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.