Conclusion
The thermal treatment significantly influenced the physicochemical, structural, functional and digestibility properties of protein in walnut kernel flour in a temperature-dependent action. SEM micrographs reveled the walnut kernel flour by roasting and steaming treatment more sensitive than the untreated, while the steam treatment had more influence on walnut kernel flour than the roasting treatment. The secondary structures of protein in walnut kernel flours were affected by heating as measured by FTIR, the protein structure changes of samples for steamed processing happened to a greater extent than that of roasted ones. It was found that the heat treatment could result in unfolding of protein, which had significantly affected the functionality of samples. The results showed that the roasting and steaming treatment affected the amino acid composition and amino acid score (AAS), such as the roast processing decreased the AAS for essential amino acids compared to the untreated sample, while the steam processing increased the AAS. But the lysine was the only limiting amino acid for all samples.
It was noted that the digestibility, water holding capacity and oil holding capacity, foaming and emulsion capacities and stabilities of walnut kernel flour during heat processing increased compared to untreated, however, with the extension of heating time, these functional properties would produce the different changes. At the same time, the walnut kernel flours prepared by heating treatment were characterized by high enthalpy (△H), storage modulus (G′), loss modulus (G′′) and viscosity compared to that of untreated sample, the order of these values from high to low was the steaming > roasting > untreated. Therefore, it could conclude that when the heating technique would be used to alter the functional properties and quality of walnut kernel flour and considered to have good utilization promises in food industry. Further, it was proved that the moisture pretreatment due to containing water in the modifications of functional properties and quality of protein based on food materials was more effective than dry-heating.
Author Contribution Xiaoyan Zhao : Project administration, Resources, Supervision, Funding acquisition, Writing – review & editing. Haifang Hu : Project administration, Resources, Supervision, Funding acquisition. Xiangrui Ren : Investigation, Writing – original draft. Xiaowei Zhang : Data curation, Methodology, Writing - original draft. Hongkai Liu : Conceptualization, Software. Meng Wang : Validation, Formal analysis.
Acknowledgments Financial support of this work by National Natural Science Foundation of China (Grant No. 21406133) and Xinjiang Key Scientific and Technological Project (Grant No. 2021A02002-2).
Data availability Xiaoyan Zhao can be contacted for the data
Conflict of Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.