Structure, Nutritional Value, Extraction, Functional Properties of Walnut Proteins and Their Application in Foods: A Review

China is the world’s largest walnut producer, with an annual output of 4.795 9 million tons. Walnut kernels have high nutritional value, containing 65%−69% of oil and 15%−20% of protein. Walnut meal, an abundant byproduct of walnut oil production, is mainly used in animal feed and fertilizers with low processing efficiency, whose potential for high-value utilization is far from being excavated. Studies have found that walnut meal contains a large amount of protein (42%−54%), its amino acid composition is rich, and the proportion of amino acids in walnut meal is suitable for human absorption. Walnut meal is a source of high-quality plant protein with a true protein digestibility score of 86.22%, and a biological valence of 98.77%. Therefore, high-efficiency utilization of walnut meal proteins is an important pathway for the high-value utilization of walnut meal. In this article, the composition, structure, and nutritional value of walnut proteins are reviewed. The extraction methods and functional characteristics of walnut proteins are summarized. The effect of modification on their functionalities and digestibility is also summarized. Finally, the application potential of walnut protein in foods is discussed.