10Rice Bran Oil

Frank T. Orthoefer

1. INTRODUCTION

Rice oil, also called rice bran oil, has been used extensively in Asian countries such

as Japan, Korea, China, Taiwan, Thailand, and Pakistan (1, 2). It is the preferred oil

in Japan for its subtle flavor and odor. Interest in rice oil in the United States was

initiated after WWII, primarily to provide an additional revenue stream to the rice

miller. More recently, interest in rice oil escalated with its identification as a

‘‘healthy oil’’ that reduces serum cholesterol (3, 4).

Three facilities were constructed in the United States to produce rice oil (5). The

first facility began operation in the late 1950s, and a second facility was started in

the 1960s. Both were shut down in the early 1980s because of economics. A third

production facility began operation in the early 1990s and continues producing both

bulk and packaged oils for the domestic and export markets. Attempts at further

development of rice oil production have not been successful because of high capital

requirement to construct an oil extraction plant and refining facility and limited

availability of stabilized rice bran (6).

Rice oil is a minor constituent of rough rice when compared with the carbohy-

drate and protein content. Two major classes of lipids are present: those internal

within the endosperm and those associated with the bran. The internal lipids con-

tribute to the nutritional, functional, and sensory qualities of rice (7).

Rice bran is the main source of rice oil. The majority of available bran continues

to be used for animal feeds without being extracted for the oil. The food industry

Bailey’s Industrial Oil and Fat Products, Sixth Edition, Six Volume Set.Edited by Fereidoon Shahidi. Copyright # 2005 John Wiley & Sons, Inc.

465

uses minor quantities of stabilized rice bran as a source of dietary fiber, protein, and

desirable oil.

This chapter reviews the source and composition of rice bran oil, its nutritional

characteristics, production, and refining of the oil and its applications.

2. COMPOSITION OF RICE AND RICE BRAN LIPIDS

The structure of the rice kernel is given in Figure 1. Lipids are present as sphero-

somes or lipid droplets less than 1.5 mm in diameter in the aleurone layer, less than

1.0 mm in the subaleurone layer, and less than 0.7 mm in the embryo of the rice

grain (7, 9). Most of the lipids in the endosperm are associated with protein bodies

and the starch granules as bound lipids (10). The lipids are broadly classified as

nonstarch and starch lipids (Table 1). The majority of the lipids are the nonstarch

lipids. Starch lipids consist primarily of lysophospholipids, triacylglycerols, and

free fatty acids (13). Major phospholipid species are lysophophatidylethanolamine

and lysophosphatidylcholine. The major fatty acids are palmitic and linoleic acids

along with oleic acid. Minor amounts of monoacylglycerols, diacylglycerols, and

sterols are also found. Glycolipids found are diglycosyl monoacylglycerols and

monoglycosyl monoacylglycerols. The component sugars are galactose and glu-

cose.

The nonstarch lipids in the aleurone, subaleurone, and germ layers were 86–91%

neutral lipids, 2–5% glycolipids, and 7–9% phospholipids, although these are vari-

able because of different milling degrees (11). The fatty acid composition of

Figure 1. Relative proportion of major rice caryopsis components (8).

466 RICE BRAN OIL

nonstarch lipids showed 22–25% palmitic, 37–41% oleic acid, and 37–41% linoleic

acid (Table 2). The brown rice non-starch lipids was 14–18% in germ, 39–41% in

bran, 15–21% in polish, and 25–33% in milled rice. The composition was 83–87%

triacylglycerol together with 7–9% free fatty acids, diacylglycerols, sterols together

with sterol esters, hydrocarbons, and wax. Oil extracted from rice bran contained

