The sterilization process in non-standard nut processing requires a precise balance between sterilization effectiveness and flavor preservation. The core of this lies in selecting appropriate sterilization technologies based on raw material characteristics, process requirements, and product positioning, while simultaneously minimizing damage to flavor compounds through process optimization and auxiliary methods. Non-standard nut processing often involves irregularly shaped, dimensionally diverse, or fragile raw materials, such as shelled nuts and irregularly shaped nut pieces. These materials have complex surface structures, allowing microorganisms to easily hide in crevices, making it difficult for traditional sterilization methods to completely cover them. Over-sterilization, on the other hand, can destroy unsaturated fatty acids, volatile aroma components, and natural antioxidants in nuts, leading to oil oxidation and rancidity or flavor loss.
High-temperature sterilization is the most widely used sterilization method in non-standard nut processing, but strict control of temperature and time parameters is necessary. For example, roasting denatures microbial proteins through high temperatures and promotes Maillard reactions to generate caramel flavor compounds. However, if the temperature is too high or the time is too long, it can cause the nut surface to char and excessive internal moisture loss, resulting in a hard texture and the volatilization of aroma components. To balance sterilization and flavor, a segmented roasting method can be used: initial low-temperature slow roasting removes excess moisture, preventing a sudden increase in internal steam pressure that could cause cracking; subsequent high-temperature short-time sterilization quickly kills microorganisms while minimizing heat damage. Additionally, spraying a small amount of honey or syrup onto the nut surface utilizes the hygroscopic nature of sugar to prolong moisture evaporation time, improving sterilization uniformity and forming a protective layer to reduce aroma loss.
Irradiation sterilization uses gamma rays or electron beams to penetrate the interior of the nut, directly destroying the microbial DNA structure. It has the advantages of strong penetration and thorough sterilization, especially suitable for shelled nuts or irregularly shaped raw materials. However, irradiation may trigger lipid oxidation in nuts, producing an "irradiated flavor." To mitigate this problem, nuts can be vacuum-packed before irradiation to isolate oxygen and inhibit oxidation; or nitrogen-purged packaging can be used after irradiation to further delay flavor deterioration. Some companies also add natural antioxidants, such as vitamin E or rosemary extract, after irradiation to reduce the impact of irradiation on flavor by scavenging free radicals.
Microwave sterilization utilizes high-frequency electromagnetic waves to vibrate water molecules inside nuts, generating heat and achieving simultaneous heating inside and out. It features rapid heating and high sterilization efficiency. However, microwave heating can easily lead to localized overheating, causing the edges of the nuts to char or the center to remain uncooked. To address this issue, microwave sterilization can be combined with a vacuum environment. In a vacuum, the boiling point of water is lowered, allowing the nuts to achieve sterilization at a lower temperature, thus reducing heat damage. Simultaneously, by adjusting the microwave power and pulse frequency, heat distribution can be controlled to prevent excessively high local temperatures, ensuring both effective sterilization and preserving the nuts' crisp texture and original flavor.
For non-standard nuts with high levels of heat-sensitive flavor compounds, such as flavored nuts or nut pieces, low-temperature plasma sterilization technology can be used. This technology uses ionized gas to generate active particles such as reactive oxygen species and nitrogen species, which disrupt microbial cell membranes and DNA, achieving rapid sterilization at room temperature. Low-temperature plasma has minimal impact on the physical structure and chemical composition of nuts, maximizing the preservation of their color, aroma, and nutrients. However, this technology has higher equipment costs and is currently mostly used in the processing of high-end nut products.
In non-standard nut processing, the sterilization process requires a focus on synergistic effects. For example, soaking in ozone water during the washing stage can remove surface microorganisms and oxidize and decompose some pesticide residues; combining hot air and infrared drying in the drying stage shortens drying time and reduces flavor loss; and modified atmosphere packaging in the packaging stage, by adjusting the oxygen-carbon dioxide ratio, inhibits microbial growth and delays flavor deterioration. Through multi-stage synergistic control, flavor and quality can be optimized while ensuring food safety.
