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Nuts are an important food commodity and an essential part of our diet. Nuts are defined as single-seeded fruits, dry in nature and containing high oil content. Botanically, exclusively a kind of dry fruit with a single seed, a hard shell, with a protective husk. The four nuts that truly fit this definition are chestnuts, hazelnuts, pecans, and walnuts. However, the term "nut" is frequently used in a broader context to include legumes (peanuts), seeds (Brazil nuts, cashews, flax, sunflower seeds, sesame seeds, and pine nuts), and drupes (almonds, pecans, pistachios, and walnuts), which resemble botanical nuts in terms of their structure and composition but strictly speaking are not nuts.
In the United States, almonds are the most popular tree nut, followed by pecans, walnuts, macadamias, and pistachios. Nuts are sold with or without the shell except cashews, which are always sold without the shell because the double shell that protects the kernel contains toxins. In addition to being consumed raw as snacks, nuts are also commonly used as ingredients in a variety of confections, baked goods, and snack foods.
Nuts can be ground to make a paste called "butter". To prevent separation, nut pastes can either contain only nuts or additional seasonings like salt and sugar. They can also contain hydrogenated vegetable oils. Peanut butter is one of the most well-known and recognizable nut pastes. Butters can also be made from almonds, macadamias, cashews, sesame seeds, pistachios and hazelnuts, among others.
Regular consumption of nuts has been linked to a number of health benefits. Nuts are a great source of antioxidants, amino acids, vitamins, and minerals. Tocopherols and essential fatty acids, which are abundant in nuts, help lower the risk of many diseases.
Most microorganisms cannot grow on the surface of nuts due to their low moisture content and water activity. The resistance to pathogens is further increased by the high fat content of nuts. For these reasons, nuts and nut products have long been regarded as low risk foods. However, nuts can be contaminated with pathogenic microorganisms at any point during production, processing, distribution, and consumption. Low water activity foods, such as nuts and nut butters, are increasingly recognized as important sources of foodborne illness outbreaks because foodborne pathogens survive at low levels in nuts for long periods of time.
Salmonella, with its long-term persistence and high heat resistance on dry foods, is considered the target organism for tree nuts and peanuts. Salmonella outbreaks have been linked to sesame seeds, raw almonds, peanut butter, pistachios, and pecans among others. Other bacterial foodborne pathogens associated with nuts are Bacillus cereus, Clostridium botulinum, Clostridium perfringens, Cronobacter, Escherichia coli O157:H7, L. monocytogenes, and Staphylococcus aureus. The prolonged storage time associated with nuts also leads to an increased risk of contamination from pathogenic fungi.
Safe food handling practices should be adopted to prevent contamination in nuts. For more information on safe handling of nuts, please see the link below:
Brar, P.K. and Danyluk, M.D. (2018). Nuts and Grains: Microbiology and Preharvest Contamination Risks. In Preharvest Food Safety (eds S. Thakur and K.E. Kniel). https://doi-org.proxy.lib.ohio-state.edu/10.1128/9781555819644.ch6
Frelka, J.C., Harris, L.J. (2014). Nuts and Nut Pastes. In: Gurtler, J., Doyle, M., Kornacki, J. (eds) The Microbiological Safety of Low Water Activity Foods and Spices. Food Microbiology and Food Safety(). Springer, New York, NY. https://doi.org/10.1007/978-1-4939-2062-4_12
Georgiadou, M., Dimou, A., & Yanniotis, S. (2012). Aflatoxin contamination in pistachio nuts: A farm to storage study. Food Control, 26(2), 580–586. https://doi.org/10.1016/j.foodcont.2012.02.014
Mir, S. A., Shah, M. A., Mir, M. M., Sidiq, T., Sunooj, K. V., Siddiqui, M. W., Marszałek, K., & Mousavi Khaneghah, A. (2022). Recent developments for controlling microbial contamination of nuts. Critical Reviews in Food Science and Nutrition, 1–13. https://doi.org/10.1080/10408398.2022.2038077
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