20.1% total lipid, 89.2% neutral lipids, 6.8% glycolipid, and 4.0% phospholipid

(14). A component of rice bran oil that has promise as a nutraceutical compound

is g-oryzanol (15). g-Oryzanol was first isolated from soapstock from rice oil

TABLE 1. Lipid Composition of Rice and its Fractions (7, 9, 11, 12).a

Nonwaxy

Nonstarch Lipids in Rice Fractions Starch Lipid in

—————————————————— ———————

Brown Milled

Property Hull Brown Milled Bran Germ Polish Rice Rice

Lipid content 0.4 2.7 0.8 18.3 30.2 10.8 0.6 .05

Saponification no. 145 181 190 184 189

Iodine no. 69 94 100 99 101

Unsaponifiable matter 26 6 6 6 34

Fatty acid composition Wt % of total

Palmitic 18 23 33 23 24 23 46 45

Oleic 42 35 21 37 36 35 12 11

Linoleic 28 38 40 36 37 38 38 40

Others 12 4 6 4 3 4 4 4

Neutral lipids, % of total lipids 64 86 82 89 91 87 28 26

Triglyceride — 71 58 76 79 72 4 2

Free fatty acids — 7 15 4 4 5 20 21

Glycolipids, % of total lipids 25 5 8 4 2 5 19 16

Phospholipids, % total lipids 11 9 10 7 7 8 53 58

Phosphotidylcholine — 4 9 3 3 3 4 4

Phosphatidylethanolamine — 4 4 3 3 3 5 5

Lysophosphatidylcholine — <1 2 <1 <1 <1 21 23

Lysophosphatidylethanolamine — — 1 — — — 22 25

aBased on 6% bran-germ, 4% polish, and 90% milled rice from brown rice.

TABLE 2. Major Lipid Classes of Crude Bran Oil Extracted from Raw Rice Bran

and Their Fatty Acid Composition (14).

Fatty Acid Composition (%)

——————————————————————————————————————————

Lipid classa wt% 14:0 16:0 18:0 18:1 18:2 18:3 20:0 saturated unsaturated

TL 20.1 .40 22.21 2.21 38.85 34.58 1.14 0.61 25.43 74.57

NL 89.2 0.43 23.41 1.88 37.24 35.29 1.07 0.68 26.40 73.60

GL 6.8 0.09 27.34 0.28 36.45 35.76 0.18 27.61 72.39

PL 4.0 0.11 22.13 0.16 38.11 39.32 0.17 22.40 77.60

aTL ¼ total lipids; NL ¼ neutral lipids (nonpolar lipid and free fatty acids); GL ¼ glycolipids; PL ¼ phospholipids.

COMPOSITION OF RICE AND RICE BRAN LIPIDS 467

refining (16). Although originally thought to be a single compound, it is now known

to be a mixture of steryl and other triterpenyl esters of ferulic acids (cycloartenyl

ferulate, 24 methylenecycloartenyl ferulate, and b sitosterol ferulate and campesteryl

ferulate) (Figure 2). It is present at 1.5–2.9% of rice bran oil with a m.p. of 138.5�C.

The oryzanol content is dependent on rice grain variety with long grain rice at

6.42 mg/g and medium grain rice at 5.17 mg/g (17).

Tocopherols and tocotrienols (tocols) are present in rice oil (Figure 3). Crude

rice bran oil was found to contain, per 100 g of oil, 19–46 mg of a-tocopherol,

1–3 mg of b-tocopherol, 1–10 mg of g-tocopherol, and 0.4–0.9 mg of d-tocopherol,

14–33 mg of a-tocotrienol, and 9-69 mg of g-tocotrienol (18, 19) (Table 3). The

mean tocol content was 93 mg/100 g for crude oil and 50 mg/100 g for refined

oil (19). Close to 370 mg/100 g has been reported (20). Rice bran stabilization

and storage (21) and method of extraction (22) affects the concentration of tocols

in the oil. g-Tocotrienol is more stable and persists to a greater extent during storage

than other tocols (21). Other factors influencing toco content are milling and variety

(17, 23). Long-grain varieties have higher levels of tocotrienols than medium grain

rice (17).

CH CH COORH

CH3O3 2

1 7 84

5 6 (ROH see below)

= campesterol= O sitosterol= cycloartenol= 24 methylene-cycloartenol= cyclobranol

ROH

Figure 2. Major ferulates in oryzanol (9).

O

R3

R1

R2

HO

Tocopherols (T)

O

R3

R1

R2

HO

Tocotrienols (T3)

α-T(3)β-T(3)γ-T(3)δ-T(3)

CH3

H3

HH

CH3

HCH3

H

CH3

CH3

CH3

CH3

R1 R2 R3

Figure 3. Structure of tocopherol and tocotrienol (9).

