stabilising rice bran through high shear extrusion

8/20/2019 Stabilising rice bran through high shear extrusion

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Rice is considered one of the world’s

most important crops and is a major

part of the food culture in Asia, Latin

America, Africa and other parts of the

world.

Global rice production is steadily

increasing to meet the growing

demand for food from a rapidly

rising global population. The Food

and Agriculture Organisation (FAO) – United Nations estimated

that 2015 production of rice is around 500 million metric

ton. Production of grains including rice is currently meetingand exceeding consumption. As more rice is being produced

and processed, another part of the paddy, rice bran, is being

generated.

Rough rice or paddy (see diagram) consists of the white starchy

rice kernel, surrounded by a tightly adhering brown coating of

bran and enclosed within a loose outer hull. During the rice

milling process, the hull and the bran along with the rice germ are

removed mechanically to access and polish the white rice, which

is the principle sustenance for the majority of world’s population.

Due to the lack of a proper method for rice bran stabilisation in

under developed and developing countries, rice bran was under-

utilised until low cost extruders were introduced in the market.

General steps in the production and utilisation of

stabilised rice bran

The separation of the hulls and the bran can be through one,

or more stages. If the dehulling took place in one stage where

both the hulls and the bran are mixed, the oil content will be

low (below 10 percent), an economical separation of the oil is

not possible. However, the use of two stage rice mills, in which

the bran and the hulls are recovered separately, allows for an

economical extraction of oil.

The hulls (about 20 percent of the rice paddy by weight)

have no signicant nutritional value as they consist mainly of

cellulose, lignin and select minerals. On the other hand, rice bran(approximately 5-10 percent of the rice paddy by weight) is rich

in protein (14-18 percent) and energy; mainly in the form of the

oil it contains (10-20 percent). The percentage of oil in the bran

depends on the milling process, the contamination of the bran

with hulls and broken kernels and whether the bran is obtained

from raw or parboiled paddy.

Besides the protein and energy, stabilised rice bran is an

excellent source of vitamins B and E and some antioxidants. It

has been used as an ingredient in poultry, pet food, and ruminant

animal diets.

The urgency for stabilisation of rice bran

Rice bran has a powerful enzyme system which is activated

during the milling process and causes rapid deterioration of the

oil by exposure to the enzyme lipase and, to a certain extent,

oxidase. Enzymatic hydrolysis of the oil into free fatty acids

and glycerol will start and the acidity increases (reduced PH)resulting in the development of a soapy off avor.

The oil in the un-milled rice paddy is stable, as the Lipolytic

enzymes are located primarily in the seed coat. Whereas most

of the oil in the un-milled paddy or brown rice is stored in the

germ and the Aleurone cells (the protein rich outmost layer of

the endosperm), upon milling, the oil is subject to the activity of

the powerful lipase enzyme causing the accelerated break down

of the oil into free fatty acids and glycerol. The free fatty acids

become susceptible to further decomposition through oxidative

rancidity that will produce free radicals, cause soapy avour and

Stabilising rice bran

through high shear

extrusion

by Dr Nabil W. Said, VP Nutrition & Extrusion Technologies,Insta-pro International

Figure 1

Figure 2

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a reduction in the nutritional value. For this reason, it is of utmost

importance to immediately stabilise the bran to prevent thedeterioration and the rancidity of the oil.

Inactivation of enzymes through extrusion

High shear dry extrusion was developed by Insta-Pro

International in the late 1960’s to inactivate the anti-nutritional

factors present in soybeans. Those anti-nutritional factors are

enzymes such as the trypsin Inhibitors, hemagglutinins, lipase,

lipoxygenases, urease and other anti-nutritional factors. If

ingested in their intact state, the anti-nutritional factors will

adversely affect the digestion of nutrients through their inhibitory

effects on the pancreatic enzymes responsible for digesting the

protein and carbohydrates. The high dry shear extruder generates

heat through friction (mechanical energy) along with pressure.

The barrel of the extruder consists of segmented chambers with a

shaft that rotates at constant speed.

Screws are tted on the shaft separated by steam locks or shear

locks designed to disrupt the conveying of the material and force

some of it to revert back through channels in the wall of the barrel

thus generating shear and friction that raises the temperature of

the extrudate in a controlled manner (see the following gures).

Upon exiting the extruder, a sudden drop in the pressure

takes place that results in rupturing cell walls, releasing natural

anti-oxidants (tocopherols), partially dehydrating the product as

some of the moisture will be ashed off in the form of steam.

This sudden drop in pressure also results into sterilisation of the

product. As concluded from numerous scientic studies, viruses,

bacteria, coccidia, mold and yeast cannot survive the extrusion

conditions.

The inactivation of the enzymes takes place through altering the

tertiary structure by breaking the disulde bonds holding the aminoacids strands together. The proper optimisation of the extrusion

parameters allows for the denaturing of those enzymes without the

adverse effect on the primary structure of the amino acids.

The cooking time takes only a few seconds and under the

proper conditions of operation, the digestibility of nutrients in

the extrudate would be at its highest level. The extruder has

been used as one of the most effective tools to stabilise products

through enzymes inactivation thus in the case of rice bran,

Figure 3

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stabilising it immediately after milling.

Fresh milled rice bran oil normally contains 1.5-2.0 percent

free fatty acids. Free fatty acids level of less than 5 percent

is desirable. If not stabilised within few hours after milling,

the free fatty acids in the oil will exceed the 5 percent level.

Exceeding 10 percent free fatty acids in the oil would indicate

less recoverable edible oil.

Utilisation and benets of stabilised rice bran

Stabilised and extracted rice bran oil is very popular as a

cooking vegetable oil in China, Japan, Taiwan and other Asian

countries. In the late 1800’s, it was discovered that East Indies

sailors suffered from what thought to be toxins or infection

generated from the white rice that they consumed. The disease

is called Beriberi. It was later discovered that when the sailors

were fed meat, vegetables and dry milk, the symptoms subsided

and disappeared. It was then recognised that it was a nutritional

deciency in the polished rice that lead to the discovery of

vitamin B1, thiamin. This vitamin stays in the bran upon

polishing the rice. By adding rice polishing to the Beriberi

inicted chicken’s diet, the fowl were cured of the disease.

Because of its content of decent protein level, oil, bre andantioxidants, stabilised rice bran can be used as an excellent

source of energy, protein and bre in animal feeds, human food

and as a functional food or nutraceutical ingredient. Many claims

have been made as to its benet in lowering the cholesterol

through its content of tocotrienols tocopherols, ß-sitosterol and

γ-oryzanol. It is also been used as a stool-bulking agent.

In the feed industry, stabilised rice bran is being used as a

palatable ingredient and excellent source of protein in pet food;

energy and bre in equine feed, poultry and ruminant animals.

References available upon request

Figure 4

Figure 5

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stabilising rice bran through high shear extrusion

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