Brookfield AMETEK recently released this application note on rice texture testing by penetration. By using the CT3 Texture Analyzer with a penetration test, the hard and stickiness of Basmati rice grains may be quantified.
In the rice family, we generally find that rice grains fall into two categories known as Indica (long grain) and Japonica (short grain). Rice contains starch (amylose and amylopectin), protein and fat. Starch mainly contains 20-30% of amylose and 70-80% amylopectin. The quality of cooked rice in terms of hardness and stickiness will depend on three chemical characteristics which are (a) amount of starch extracted into the cooking water (i.e., amylose content); (b) the gelatinisation temperature (i.e., temperature at which a particular rice variety will absorb water and the starch grains begin to swell), and lastly (c) the gel consistency (i.e., tendency of the cooked rice to harden on cooling).
The amylase content will determine whether the cooked rice grains are dry and flaky or moist and sticky. This is because high amylose contents gives rise to a high volume expansion of the rice during cooking, a greater extent of flakiness, and less tender grains with a low gel consistency meaning that the grains become hard upon cooling. On the other hand, if the amylose content is low, the cooked rice grains are moist and sticky.
Japonica rice is generally known to be stickier than the Indica type as it contains low amylose content. Indica rice on the other hand has higher amylose content which makes the cooked rice harder and less sticky. However, the texture of rice can not be entirely attributed to the amylose content but also to the protein and fat content. It follows that the higher the protein content in extracts from the surface of cooked rice, the harder and less sticky the cooked rice.
The CT3 Texture Analyzer using a cylinder probe can measure the hardness and stickiness of the rice grains by compressing the grains over a target distance and measuring the force to crush and withdraw from the grains. The texture analyser can therefore be used to monitor the effects of cook time, change in formulation, change in process control and even shelf life studies as well as help to ensure consistent quality and texture in production.
Test type: Compression
Pre-Test Speed: 1.0 mm/s
Test Speed: 1.0 mm/s
Post-Test Speed: 1.0 mm/s
Target Type: % Deformation
Target value: 60%
Trigger Load: 4.5 g
- Boil one cup of rice in two cups of water for 20 minutes on medium heat. Ensure that the rice is fully cooked by pressing a few grains with a fork.
- Place the cooked rice in a container and allow about five minutes for the rice to cool down
- Take the samples from a similar location each time for samples you wish to compare also ensure that the temperature of the samples is standardised.
- Attach the cylinder probe to the M6 thread of the probe shaft of the instrument
- Place the fixture base table onto the base of the instrument and loosely tighten the thumb nuts to enable some degree of mobility
- Insert a base plate onto the fixture base table and fasten into position using the thumb screws
- As this test is a percentage deformation test, the instrument will need to locate the position of the base plate in order to measure the sample height. To do this, lower the instrument arm to a few millimetres from the base plate and click on the “locate base” icon in the adjust beam box of the software. The instrument arm will be automatically lowered until contact has been made with the base plate before withdrawing
- Place three grains of cooked rice onto the centre of base plate. A spatula or spoon may be used to move the grains on the base plate.
- Lower the arm of the instrument such that the probe is a few millimeters from the rice grains on the base plate.
- Align the rice grains centrally under the probe by repositioning the fixture base table
- Once alignment is complete, tighten the thumb nuts of the fixture base table to prevent further movement.
- Enter the test parameters (see Test settings) and ensure that the “Measure Length” option is activated/ticked on the software
- Commence the test
- When proceeding to test a fresh sample, wipe clean the base plate and probe
- Sample orientation must be consistent for all tests performed
- The same number of grains must be used for all tests
The Load/Time graph for the hardness and stickiness of 12 g cooked rice using a cylinder probe
The maximum peak force is a measure of sample hardness. The hardness values are relative to the penetration depth selected; the greater the distance or % deformation, the higher the hardness value. Distances greater than 60% of sample depth is not recommended as base effects are likely to occur when the probe begins to press against the table. The adhesive force of the sample is minimal and therefore a negative peak is not observed from the graph.
The Load/Distance graph for the hardness and stickiness of 12g cooked rice using a cylinder probe
The maximum peak force is a measure of sample hardness. The area under the graph is a measure of work done. The stickiness of the sample is minimal and therefore no negative peak is observed.
When a trigger of 4.5 g has been detected at the sample surface, the probe pauses at the sample surface for a few seconds as the instrument measures the sample length (based on the distance from the sample surface to the base plate). Once the sample height has been measured, the probe proceeds to compress the sample of the target % deformation. As the probe compresses the sample, the force is seen to rise with increasing % deformation. Once the target % deformation has been attained the probe withdraws from the sample and returns to its starting position a few millimetres from the sample surface. As the probe withdraws from the sample, the stickiness (adhesive force) and the adhesiveness (energy to separate the sample from the probe) are measured. These are the negative portions of the graph, the adhesive force is the maximum negative value and the adhesiveness the area under the negative portion of the graph. The adhesive force is the force required to overcome the attractive forces between the sample and the probe in contact with it simulating the stickiness of the rice to the teeth. From the graphs the stickiness of the sample is very low (see values in the table below).
The maximum force value on the graph is a measure of sample hardness. This value correlates with the amount of force required by the teeth to compress the sample; the higher the value, the firmer the sample (see Figure 1 and 2). The energy used to compress the sample over the target % deformation is measured as the area under the positive peak (Fig.2) Work done can only be observed on the load/distance graph as it involves applying a force over a given distance. The work done value correlates with the amount of energy required to overcome the strength of the internal bonds within the sample when chewing. The higher the value, the more energy required to breakdown/chew the sample.
The table below summarises the results taken from 12 samples:
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