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Dehydrated onions are commonly dried with convection heating, which is inefficient and costly. This study compared the drying and quality characteristics of onion dried with catalytic infrared (CIR) heating and forced air convection (FAC) heating. Sliced high‐solids onions were dehydrated under 9 conditions: CIR heating with and without air recirculation, and FAC each operated at 60, 70, and 80 °C. In general, CIR both with and without air recirculation had higher maximum drying rates, shorter drying times, and greater drying constants than FAC at moisture contents greater than 50% (d.b.). Dried onion quality, measured as pungency degradation, was similar for both the drying methods at 60 and 70 °C. The color analysis showed better product color (whiter and less yellow) at lower temperatures for CIR and higher temperatures for FAC. The browning could have been caused by the higher surface heat flux of the CIR heating and longer process times of FAC drying. Aerobic plate counts and coliform counts were not significantly different for either product from the CIR or FAC drying. However, samples dried by the CIR had significantly lower yeast and mold counts than those dried by the FAC. It is recommended that CIR be used in the early stages of onion drying.

Timing of field draining and harvesting of rice with meteorological conditions can allow growers to foster conditions for high head rice yield (HRY). The effects of timing of draining and harvesting on rice sensory and physicochemical properties are not well understood. The objective of this study was to determine the effects of varying drain and harvest dates on the sensory and physicochemical properties of M‐202 grown in California under controlled field conditions. Drain date had a significant (P < 0.05), but very small, effect on amylose and protein contents, with amylose being highest at the late drain date and protein being the lowest at the early drain date. Breakdown and setback were lowest for early and normal drain dates, respectively; however, no significant (P > 0.05) differences in texture were measured as a result of these parameters being low. Drain date did not affect the volatile composition or flavor of the rice. Harvest date had no effect (P > 0.05) on amylose content and a significant (P < 0.05), but very small, effect on protein content. Harvesting at the earliest date (9/30) resulted in rice with higher setback and lower breakdown than at the last date (10/16) and, subsequently, the early harvested rice, when cooked, was harder, more cohesive, and absorbed less saliva in the mouth. However, the differences in texture measured by the panelists were very small and would possibly not be noticed by untrained palates. The lowest levels of the lipid oxidation products 1‐pentanol, hexanal, and nonanal occurred in rice with the lowest harvest moisture content (HMC): rice harvested on 10/13 and 10/16. Differences in levels of lipid oxidation products and branched chain hydrocarbons did not lead to significant (P > 0.05) differences in flavor. In summary, M‐202 demonstrated stable composition, physicochemical properties, flavor, and texture across drain and harvest dates.

ABSTRACT. California's medium-grain rice industry experiences a wide range of head rice yield (HRY).he unit value of rice is based primarily on its head rice yield (HRY, the proportion of kernels greater than 75% of intact length; USDA-FGIS, 1994). Improving HRY is an ongoing goal for rice growers. The average moisture content of the paddy rice at harvest (HMC, expressed on a wet weight basis) influences HRY. For medium-grain rice grown in Italy, dry conditions allowed HMC to drop below about 15%, and a subsequent rain caused a significant drop in HRY (Finassi et al., 2002). However, when repeated rain events kept HMC above 20%, HRY was not influenced by HMC. In Louisiana, where rain events are common during rice maturation, long and medium grain rice experienced a significant reduction in HRY when rice dropped below 15.0% to 19.8% HMC depending on variety and year of harvest (Jodari and Lindscombe, 1996). A warmer and drier harvest season caused fissuring to begin at higher HMC than a cooler and more humid season. In two harvest seasons in Arkansas with numerous rain events, HRY was not affected by harvest moistures between 15% and 22% (Siebenmorgen et al., 1992).In California's rice production area, rain is rare and maximum HRY for medium-grain rice is obtained at high HMC. Kester et al. (1963) concluded that the highest HRY is obtained at 25% to 32% HMC. Morse et al. (1967) indicated that HRY peaks between 26% and 30% HMC. Geng et al. (1984) analyzed commercial data, and they concluded that high variability prevented them from finding a narrow range of HMC for high HRY and that HRY was maximum between 25% ±5% HMC. Commercial quality data for California medium-grain rice (D. Jones, Farmers Rice Cooperative, Sacramento, Cal., personal communication, 1999) clearly showed that HMC explains only a small portion of the variability in HRY, as described by the low regression coefficient for a second-order polynomial regression line ( fig. 1). While maximum HRY was obtained at HMCs above 20%, lots below 18% HMC had nearly the same quality, and lots above 22% moisture had HRY values of less than 50%. The data show that in commercial practice HMC was not a good predictor of HRY, and apparently there must be other variables influencing rice quality.A great deal of research beginning the 1930s showed that paddy rice kernels fissure when dried below a critical moisture content and then rehydrate because of exposure to free moisture or high humidity, (Kunze, 1993;Siebenmorgen et al., 1998;Lan et al., 1999). The fissured kernels break during milling, causing low HRY in lots with high amounts of fissured kernels. At harvest, individual kernels vary widely in moisture (Siebenmorgen et al., 1992), and if the pattern of moisture distribution varies because of varying cultural practices or weather conditions, the proportion of kernels below the critical moisture may be a better indicator of HRY than average moisture. Geng et al. (1984) indicated that cycles of drying and moisture absorption may influence quality, but they did no...

