Changes in the Drying Characteristics and Water Activity Values of Selected Pistachio Cultivars During Hot Air Drying

Abstract

The drying characteristics and changing of water activity (Aw) values of Kirmizi and Siirt pistachio cultivars were determined experimentally as a function of drying air temperature and initial moisture content during hot air drying. The drying experiments were conducted using two levels of initial moisture content (32 and 38%) and three drying air temperatures (40, 50 and 60C) at 1.5 m/s drying air velocity at 12% air relative humidity. The drying curves obtained from the experimental data were then fitted to the 10 different semitheoretical and/or empirical thin‐layer drying models. The moisture ratios at any drying time were compared by the Newton, Page, Henderson and Pabis, Logarithmic, Two‐term, Wang and Singh, Diffusion approximation, Modified Henderson and Pabis, Verma et al. and Midilli et al. models. The effect of drying air temperature and initial moisture content on the coefficients of the best‐suited moisture ratio model were determined by multiple regression method. Root mean square error and reduced chi‐square were used for the determination of the best suitable model. In addition to these statistical parameters, the modeling efficiency was also determined. The Modified Henderson and Pabis model was found to be the most suitable for describing the drying behavior of both pistachio cultivars. As a criterion of shelf life, Aw changes of dried pistachio samples were investigated. PRACTICAL APPLICATIONS Proper harvesting and postharvest handling are basic parts to achieve maximum yield of good quality nuts that determine marketability and profit. Processing right after harvesting is very important to pistachio quality. Drying is an important operation in pistachio processing. During the drying process, nuts can undergo reactions that cause loss of nutritional value and enzymatic activity. It is therefore necessary to design and simulate accurately the drying systems for pistachio nuts. For this reason, knowledge of such nut properties as thin‐layer drying characteristics under different drying conditions is required. Also changing of water activity values is also important for different drying and storage conditions in practical applications because the most effective preventative control is to dry pistachio nuts to a water activity of 0.82 for short‐term or 0.70 for long‐term storage to prevent mold growth and aflatoxin contamination.

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