process of roasting, such as hydrolysis, oxidation, reduction, and resulting reactions from pyrolysis [11]. These changes also will be helpful to enhance the color, flavor, taste active compounds, and antioxidants and decrease the microbiological pollution, natural poison and enzyme inhibitors in food [11]. The steaming method is considered as one of the most common mechanical heating methods with the features of low cost and convenience in food industry, which can affect the texture, color, digestibility, structure and antioxidant compounds in food material [12-14]. Moreover, it can also avoid the leaching of nutritional compositions [12].
Meanwhile, there is absent from the systematic report on the effects of roasting and steaming methods on the physicochemical and functional properties of walnut meal flour [6]. Such information is needed to strengthen the study for this species from walnut meal as a major source of protein and other nutritional requirements for healthy growth. Therefore, the purpose of this study was to compare the effects of roasting and steaming methods under different treatment time at the same heating temperature on the vitro digestibility, physico-chemical and functional properties of walnut meal flour.