468 RICE BRAN OIL

Waxes are present as long-chain fatty acid esters with fatty alcohols, methanol,

and ethanol. Fatty acid analysis showed that behenic (C:22), lignoceric (C:24), and

palmitic acids (C:16) are the major fatty acid for longer alkyl esters and oleic and

palmitic for the shorter alkyl esters (Table 4) (24). The major alcohols found are for

longer alkyl esters. These are as follows:

TABLE 3. Tocopherol and Tocotrienol Concentrations (mg/100g) in Raw Rice Bran

and Commercially Available Refined Oil (14).

Source a-T b-T g-T d-T a-T3 g-T3 d-T3

Rice bran 6.3 0.9 3.20 0.20 3.8 12.0 0.7

Brown ricea 0.63 0.09 0.32 0.02 0.38 1.2 0.07

Crude oila 31.50 4.50 16.00 1.00 19.0 60.0 3.5

Refined oil 8.2 12.80 1.3 2.1 42.9 3.5

aCalculated values.

TABLE 4. Fatty Acids of Sterol and Alkyl Esters, Alcohols of Longer Alkyl Esters,

and Alkanes and Alkenes of Rice Bran Waxy Lipids (24).

Fatty Acids Composition of:

——————————————————————————————————————————

Carbon and Alcohols of

Double Bond Longer Alkyl Shorter Alkyl Longer Alkyl

No. Sterol Esters Esters Esters Alkane Alkenes

14.0 0.6 1.8 2.2 — — —

16.0 11.1 23.8 35.5 — — —

18:0 1.0 3.8 0.8 — — —

18:1 33.1 2.9 60.2 — — —

18:2 51.1 0.3 1.5 — — —

18:3 2.0 — — — — —

20:0 0.7 3.6 — 0.1 — —

22:0 — 32.6 — 2.0 — —

23:0 — 31.2 — — 1.3 —

24:0 — — — 11.2 0.2 —

25:0 — — — — 2.0 —

26:0 — — — 6.3 0.8 —

27:0 — — — — 9.5 7.9

28:0 — — — 12.5 3.6 1.3

29:0 — — — — 46.5 38.8

30:0 — — — 19.1 3.0 0.9

31:0 — — — — 23.7 20.8

32:0 — — — 10.5 (5.1) 1.4 0.5

33:0 — — — — 6.5 18.8

34:0 — — — 6.6 (18.3) 0.7 1.4

35:0 — — — — 0.8 8.4

36:0 — — — 3.0 (5.4) — 0.1

37:0 — — — — — 1.6

COMPOSITION OF RICE AND RICE BRAN LIPIDS 469

Major alcohols:

Tetratriacontanol C34:0

Triacontanol C30:0

Dotriacontanol C32:0

Octacosanol C28:0

Tetracosanol C24:0

Straight-chain alkanes, alkenes, and branched-chain alkenes (squalene) are detected

in the hydrocarbon fraction. The squalene content is 120 mg/100 g.

Hard and soft waxes are recovered from crude rice bran oil with m.p. of 79.5�Cand 74�C (25). The hard wax consists of 64.5% fatty alcohols, 33.5% fatty acids,

and 2% hydrocarbons. Soft wax includes 51.8% fatty alcohols, 46.2% fatty acids, and

2% hydrocarbons.

3. MILLING OF RICE

Today’s modern rice mills efficiently separate hulls from paddy rice followed by

bran removal (Figure 4) (6). Milling consists of rubber roll dehullers, paddy separa-

tors, abrasive milling (whitening), and possibly friction mills. The bran and polish

consist mainly of the outer layers of rice caryopsis. These include the pericarp, seed

coat, nucellus, aleurone layer, germ, and part of the subaleurone layer of the starchy

endosperm. Rice bran makes up 5–8% of rough rice, and the polish may account

for an additional 2–3% (5). Commercial rice bran is a fine, floury material made

up of the outer layers of the brown rice plus pulverized germ, some hull fragments,

and some endosperm (white rice fragments) (8). The particle size distribution of the

bran is shown in Table 5 (26). The particle size of the bran varies significantly with

type of milling and milling condition. The composition of the bran also varies as

a function of milling degree (Table 6) (27). Generally, a low degree of milling is

practiced.

