Abstract
In this study, walnut butter was produced by mixing functinal lipids with defatted walnut meal. Three kinds of functional lipids(FL), medium-chain triglycerides (MCT), diacylglyceroe (DG), and conjugated linoleic acid glycerides (CLA), were used to make functional lipids walnut butter(FLWB) and their physical properties as well as microscopic morphology were compared with commercial walnut butters. The FLWBs were prepared by grinding FL and defatted walnut meal through the ball milling technique. The mixing ratios of FL and defatted walnut meal were 6:4, 6.5:3.5, 7:3. It was found that the fluidity of FLWB was increased with the addition amount of FL, but the particle size of FLWB was decreased with the addition amount of DG or CLA. As the additions of DG and CLA increased from 60% to 70%, the D (4,3) of DG-WB and CLA-WB decreased by 36.23% and 20.88%; the flowability index increased by 15.74% and 168.91%. As the addition of MCT increased from 60% to 70%, there was no significant difference in D (4,3) of MCT-WB, but the flowability index increased by 717.34%. Microrheological properties indicated that both FLWB and commercial walnut butter exhibited viscoelastic characteristics. The rheology showed that the FLWB was non-Newtonian pseudoplastic fluids, which was similar to the commercial walnut butter. The DG-WB and CLA-WB were closer to the commercial walnut butter, compared with MCT walnut butter. The microstructure further indicated that the walnut butter with 65% CLA addition was closer to the commercial walnut butter.
Keywords : Defatted walnut meal; Walnut butter; Functional lipids; Vibratory ball mill; Processing characteristics
Introduction
Walnut is one of the most widely grown nut food in the world, which is popular due to its high economic value. China accounts for 48% of the world’s walnut productions (Qu et al., 2016). Most walnut is used for walnut oil (WO) extraction, while many defatted walnut meals are produced during this processing. Defatted walnut meal contain more than 40% protein and have high levels of arginine and glutamic acid, while other essential amino acids for the human body (Sun et al., 2019). Defatted walnut meal are often used as animal feed or directly wasted in landfills (Feng et al., 2021). At present, there have been studies on the extraction of active ingredients from defatted walnut meal, such as walnut protein or active peptide (Feng et al., 2021). In addition to this utilization, there is still a need to further improve the comprehensive utilization of defatted walnut meal.
Traditionally, walnut kernels were roasted and then ground to make walnut butter. (Zhaohua et al., 2020). However, the fat content of walnut butter is high, while excessive intake of high-fat foods can increase the risk of obesity and cardiovascular disease (Lu et al., 2019). Therefore, the development of Low-fat and functional walnut butter is becoming a new trend in the nut industry. In order to reduce oil amount of walnut butter, some previous studies produced the product by adding water rather than oil. However, the higher the moisture content of the walnut butter, the more likely it is to have a negative impact on the sensory properties (Mehdi et al., 2019). The oils in walnut butter are beneficial to enhance the processing characteristics and stability of the product, so it is better to use oil as the mobile phase of walnut butter. Therefore, using choice of functional lipids (FL) to replace the oil in traditional nut butters may maintain the quality of nut butter.
FL, such as medium-chain triglycerides (MCT), diacylglyceroe (DG), and conjugated linoleic acid glycerides (CLA), could meet people’s need for flavorful oils and reduce fat energy intake. Moreover, FL has many beneficial physiological effects such as losing weight and reducing blood lipid. MCT has good antioxidant stability, which is naturally found in foods such as palm oil and coconut oil (Walker et al., 2017). DG is a trace component of natural vegetable oils, which can be used as a substitute for animal fat in meat processing. These meat products usually had better stability (Miklos et al., 2011). CLA is a physiological active material, which have potentially antitumor and anti-atherosclerosis effects (Belury, 2002).
Traditionally, walnut butter were usually ground by stone mills, high-speed grinders, and colloid mills (Wagener and Kerr, 2018). It is hard to obtain small walnut butter particle sizes by one time of grinding using the above mentioned traditional grinding methods. So a secondary grinding is necessary for walnut butter to reach a specific particle size range. Vibration ball milling is a new ultrafine grinding technology. Ball milling could exert collisional, compressive, shear, and frictional forces on the sample during the collision processing. The frictional process could lead to local heating, causing thermochemical reactions (Steiner et al., 2016). Therefore, after ball milling treatment, the processed material undergoes reversible internal structural rearrangement. This process can create new active surfaces or result in covalent bond breakage, and thus changing the molecular conformation and chemical bonding of the material (Karinkanta, 2014; Marti, 2004). Based on the above mentioned information, ball milling technology is gaining more and more attention as a new, efficient and green grinding technology in the food field. Vibration ball milling may effectively reduced the particle size of walnut butter with only one time of grinding. Consequently, this study aimed to make comprehensive utilization of defatted walnut meal to make a healthier and marketable FLWB by vibration ball milling technology.
Materials and Methods