Incorporating glass transition concepts to explain rice milling-quality reductions during the drying process

Previous research has indicated that while drying rough rice using air temperatures above the glass transition temperature (Tg), head rice yield (HRY) reductions are incurred if a state transition occurs when severe intra-kernel moisture content (MC) gradients are present. State transitions can occur by extended drying using high-temperature air or by cooling kernels below Tg before sufficient tempering has occurred. The objectives of this experiment were to determine the maximum MC removal per initial drying pass and the associated tempering durations required to prevent HRY reduction. Two long-grain cultivars, ‘Francis’ and ‘Wells’, at two harvest moisture contents (HMC) were used. Samples were dried with air conditions of either 60°C/17% RH or 50°C/28% RH for various durations to create a range of intra-kernel MC gradients and were subsequently tempered in sealed bags for durations ranging from 0 to 160 min. After tempering, samples were cooled to cause a state transition, and then slowly dried to 12.2% MC. Samples were then milled to determine HRY. Control samples were dried at 21°C/60% RH. Results showed that the amount of moisture that could be removed in the initial drying pass was directly related to the HMC and the drying air condition. The tempering duration required to prevent HRY reductions increased with the amount of MC removed from the kernel in a drying pass. The HRY reduction patterns concur with a hypothesis that explains fissure formation during the drying process based on the Tg of rice kernels

Drying of post-harvest rough rice with silica gel: A preliminary investigation

Rice drying operations can encounter problems of over drying and losses in head rice yield (HRY) through the formation of fissures. Typical rice drying methods also utilize large volumes of expensive fossil fuels to dry the kernels. Drying of rice with a solid desiccant such as silica gel has several potential advantages that avoid some of these problems. Two cultivars of long-grain rough rice, ‘Cheniere’ and ‘Wells’ with harvest moisture contents of 17.8% and 22.0%, respectively, were dried over a 48-h period with various ratios of rough rice-to-silica gel. It was found that an intimate mixture of 3:1 rough rice to silica gel was sufficient to dry these rice lots to 12.5% and 14.3% within 12 h, respectively. Head rice yields of desiccant-dried rice showed no considerable differences from the control. Rough-rice drying curves for all rough rice-to-silica gel mixtures followed exponential relationships

A comparison of recently introduced instruments for measuring rice flour viscosity

The Rapid Visco-Analyser (RVA) and the Micro Visco-Amylograph (MVA) were compared in measuring the viscosity properties of rice flours. A total of 72 rice samples were procured from three cultivars harvested at two locations and three moisture contents and separated into thin, medium, and thick kernel-thickness fractions. A fast and a slow heating rate was used in the procedure for both instruments. Cultivar, kernel thickness, and harvest location affected rice viscosity. The RVA viscosity profiles using a fast heating rate were best correlated with those of the MVA using a slow heating rate. The RVA slow heating rate resulted in lower final viscosities than those using the MVA because of the spindle structure of the RVA. For both the RVA and the MVA, greater rice flour peak viscosities and less trough and final viscosities were obtained with a slow rather than a fast heating rat

Correlating fissure occurrence to rice quality for various drying and tempering treatments

When a rice kernel fissures, it can break in subsequent food processing operations and lose its commercial value. Head rice yield (HRY) is a measure of the percent of kernels that remain whole (at least three-fourths of original length) after rice has been milled. Our experiment was designed to test the effect of a rapid state transition during drying and tempering processes using cultivars Bengal and Cypress. ‘Bengal’ is a medium-size kernel and ‘Cypress’ is a longsize, thinner grained cultivar. Immediately after drying, the rice samples were separated into four sub-samples and tempered for 0, 80, 160, or 240 minutes at the temperature of the drying air. Tempering is a process to allow kernel moisture content gradients to decrease, thereby reducing the stress within the kernel. From each sample, 400 kernels were randomly selected, visually observed, and the percentage of fissured kernels determined. Results showed that the percentage of fissured kernels generally decreased with tempering. However, some samples still showed many fissures even after extended tempering, yet had a high HRY. While HRY is currently the primary index of rice quality, it is known that fissured kernels can severely and detrimentally affect end-use processing operations such as cooking or puffing. Thus, the tempering duration required for preventing kernel fissuring might be longer than the tempering duration required for maintaining a high HRY