Rice bran is rich in lipids, proteins, minerals, vitamins, phytin, trypsin inhibitor,

lipase, and lectin (hemeagglutinins) (5). Compared with other cereal brans, rice

bran with germ is a little higher in fat content but comparable in protein, fiber,

and ash (Table 7). The high phosphorous content is among the highest of the cereal

grains. Rice bran is also high in silica probably because of the presence of rice hull

fragments. Bran is high in B vitamins and tocopherol, but it contains only a little

Vitamin A and C (28).

Rice bran and germ are used in animal feeds as a low-cost source of protein and

oil (6). ‘‘Rice mill feed’’ is a combined product produced by huller mills, where

dehulling and milling is a single processing step (5). Raw rice bran, when dehulling

is a separate processing step, has about four times the oil content (17–20%) of rice

mill feed (6). Parboiled rice bran produced by cooking of rough rice prior to milling

has a greater oil content, usually above 20%, than raw rice bran. The higher oil con-

tent may be caused by less endosperm contamination, better extractability of the oil

470 RICE BRAN OIL

TABLE 5. Particle-Size Distribution (%) of Raw and Heat-Stabilized Brans (26).

Mesh Particle Size (mm) Raw Bran Moist Heat-Stabilized Bran

18 >1000 0 0

18–30 1000–595 2.4 18.6

30–50 595–297 30.0 32.7

50–80 297–177 12.2 18.5

80–100 177–149 8.5 10.8

<100 <149 46.7 19.4

TABLE 6. Variation in Rice Bran Composition as a Function of the Degree of Milling (27).

Degree of Bran Composition (%)

Milling (%) Protein Fat Fiber Ash NFEa

1st Cone 0–3 17–0 17.7 10.5 9.8 45.0

2nd Cone 3–6 17.6 17.1 10.3 9.4 45.2

3rd Cone 6–9 17.0 16.5 5.7 8.4 52.5

4th Cone 9–10 16.7 14.2 5.7 7.5 55.9

aNitrogen-free extract.

Steps in Rice Milling

Rough Rice

Screening

Destoning

Dehulling

Polishing

Grading and sizing

Milled Rice

Bran

Figure 4. Steps in rice milling.

MILLING OF RICE 471

TABLE 7 Composition (% at 14% Moisture) of Rice Bran and Polish and Other Cereal Brans (26).

Rice

——————————————

Bran Polish Wheat Corn Barley Rye Oat Bran Sorghum Millet

Constituent Bran Bran Bran Bran Shorts Bran Bran

Crude protein (% N � 6.25) 12.0–5.6 11.8–13.0 14.5–15. 7.8–11.5 11.5 14.6 8.8–16.2 7.7–15.0 11.5

Crude fat (%) 15.0–19.7 10.1–12.4 2.9–4.3 4.4–8.1 2.8 2.6 3.0–6.8 4.6–4.7 8.0

Crude fiber (%) 7.0–11.4 2.3–3.2 6.8–10.4 2.6–9.4 9.6 6.6 20.5 7.4–9.1 —

Available carbohydrates (%) 31.1–52.3 51.1–55.0 50.7–59.2 58.9–62.6 58.4 58.0 61.4 54.3–64.1 56.0

Crude ash (%) 6.6–9.9 5.2–7.3 4.0–6.5 1.9–3.4 3.6 4.2 6.3 2.1–3.0 10.5

Calcium (mg/g) 0.3–1.2 0.5–0.7 1.2–1.3 0.3–0.4 2.8 0.9–1.2 0.9 — 0.8

Magnesium (mg/g) 5–13 6–7 5.6 2.5 — — 3.0 — 4.0

Phosphorus (mg/g) 11–25 10–22 9–13 1–6 5–8 7.2–10.5 8.1 —

Phytin phosphorus (mg/g) 9–22 12–17 10 — 3.1 6.9 — — —

Silica (mg/g) 6–11 2–3 2 — — — — — —

Zinc (mg/g) 43–258 17–60 105 — 21 56 — — —

Thiamine (B1) (mg/g) 12–24 3–19 5.4–7.0 4.2 — 2.5 4.1 — 10.6

Riboflavin (B2) (mg/g) 1.8–4.3 1.7–2.4 2.4–8.0 1.5 — 0.2 3.3 — —

Niacin (mg/g) 267–499 224–389 181–550 — — 22.6 1.5 — —

by solvents, and outward movement of the oil from aleurone and germ cells to the

bran layer (28).

