Postharvest processing of tree nuts: Current status and future prospects—A comprehensive review

Abstract

Tree nuts are important economic crops and are consumed as healthy snacks worldwide. In recent years, the increasing needs for more efficient and effective postharvest processing technologies have been driven by the growing production, higher quality standards, stricter food safety requirements, development of new harvesting methods, and demand to achieve energy saving and carbon neutralization. Among all, the technologies related to drying, disinfection, and disinfestation and downstream processes, such as blanching, kernel peeling, and roasting, are the most important processes influencing the quality and safety of the products. These processes make up the largest contribution to the energy consumptions and environmental impacts stemming from tree nut production. Although many studies have been conducted to improve the processing efficiency and sustainability, and preserve the product quality and safety, information from these studies is fragmented and a centralized review highlighting the important technology advancements of postharvest processing of tree nuts would benefit the industry. In this comprehensive review, almonds, walnuts, and pistachios are selected as the representative crops of tree nuts. Current statuses, recent advances, and ongoing challenges in the scientific research as well as in the industrial processing practices of these tree nuts are summarized. Some new perspectives and applications of tree nut processing waste and by‐products (such as the hulls and shells) are also discussed. In addition, future trends and research needs are highlighted. The material presented here will help both stakeholders and scientists to better understand postharvest tree nut processing and provide technological recommendations to improve the efficiency and sustainability, product quality and safety, and competitiveness of the industry.

Understanding chemical pathways of brown centre formation in laboratory induced and conventionally dried nut-in-shell macadamia kernels

“…Brown centre formation in macadamia kernels stored as nut-in-shell can be induced in the laboratory when they are maintained at high moisture of approximately 25 % concentration for five days or more at temperature above 30 °C [ 13 ]. High moisture concentration accelerates the breakdown of sucrose into reducing sugars that are needed for the Maillard reaction [ 9 , 12 , 21 ]. However, the chemical differences associated with browning in raw macadamia kernels, both in nut-in-shell samples kept at high moisture concentration and those produced from standard postharvest handling practices, have not previously been explored.…”

Drying Kinetics and Mass Transfer Characteristics of Walnut under Hot Air Drying

“…Walnut has an essential economic and medicinal value due to its high content of unsaturated fatty acids, proteins and amino acids and health benefits [1,2]. Due to the high initial moisture content (IMC) in fresh walnuts, drying is important for the primary processing of walnuts to remove water, avoid microbial spoilage, maintain quality, reduce transportation costs, and increase consumption diversity [3,4]. Over recent decades, drying technology has been increasingly used in the walnut industry.…”

Presence of Testa and Shell Maintains Oil Stability in Almond and Canarium Nuts

“…Oil rancidity leads to volatile compounds and off-flavours [7,8] which are undesirable for consumption. Poor post-harvest storage practices such as elevated humidity and temperature are two of the main factors initiating oil oxidation [2,[9][10][11]. Post-harvest storage practices that alleviate or slow down oil oxidation under ambient conditions are needed to prolong the shelf life of nuts.…”

“…For example, when kernels are exposed to high (120 • C) roasting temperatures for up to 20 min, testa remaining on a kernel can reduce oil oxidation compared with kernels that have the testa removed [24]. The naturally occurring kernel covers such as shell or testa are protective outer layers of kernels in tree nuts especially after proper post-harvest drying methods are practiced [9,25]. However, only few studies have examined how storage in shell or in testa affects oil stability.…”