Fissure Characterization of Rice Kernels Using Video Microscopy

Fissures are fractures of a rice kernel that can be created during the drying and tempering process. They cause tremendous postharvest losses in milling yield. Understanding why and how rice kernels fissure will lead to optimal drying and tempering operations. This information could also provide input to plant breeders for producing rice cultivars that are more resistant to fissuring. Rice kernels were dried using various air conditions in a controlled environment chamber. The kernels were viewed by video microscopy to observe the occurrence of fissures. A videocassette recorder recorded the images for a 24-hour period after the drying process. The tapes were reviewed to reveal characteristics of the fissures. The rice cultivars used in this experiment were ‘Bengal’, ‘Cypress’, and ‘Drew’. The tests showed that Cypress kernels were more resistant to fissuring than were the other two cultivars. The recorded images from the microscopy chamber showed that fissures begin from the inside of the kernel. Also, fissures were observed to form almost instantaneously

Energy use and efficiency of rice-drying systems II. Commercial, cross-flow dryer measurements

Energy use and efficiency of a commercial, cross-flow dryer were measured when drying rough rice across a range of ambient conditions and drying air temperatures. Four tests were conducted during the 2011 harvest season using rice with initial moisture contents ranging from 19.0% to 21.7% wet basis and three tests were conducted during the 2012 harvest using rice with initial moisture contents from 15.4% to18.3%. To obtain thermal energy requirements in terms of energy per unit mass water removed, the energy consumed by the burner was divided by the total amount of water removed. In addition, electrical energy requirements were determined by multiplying the average power draw of the fan motor by the fan operating duration. Thermal energy efficiency was calculated by dividing theoretical energy requirements by the measured thermal energy requirements. Thermal energy requirements to dry rice ranged from 6,900 to 9,670 kJ/kg water removed in 2011 and from 8,810 to 9,620 in 2012. Electrical energy use, which ranged from 300 to 400 kJ/kg water removed in 2011 and from 410 to 630 in 2012, accounted for ~4% to 5% of the total energy used to dry rice. Thermal energy requirements were linearly correlated to the difference between drying air temperature and ambient temperature and linearly and inversely correlated to the amount of water removed per mass dry matter. Thermal energy efficiency ranged from 26% to 36% in 2011 and from 27% to 29% in 2012

Estimating the theoretical energy required to dry rice

The total heat of desorption of rice (Qt) was determined for several rice types as a function of moisture content (MC), and kernel temperature, using a semi-theoretical approach in which desorption isotherms were used in conjunction with the Clausius–Clapeyron equation. Qt decreased exponentially as MC increased, decreasing sharply for MCs above 15% and approaching the latent heat of vaporization of free water at MCs around 20%. Qt of parboiled rice at 12.5% MC was significantly less than that of nonparboiled lots. Qt of medium-grain ‘‘Jupiter’’ was significantly greater than that of long-grains at 12.5% MC. Equations that predict the energy required to dry a unit mass of rice from an initial MC to a final MC were derived

Using thermogravimetric analysis to measure the drying rates of individual rice kernels

Individual kernel drying rates could be used to better estimate intra-kernel MC and material state gradients, which in turn would help estimate the occurrence of kernel fissuring inside rice dryers. Drying curves of individual kernels were obtained using a thermogravimetric analyzer for three long-grain cultivars (Wells, CL XL745, and CL 142) and two medium-grain cultivars (CL 261 and Jupiter) at 40°C, 50°C, and 60°C. Drying rates of individual kernels were obtained from drying curves using a three-parameter, exponential decay model. Prior to drying experiments, kernel thickness, width and length of individual rough rice kernels were measured using a digital micrometer. Total lipid content, amylose content, protein content and initial moisture content were determined for each cultivar. In general, drying rate increased as kernel thickness decreased for both long-grain and medium-grain cultivars; this effect was more pronounced at 60°C. Kernel width was also a factor that affected drying rates; as kernel width increased, drying rate increased

Energy use and efficiency of rice-drying systems I. On farm cross flow dryer measurements

Energy use and efficiency of an on-farm, cross-flow dryer were measured by performing five tests during the harvest season of 2011 and three tests during the harvest season of 2012. Thermal energy requirements were expressed in terms of energy per unit mass water removed, by dividing the energy requirements of the burner by the total mass of water removed for each drying run. Energy efficiency was calculated as the ratio of theoretical energy requirements to the measured energy requirements. In 2011, energy requirements to dry rice ranged from 2,840 to 5,310 kJ/kg water removed, with harvest moisture contents ranging from 16.6% to 21.7%, and in 2012 from 3,730 to 5,840 kJ/kg water removed, with harvest moisture contents ranging from 17.4% to 18.2%. Thermal energy efficiencies ranged from 47% to 90% in 2011 and from 44% to 69% in 2012. The difference between drying air temperature inside the dryer and ambient air temperature as well as the amount of water removed, expressed on a per unit mass of rice dry matter, significantly impacted energy use. Equations were developed to predict energy use and efficiency as a function of these two parameters