The final physical and chemical nature of bran depends on the following:

1. Rice variety

2. Treatment of the grain before milling

3. Type of milling system

4. Degree of milling

5. Fractionation that occurs during milling (29).

The preferred method for milling of rice that gives hulls, bran, and milled rice is

referred to as ‘‘multistage’’ or ‘‘multiple break’’ where shellers (dehullers), pol-

ishers, and whiteners are used. The hull is first removed in shellers, and the dehulled

brown rice undergoes subsequent whitening operations. The amount of contami-

nants in the bran affects the total lipid content. Contaminants are broken rice and

layers from the endosperm. Addition of calcium carbonate, usually at 0.25% of

rough rice as a milling aid during whitening, further reduces the oil content. Other

milling aids such as diatomaceous earth and ground limestone have also been

used.

In developing countries, most rice is milled in a one-stage (huller) mill that

removes hull, bran, and germ as a single mixture. It is estimated that less than

25% of rough rice is fractionated into hull and bran fractions (29).

4. ENZYMES IN RICE BRAN

Rice bran contains active enzymes (30). Germ and the outer layers of the caryopsis

have higher enzyme activities. Some enzymes that are present include a-amylase,

b-amylase, ascorbic acid oxidase, catalase, cytochrome oxidase, dehydrogenase,

deoxyribonuclease, esterase, flavin oxidase, a and b-glycosidase, invertase, lecithi-

nase, lipase, lipoxygenase, pectinase, peroxidase, phosphatase, phytase, proteinase,

and succinate dehydrogenase.

Particularly lipase, but also lipoxygenase and peroxidase, are probably most

important commercially because they affect the keeping quality and shelf life of

rice bran.

Lipase promotes the hydrolysis of the oil in the bran into glycerol and free fatty

acids (FFA) (5). The lipase has been studied extensively. In the intact grain, the

lipases are localized in the testa-cross layer of the rice grains while the oil is in

the aleurone and subaleurone layers and in the germ (26, 26a). The germ, where 60%

of the lipase occurs, is similarly compartmentalized. During milling, the enzyme

and substrate are bought together. The rate of FFA formation is highly dependent

on environmental conditions. Formation of 5–7% free fatty acids per day has been

reported (29). Up to 70% FFA has been reported for a single month of bran storage.

Production of FFA in a clean U.S.-produced bran is shown in Figure 5 (31). Rice

ENZYMES IN RICE BRAN 473

bran oil contains 2–4% FFA at the time of milling. Less than 5% FFA is desirable

for producing rice bran oil because high FFA results in high refining losses. The

composition of crude rice bran oil produced by hexane extraction of stabilized

bran is shown in Table 8.

Lipase has a molecular weight of about 40,000 Da and an isoelectric point (pI) of

8.56 (32). It is activated by calcium and inhibited by heavy metals. The optimum

pH is 7.5–8.0, and the optimum temperature is 37�C. It is inactivated by heating at

60�C for 15 minutes. Rice bran lipase preferentially hydrolyzes fatty acids from the

0

10

20

30

40

50

60

70

80

90

20 40 60 80 100 120 140

Time (number of Days)

FF

A (

%)

Figure 5. Free fatty acid (FFA) increase in raw bran during a 135-day storage period (8).

TABLE 8. Crude Rice Bran Oil Composition (8).

Lipid Type Percent

Saponifiable lipids 90–96

Neutral lipids 88–89

Triacylglycerols 83–86

Diacylglycerols 3–4

Monoacylglycerols 6–7

Free fatty acids 2–4

Waxes 6–7

Glycolipids 6–7

Phospholipids 4–5

Unsaponifiable lipids 4–2

Phytosterols 43

Sterol esters 10

Triterpene alcohols 28

Hydrocarbons 18

Tocopherols 1

474 RICE BRAN OIL

1 and 3 positions in the triacylglycerol molecules. Two subunits are suggested for

lipase, and these are held together by disulfide bonds.

