Graduate Thesis Or Dissertation
 

Investigation of Hot-Air (HA) Drying Conditions and Shell Cracking Mechanism and Development of Radio-Frequency (RF) Dielectric Heating for Ensuring Quality and Reducing Shell Cracking of Oregon Hazelnuts (Corylus avellana L.)

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/qv33s3674

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  • Oregon produces 99% US hazelnuts (Corylus avellana L.), occupying 3% world production. To ensure the quality and food safety of hazelnuts (inshells or kernels), industry and federal standards require freshly harvested nuts to be cleaned and dried to reach a final moisture content (MC) of inshells < 10%, MC of kernels < 6% and water activity (aw) of kernels <0.70 (at 25 oC). Since fresh nuts have MC from 14 to 30%, drying is thus essential to reduce moisture content and water activity, as well as preserving nut quality. Hot-air drying at a temperature of 38-49 oC is standard for commercial hazelnut drying facilities in Oregon. However, this drying method can take days to complete. Considering the high fat content (60%), especially high mono-unsaturated fatty acids, and bioactive compounds (vitamin E and phenols) of hazelnut kernels, such drying conditions can induce severe autoxidation of kernels, resulting in rancidity and browning discoloration of kernels. In addition, some cultivars are highly susceptible to shell cracking during drying, especially when the nuts have high moisture content, which can accelerate quality deterioration of nuts during storage. Therefore, it is critical to develop drying methods that are not only drying fast, but also retain nut quality with lower shell cracking. Three specific studies were conducted in this dissertation research. The first study evaluated the chemical composition and bioactive compounds in 3 Oregon hazelnut cultivars (Barcelona, Yamhill, and Jefferson) and studied the effect of temperature and relative humidity (RH) during hot-air drying on the quality of nuts. It was found that chemical compositions and bioactive compounds varied among the cultivars, in which Barcelona contained the highest protein, Yamhill had the highest lipid, phenolic content, and vitamin E, and Jefferson had the highest carbohydrate. Overall, maintaining low RH (40%) with a moderate drying temperature (43 or 49 °C) might be the ideal drying conditions for retaining nut quality with improved drying efficiency and reduced enzyme activity and lipid oxidation for Oregon hazelnuts. The second study investigated the mechanism of shell cracking during hot-air drying (HA) and developed strategies to reduce cracking by correlating the cracking with physiochemical, structural, morphological, and desorption isotherm properties of nuts in 4 Oregon hazelnut cultivars (Barcelona, Yamhill, Jefferson and Wepster). It was found that Jefferson nuts had the highest cracking ratio due to lower shell density, larger air gap between shell and kernel, more hygroscopic shells with hydrophilic and high lignin contents, and thick, strongly bound inner fibers attached to the shell. Using a systematic experimental design for Jefferson nuts, RH was identified as the most important factor influencing the shell cracking ratio and drying time in comparison with temperature and air velocity. Use of a gradient drying in which increasing RH or decreasing temperature at a critical MC of inshells (∼16%) effectively reduced cracking ratio to <30% within a drying time of 15 h to reach target MC of inshells (∼10%). The third study assessed the use of a novel heating technology, ‘hot-air assisted radio-frequency (HARF)’, for drying hazelnuts. Radio-frequency (RF) offers a unique heating mechanism, in which under electromagnetic waves of 10-300 MHz, heat is generated volumetrically in the materials with the polar dielectric molecular (mainly water) friction from the continuously changing of the field direction. With the assistance of HA, the RF operation is expected to reach a rapid heating in core of the material with improved heating uniformity. An intermittent mode of HARF drying process (pausing the RF heating when core temperature of nuts reached 49 oC with the HA circulation) was studied for drying two nut cultivars (Barcelona and Jefferson). It was found that 15 cm electrode gap and 5 cm sample thickness for Barcelona, and 15 cm electrode gap and 7 cm sample thickness for Jefferson, respectively, are the ideal conditions to obtain high drying efficiency, heating uniformity, nut quality, and lower shell cracking. Furthermore, a 2-step HA-HARF drying using 12% MC as intermediate MC for changing the drying method from HA to HARF was discovered to further reduce shell cracking to <15% along with improved heating uniformity for Jefferson nuts. In addition, the continuous HARF mode (continuous RF heating until reaching core temperature equilibrium) resulted in a faster drying rate. Therefore, the continuous HARF and 2-step HA-HARF drying could produce nuts with better kernel quality, while the longer HA pre-drying in the 2-step HA-HARF drying might lead to undesirable lipid oxidation. Hence, different drying conditions should be considered depending on the main purposes of drying, i.e., fast drying rate, low shell cracking, or better nut quality based on the industry needs. More studies are necessary for improving heating uniformity and reducing shell cracking, as well as inactivating concerned microorganisms of hazelnuts using HARF drying. The potential strategies may involve applying different hot-air temperatures, adjustment of electrode gap distance, and intermittent stirring of nuts during pauses in RF heating. Comparison of the storability of hazelnuts from different drying methods (HA, HARF and 2-step HA –HARF) should be investigated in future research. Furthermore, the required HARF conditions for inactivation of microorganisms for improved food safety of hazelnuts should be further studied.
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  • Pending Publication
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  • 2019-12-12 to 2021-01-17

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