Methods for stabilizing rice bran

by Susan Reidy
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Traditional heating methods such as hot air, drum drying and dry extrusion subject rice bran to severe and non-uniform heating. This damaged valuable components and results in high energy consumption.


Researchers in the Healthy Processed Foods Unit of USDA-ARS and Department of Biological and Agricultural Engineering at the University of California, Davis, looked to develop alternative techniques for stabilizing rice bran. They looked at infrared radiation (IR), ultraviolet (UV) and pulsed electric field (PEF).

As part of the study, researchers looked at the effects of IR heating and tempering treatments on moisture loss, milling quality and enzyme inactivation of rough rice with different initial moisture contents. They also looked at the impact of UV and PEF treatments on enzyme inactivation of brown rice and rice bran. Finally, they provided recommendations for conducting the most effective stabilization approach.

The study results showed that high drying rates, good milling quality and effective lipase inactivation of rough rice is possible with a short IR heating time followed by tempering treatment for at least three hours.

The project leader was Zhongli Pan, Processed Food Research Unit, and Pan and Griffiths Atungulu were the lead investigators for the study.

IR heating, tempering

Medium grain rice at three moisture contents (32.51%, 25.54% and 20.07%) were heated using an IR device developed in the Food Processing Laboratory at UC Davis. The samples were heated for one and two drying passes under radiation intensity of 5,000 W/m2. They were heated for 55 seconds to reach a surface temperature of 60 degrees C during each drying pass.

The mass loss during IR heating and the initial moisture content were used to calculate the moisture loss during the heating period. The moisture loss was calculated as the difference between the initial moisture content and the moisture content after IR heating and is reported as percentage points.

After IR heating, the tempering treatment was conducted by keeping rice samples in closed containers placed in an incubator set at temperature of 60 degrees C for various durations (1, 2, 3, 4 and 5 hours). After the tempering treatment, samples were allowed to cool naturally to the room conditions.

Moisture loss. After one drying pass, the moisture loss during IR heating only was 1.7%, 2.2% and 2.9% for samples with initial moisture contents of 20.06%, 25.53% and 32.50%, respectively. After two drying passes, the moisture loss during IR heating was 5.8% for samples with initial moisture content of 32.5%.

After one drying pass, the total moisture loss during IR heating, tempering and cooling treatments, were 3.3%, 3.8% and 5.6% for samples with initial moisture contents of 20.06%, 25.53% and 32.5%, respectively. The total moisture loss during two drying passes was 12.2% for samples with initial moisture content of 32.5%.

The results show that high drying rate can be achieved using IR heating for a short time. Tempering treatment significantly improved the moisture removal during cooling. It reduced the moisture gradient in rice kernels and allowed the moisture to equilibrate before the rice kernels were cooled. Tempering is a critical step to increase the moisture removal during cooling of the rough rice, researchers said.

Milling quality. After IR treatment, the milling quality was evaluated. Treated and untreated rice samples were dehulled and milled three times to achieve well-milled rice as defined by the U.S. Federal Grain Inspection Service. Total rice yield, head rice yield and whiteness index were used to evaluate the effects of different treatments on milling quality.

IR dried rice with tempering followed by natural cooking had similar and higher total rice yield and head rice yield compared to the control. On average, the total rice yield was 63.1%, 66.5% and 67.4% for samples with initial moisture contents of 20.06%, 25.53% and 32.50%, respectively.

The averages of head rice yield were 48.0%, 51.5% and 49.4% for samples with initial moisture contents of 20.06%, 25.53% and 32.50%, respectively.

The whiteness index of the treated samples was 35.1 units, which was slightly less than the control. Researchers said that storage of fresh rough rice samples in the refrigerator for five weeks before starting the test could have contributed to the low whiteness index.

Researchers said the results clearly demonstrate that high drying rate and good milling quality are possible by heating rice to about 60 degrees C followed by tempering and natural cooling. The same is true by heating the rough rice under two drying passes, especially for rice with high initial moisture content.