A second rice bran lipase has a pI of 9.1 and an optimum temperature of 27�C(33). It has a high specificity for triacylglycerols having short-chain fatty acids.

The enzyme, lipoxygenase, is associated with the oxidation of the polyunsatu-

rated fatty acids (PUFA) having a cis, cis-pentadiene structure. The carbonyl pro-

ducts from the degradation, particularly hexanal, have been implicated in the stale

flavor of rice. Lipoxygenase activity is highest in the germ fraction. Three forms of

lipoxygenase have been isolated differing in pH optimum and specificity (34).

5. STABILIZATION OF RICE BRAN

The instability of rice bran has long been associated with lipase activity (35). As

long as the kernel is intact, lipase is physically isolated from the lipids (29).

Even dehulling disturbs the surface structure allowing lipase and oil to mix. Oil

in intact bran contains 2–4% free fatty acids (2). Once bran is milled from the ker-

nel, a rapid increase in the FFA occurs. In high humidity storage, the rate of hydro-

lysis is 5–10% per day and about 70% in a month as shown earlier. The objectives

of rice bran stabilization are as follows:

� Arrest lipase and lipoxygenase activity.

� Improve oil extraction efficiency.

� Reduce fines in crude oil.

� Sterilize the bran.

� Reduce color development.

The lipoxygenase and peroxidase enzymes also have a negative impact on the

oxidative state of the bran (Table 9). Further degradation of the oil occurs as

reflected in an increase in peroxide and thiobarbituric acid value and a decrease

in iodine value. Both lipoxygenase and peroxidase enzymes are inactivated with

lipase inactivation.

TABLE 9. Changes in the Composition of Bran Lipids During Storage of Milyang 23 Rice

Bran at 30�C and 80% RH (28).

Storage Period (weeks)

——————————————————————————————————————————

Oil Property 0 1 2 3 4 5

Free fatty acids (% as oleic acid) 3.6 33.0 40.3 45.8 61.8 68.2

Peroxide value (meq/kg) 32.8 73.2 96.0 109.3 90.6 91.0

Iodine value (%) 96.8 90.2 85.4 83.2 79.0 74.7

TBAa (mg of malonaldehyde) 0.5 0.8 1.1 0.7 0.7 0.6

equivalents/Kg

aThiobarbituric acid.

STABILIZATION OF RICE BRAN 475

Lipase activity results in hydrolytic rancidity. There is little or no change in

flavor of the bran with an increase in FFA (5). Lipoxygenase activity, however,

increases with the presence of FFA resulting in oxidative rancidity (36). It is

oxidative deterioration that is responsible for the flavor and odor of rancid rice

bran.

Peroxidase is used as a convenient index of lipase activity. The inactivation tem-

perature for lipases and associated enzymes is dependent on the moisture content.

At 4% moisture, inactivation temperature for lipoxygenase is 40�C, lipase is 55�C,

and peroxidase is 70�C (28).

Methods for stabilization of rice bran have been reviewed (37). These include

dry heating, wet heating, and extrusion. The most practical method has been the

use of extrusion or expansion methods.

In retained heating methods (dry heat), a simple hot air drying reduces the moist-

ure content to 3–4%. The bran must be kept dry in moisture-proof containers, or the

rehydrated bran will regain its lipase activity. If the bran is heated in the presence of

moisture, the lipase is permanently denatured.

The types of retained-moisture heating methods include extrusion cookers and

sealed rotating drums. Extrusion cooking results in both lipase denaturation and

bran sterilization. When pressure is released, part of the superheated moisture eva-

porates with little or no drying being required. Expanders or expellers are also used

to permit addition of moisture (wet heating) through steam and the formulation of

colletts or pellets from the bran. The colletts aid handling and oil extraction.

Extrusion (dry heat) cookers have been ideal for stabilization because excess

moisture is not added, eliminating the need for drying. The heating of the bran

occurs through conversion of mechanical energy of the screw drive to heat the

bran. Temperatures used for stabilization vary from 100� to 140�C. The bran is

kept hot for 3–5 minutes after extrusion to ensure lipase inactivation. The hot

bran is then cooled using ambient air.

Extrusion cooking of the bran was pioneered by the Western Regional Research

Laboratory (28, 29, 29a). Dry extrusion was found more suitable for stabilizing

bran to be used as a food ingredient (38). Stabilization within 1 hour after milling

is considered ideal for bran quality.

