Drying and Quality Characteristics of Almonds and Walnuts with Different Drying Conditions
California produces over 99% of US commercial almonds and walnuts. Conventionally, almonds are dried on the ground while walnuts are partially dried on ground and then dried in heated air dryers. Both drying methods are essential to increase product shelf-life and safety as well as ensure yearlong availability. However, these conventional drying practices are inefficient due to the slow process and intensive energy use, and on-ground drying often is associated with food safety risk of the finished products. This research investigated the drying and quality characteristics of almonds dried by stepwise high temperature air drying and walnuts dried by sequential infrared and heated air drying (SIRHA) as alternative methods to the conventional drying. The studied quality characteristics include kernel color change, concealed damage, and oil quality analysis. Almond in-hulls and in-shells of Nonpareil (NP) and Wood Colony (WC) varieties were dried with air velocity of 1.0 m/s (air flow rate of 1.75 m3/s per m3 of fresh almonds) under two approaches: at a constant temperature of 40°C, 50°C, and 60°C and at a stepwise high temperature with the first stage at 80°C and 90°C, a holding time of 1, 2, and 3 hours, and the second stage at 60°C. The almond samples were dried until the kernels reached a final wet basis moisture content of 6%. The total processing times of almonds dried by heated air at 40, 50, and 60°C ranged from 7 to 16 hours while the total processing time (including holding) of almonds dried by stepwise high temperature ranged from 7 to 10.5 hours, both of which were significantly less than the drying time of 36 hours for almonds dried at ambient temperature (p < 0.05). Under the same drying condition, almond in-shells required significantly less drying time than almond in-hulls. Overall, the drying curves for almonds were best represented by the Page model. To evaluate the almond kernel quality, the whiteness index (WI) of almond flesh was measured with a colorimeter, and the concealed damage after roasting at 135°C for 90 minutes was evaluated with the color development score (CDS). To determine the oil quality, the peroxide value (PV) and acid value (AV) of extracted almond oils were measured by titration methods. In general, the different drying conditions did not cause a significant color change in the almond kernels. The WI values of all dried almond samples were relatively similar at 77.1±2.3 for NP and 79.1±2.1 for WC. After roasting, the concealed damage scores appeared to be consistent regardless of the drying conditions and the variety, averaging at the CDS of 2.8±0.6 in which indicated no concealed damage. In terms of oil quality, both PV and AV of all almond samples increased along with the rising of drying temperature. However, the PVs of both varieties at different drying conditions were not significantly different, averaging at 0.585±0.100 meq/kg. Meanwhile, the AVs of NP samples were significantly lower than the WC samples at p < 0.05. Overall, the oil quality of almonds dried under all of the drying methods was far below the industry standards (PV < 5 meq/kg and AV < 3.0 mg KOH/g). In shell walnuts with low initial wet basis moisture content of 10.06±0.75 % (LMC) and high initial wet basis moisture content of 27.67±2.74 % (HMC) were acquired for this study. Both kinds of walnuts were cleaned by dipping and soaking in clean water for 2 minutes to mimic the industrial washing procedure. Then, the half of the walnut samples were infrared pre-dried for 25 seconds for LMC walnuts and 60 seconds for HMC walnuts, based on the time for the internal temperature of the walnuts to reach about 43°C. After the IR pretreatment, the walnuts were conditioned into in-shell, cracked shell, and kernel using a nut cracker. Then, the samples were dried by heated air at 45, 50 or 55 degrees Celsius with air velocity of 1.0 m/s to an average final wet basis moisture content of 7% for walnuts dried in in-shell and cracked-shell conditions and 3% for walnuts dried in kernel condition. The drying time of LMC walnuts ranged from 0.5 to 2 hours while the drying time of HMC walnuts ranged from 5 to 16 hours. Overall, walnut samples dried the fastest under heated air temperature of 55°C, followed by 50°C and 45°C, consecutively. When compared to in-shell walnuts, walnuts dried in cracked-shell and kernel only conditions could reduce the drying time by up to 28% and 54%, respectively. An accelerated shelf-life study was conducted by storing dried walnut samples in an incubator set at 35oC with relative humidity of 53% for 20 days, representing 2 years of storage at 4oC. Samples were collected after 0, 5, 10, 15, and 20 days of storage for quality analysis. To evaluate the product quality, the lightness value (L*) of the kernels was measured with a colorimeter, and the PV and AV of extracted walnut oils were determined by titration methods. Under all of the tested conditions, the L* values decreased over the storage time. The application of IR pretreatment and the different drying temperatures did not significantly alter the lightness values. However, the L* values of the kernels from dried in-shell walnuts were significantly different from that of direct kernel dryings at p <0.05. In terms of oil quality, the PVs were generally increased over the storage time, but the AVs were relatively stable over the storage time at 0.147±0.125 mg KOH/g. The application of IR pretreatment on LMC walnuts resulted in lower PVs, possibly due to the deactivation of catalytic pro-oxidant in the walnuts due to the high temperature heat treatment. However, the application of IR pretreatment on HMC walnuts significantly increased the AVs, indicating high lipid hydrolysis activities. Overall, the product qualities over storage time were within the industry standards (L* > 40, PV < 1.0 meq/kg oil, AV < 1.2 mg KOH/g). Based on the experimental data and analytical observation, almonds dried by stepwise high temperature air drying and walnuts dried by sequential infrared and heated air drying can be applied as alternative methods to the conventional drying practices without causing any quality concern. Also, this study showed that heated air drying can be used for walnuts in cracked-shell and kernel only conditions without affecting the quality and shelf-life of the dried products.