Enzyme inactivation. To evaluate the effectiveness of IR heating on inactivation of lipase enzyme, researchers determined the free fatty acid (FFA) concentration in rice bran produced from the rough rice samples and stored for different durations.

To measure FFA concentration, a rice bran sample of 2 grams was mixed with 40 mL hexane and shaken for one hour at 20 degrees C using a mechanical shaker. The mixture was centrifuged for six minutes, and the amount of extracted oil was measured gravimetrically. Total concentration of total FFA in rice bran oil, expressed as oleic acid percentage, was evaluated according to alcoholic alkali titration method.

The results showed that FFA concentration was affected by initial moisture content and tempering duration. The concentration of FFAs increased in the course of storage time under all tested conditions.

The concentration profiles of treated samples and control are different, researchers said. For the control samples, the FFA concentration increased sharply to more than 10% during less than seven days of storage. This means that FFAs formed as a result of hydrolysis reaction in the rice bran and lipase enzyme catalyzed this reaction to process rapidly. Untreated rice bran cannot be used for oil production after one week from the date of milling.

However, in samples treated with IR followed by tempering, the FFA concentration increased gradually to 10% after 17, 19 and 30 days. The FFA concentration in the two-pass treatment rice reached 10% after 38 days. This means IR heating followed by tempering has a destructive effect on lipase activity. The activity declined with an increase in tempering time and initial moisture content of rough rice.

Researchers said IR heating of rough rice followed by tempering treatment is an effective approach to inactivate the lipase and extend the storage stability of rice bran for up to 38 days after milling. Further studies are needed to determine the potential of using IR heating and tempering to improve the storability of brown rice.

UV, PEF treatments

UV treatment. Rough rice samples were dehusked to produce brown rice. The brown rice samples were treated under a double side UV heating system, and preheated to 30 degrees C. The samples were heated for different exposure times of 10, 20 and 30 minutes. They were mixed to achieve uniform exposure.

After UV treatment, the milling quality was evaluated. The enzyme inactivation and FFA of rice bran from the milling the brown rice samples over one month storage period was determined and quantified and compared to those of the control samples.

The concentration of FFAs increased sharply in the course of storage time for the treated and control samples. There was no significant difference in terms of FFAs between the samples under all treated conditions. Researchers said this means that the UV doses have no significant impact on lipase activation.

PEF treatment. Rough rice samples were dehusked and milled to produce rice bran. The samples were treated in PEF chamber at 0, 200, 400, 600, 800, and 1,000 pulses at frequencies of 0.25, 0.50 and 1.00 Hz.

After PEF treatment the rice bran was stored and the enzyme inactivation and FFA studied during the storage period of 23 days. The concentration of FFAs increased sharply in the course of storage time for the treated and control samples under different number of pulses.

The same trend was noticed for bran rice samples treated under different frequencies. There was no significant difference in terms of FFAs between treated and untreated samples of rice bran under all tested conditions of number of pulses and frequencies, researchers said. This means that the PEF treatment protocol used in the study does not have a significant effect on lipase activation.

Recommendations

Researchers recommend using IR to heat rough rice to 60 degrees C followed by tempering treatment for at least three hours. This can be an effective approach to inactivate the lipase and extend the storage stability of rice bran for up to 30 days after milling, they said.

It took only 55 seconds to achieve about 60 degrees C rice temperature and remove 1.7 to 5.8 percentage points of moisture content during IR heating along. The amount of moisture removal depends on the original moisture content of the rice and the number of drying passes.

Total moisture removal after tempering and cooling was up to 5.6 and 12.2 percentage points for one and two drying passes, respectively. IR heating followed by tempering treatments inactivates the lipase enzyme and extends storability.

Tempering is essential to achieve high rice milling quality, improve the amount of moisture removal during cooling and enhance the enzyme inactivation, researchers concluded.

The UV and PEF treatments for the brown rice and rice bran did not have a significant effect on lipase activation.

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