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Between harvest and the start of drying, paddy rice may be held for more than 24 hr at moisture contents ranging from 16% to >26%. Microbes found on the freshly harvested rice grow under these conditions and produce a wide variety of volatile compounds that impact the flavor/aroma of the white rice obtained after drying and milling of the paddy rice. The contents of 10 volatile microbial metabolites were compared in white rice obtained from paddy rice harvested at differing moisture contents and immediately dried (0 hr) or held for 48 hr before drying. No increases in volatile microbial metabolite levels were observed in white rice obtained from paddy rice that was stored at 17–21% moisture contents for 48 hr. In white rice from paddy rice stored at ≥24% moisture content, 3‐methyl‐butanol, 2‐methyl‐butanol, acetic acid, 2,3‐ butandiol, and ethyl hexadecanoate increased markedly with time. Also in these samples, as determined by a descriptive panel, sour/silage and alfalfa/grassy/green bean flavors significantly increased (P < 0.1) in intensity. Sour/silage, the predominant off‐flavor note in the stored samples, correlated highly (r = 0.98) with 2,3‐butandiol. Ethanol concentration measurements on the paddy rice correlated highly with sour/silage (r = 0.99) and 2,3‐butandiol (r = 0.97), and correlated well with several other volatile microbial metabolites. Carbon dioxide measurements taken on the paddy rice did not correlate as highly (r =.78) with sour/silage. Measurements of ethanol produced in paddy rice may serve as an indicator of off‐flavor/aroma development in the resultant white rice.

The objective of this study was to investigate the moisture removal characteristics of thin layer rough rice heated by infrared (IR) and cooled with various cooling methods. Thin layer rough rice samples with different initial moisture contents (MCs) were heated using a catalytic IR emitter for four exposure times and radiation intensities. High heating rate and moisture removal were achieved during the IR heating period. After heating, more moisture removal was achieved during the cooling period. The achieved grain temperatures ranged from 35.1 to 68.4C under the tested heating conditions. The vacuum and forced air cooling methods removed more moisture than did the natural cooling. When rice with 25.7% MC was heated by IR, MC was reduced by 3.2, 3.5, and 3.8 percentage points for rice heated to 63.5C at the IR intensity of 5348 W/m2 for120 s followed by natural cooling for 40 min, forced air cooling for 5 min and vacuum cooling for 10 min, respectively. Practical Applications To design efficient infrared (IR) dryers for rough rice, it is important to optimize the operating parameters of IR dryer to achieve high heating rate, fast drying and good quality of end‐products. To achieve the aforementioned objectives, we have been conducting several studies including our previous publications (Pan, Khir et al. and this study). The outcomes of our studies have clearly indicated that a high heating rate, fast drying, good quality and simultaneous drying and disinfestation can be achieved by IR heating of rough rice to bout 60C followed by tempering and natural cooling with tested bed thickness up to 10 mm. Consequently, IR heating followed by cooling could be an effective approach for designing IR rough rice dryers. It is expected that this alternative approach could be used as an energy saving drying method with improved drying efficiency, space saving, clean working environment and superior product quality compared with the conventional heated air drying method.