Dr. Antonella Grosso is a professor at the National University of Cordoba and a doctor in Agricultural Sciences in Argentina. She obtained her Agricultural Engineering degree in 2014. In the same year, she began a research line on walnut preservation, supported by the National Council of Scientific Research and Technology (CONICET). She obtained her PhD in 2018. She is the corresponding author of the research article Grosso, A. L., Asensio, C. M., Grosso, N. R., & Nepote, V. (2020). Increase of walnuts' shelf life using a walnut flour protein-based edible coating. LWT, 118, 108712.
Walnuts are among the most consumed nuts and are selected by consumers due to their sensory characteristics and health-promoting properties. Walnuts contain high amounts of lipids (68-72%), polyunsaturated fatty acids (PUFAs, 70%) and also contain tocopherols. Due to the composition of their oil, they are highly susceptible to lipid deterioration (oxidation) and have a limited shelf-life. This process results in the loss of essential fatty acids and leads to the emergence of rancidity and off-flavors, the development of toxic compounds, and adverse color changes. All of
these changes reduce the product’s sensory, chemical, nutritional and physical quality and leads to consumer rejection. An edible coating is a thin layer of biodegradable material that can be formed on a wide range of foods, with the ability to enhance their safety, transportation and storage. Edible coatings have also been shown to protect the quality of food products by decreasing their deterioration. Hence, the application of edible coatings on walnuts could improve their lipid preservation and extend their shelf-life.
these changes reduce the product’s sensory, chemical, nutritional and physical quality and leads to consumer rejection. An edible coating is a thin layer of biodegradable material that can be formed on a wide range of foods, with the ability to enhance their safety, transportation and storage. Edible coatings have also been shown to protect the quality of food products by decreasing their deterioration. Hence, the application of edible coatings on walnuts could improve their lipid preservation and extend their shelf-life.
Coatings Prepared Using Commercial Ingredients
Methylcellulose (MC), carboxymethylcellulose (CMC) and whey protein (WP) coatings can be applied to walnuts through an immersion process. In general, the use of CMC, MC and WP edible coatings on walnuts helps to prolong the shelf-life of the kernels by preserving their chemical, physical and sensory quality. MC is the coating which presents the best preservative effect. One reason for this is that it has the most powerful protective effect against the development of peroxide value and hexanal content, which are primary and secondary lipid oxidation indicators, respectively. Additionally, MC reduces the oxidized flavor intensity and triples the preservation of walnut flavor intensity. However, when the coatings were subjected to a consumeracceptance test, walnuts coated with MC (Figure 1. A.) scored lowest both for color and overall acceptance on a 9-point hedonic scale (Figure 1. B.).
Figure 1. A. Figure 1. B.
Coating Prepared Using Walnut Flour
Walnut oil is obtained by cold pressing and is used in many countries as a healthy alternative oil. Walnuts contain 12-15% protein content and therefore after pressing, it is possible to obtain a protein-rich walnut oilcake that can be utilized for several purposes.
When walnut oilcake is subjected to a solid-liquid lipid and soluble-carbohydrate extraction process, a walnut flour rich in protein is produced and it is a suitable material for the preparation of an edible coating. The walnut flour-based coating (WFC) provides protection against lipid deterioration. The use of this coating displays a more efficient effect on the preservation of walnut sensory properties like the characteristic walnut flavor (Figure 2. A.), and more efficiently prevents the development of rancid flavors such as oxidized and cardboard, in comparison with WC and WMC. Furthermore, walnuts covered with WFC exhibit similar consumer acceptance when compared to WC and are preferred over walnuts coated with MC. Meanwhile, MC presents similar or lower protection against lipid oxidation. One reason for this is because WFC presents a better preservation of PUFAs, the fatty acids responsible for the health benefits, and gamma-tocopherol, one of the main antioxidants responsible for the walnut’s natural preservation (Figure 2. B). To conclude, both coatings help to prolong shelflife. However, the walnut flour-based edible coating is more advantageous because this natural and protective coating prevents allergen cross-contamination when used on walnuts.
Figure 2. A.
Figure 2. B.
The application of WFC to walnuts by the food industry constitutes a feasible process from an economical and practical point of view. Concerning the economic side, walnut oilcake, a by-product of the walnut oil industry, is the main ingredient used for preparing walnut flour. This residual material has a low cost and could be exploited by the oil industry itself or other industries to obtain this flour which is used to manufacture walnut-flour coatings.
Considering the practical feasibility, the preparation of the walnut flour and WFC requires simple industrial processes and machinery already used by the food industry, such as solid/ liquid extraction systems. With respect to the application of the walnut coating, the recommended procedure is spraying liquid inside a rotating pan, which is very simple and a process already widely used by the industry for many food products.
For all these reasons, I believe that it is possible to transfer this knowledge to a practical application in the food industry, which will contribute to solving several of the main issues around walnut kernels, such as lipid oxidation and rancidity, in a natural and environmentally friendly way.