Wet heating is more effective for bran stabilization for oil extraction than is dry

heating. Lipase is inactivated in 3 minutes at 100�C (37). The equipment that can be

used include steam cookers, blanchers, autoclaves, and screw extruders with

injected steam and water (30). Extrusion with steam injection and up to 10% added

water reduces the temperature required for lipase inactivation. Temperatures are

reduced to 100–120�C. Product may be held at 100�C for 1.5–3.0 minutes before

drying to a stable moisture content. Bran expands as it exits the extruder, and water

flashes to steam (8). Porous pellets assist in solvent percolation during oil extrac-

tion. Fines are agglomerated as well.

Addition of water/steam to bran during wet extrusion requires drying after sta-

bilization. Hot air is simply passed through a bed of pellets. Although this increases

the cost of stabilization, lipase inactivation is permanent with less nutritional

damage to the bran. The recovered oil is lighter in color with lower refining losses.

476 RICE BRAN OIL

The stabilized bran may be stored for extended periods, although extraction should

be completed within 1 month for best quality oil (39).

Parboiling of rice is also an example of wet heat stabilization. The lipase in

rough rice is completely inactivated by either autoclaving for 3–20 minutes or by

parboiling.

Other stabilization methods that have been investigated are as follows:

1. Refrigeration to reduce the rate of hydrolysis (8)

2. Lowing pH to reduce lipase activity (4)

3. Chemical additions such as sodium metabisulfite (39a)

6. RICE BRAN TO RICE BRAN OIL

Rice bran is the source of rice bran oil (30). Various commercial efforts to extract

the oil have been made over the past 50 years. Initially, use of the oil in traditional

foods was targeted. More recent efforts have emphasized the nutritional benefits of

rice bran oil.

Rice bran oil with a low free fatty acid content can be extracted with hexane

from extrusion stabilized bran. The process flow is shown in Figure 6. Nonstabi-

lized bran, although having a high free fatty acid, can also be used for production

of oil. With nonstabilized bran, the extraction is similar to that of extracting a fine

powder. Preprocessing of the bran through an extruder, expander, or expeller may

be used to form either a flake or pellet that results in improved solvent flow through

an extraction bed (40). Flaked bran with only 7–12% passing a 25 mesh screen gave

a percolation through a 60-cm bed of 563–620 L/m2/min. The oil extraction rate

Rice bran

Stabilization

Pellets

Hexane extraction

Desolventizing

Crude rice bran oil

Figure 6. Process for rice oil production (8).

RICE BRAN TO RICE BRAN OIL 477

was rapid, with 96% of the oil being removed in 5 minutes and only 0.7% residual

oil remaining after 1 hour of extraction.

Earlier methods to recover the oil used hydraulic pressing (28). In a Japanese

system for pressing, the raw bran is cleaned by sifting and air classification to

remove whole and broken grains and hulls, and, in some instances, to recover rice

germ. The bran is then steam cooked, dried, prepressed, and finally expeller pressed.

Hexane extraction may be batch, battery, or continuous type (12). All three sys-

tems were recently operating in Japan. Continuous systems operate in Brazil,

Burma, Egypt, India, Mexico, Taiwan, Thailand, and the United States. The bran

in the most efficient systems is stabilized, pelletized, and, if required, dried. After

the pretreated bran is placed in the extractor, hexane is pumped in and allowed to

percolate through the bran to extract the oil. Countercurrent extraction is used.

The miscella (solvent plus oil) is passed through filters to remove the bran fines

before evaporation for solvent and crude oil recovery. The production of fines

from expander stabilized bran depends on stabilization condition (38). Flake size

is larger if expanded at 120�C, but the flakes are fragile and easily broken. Flakes

with high moisture content were more resistant to breakage. Final bran moisture

was about 6%.

Pelletizing of the bran improves percolation and minimizes fines in the miscella.

Pellets are 6–8 mm in diameter. Moistening during palletizing reduces the fines pro-

blem. Parboiled bran does not produce the hard pellets found for raw bran possibly

because of protein denaturation during parboiling (33). Binding of the fines in the

pellet is assisted by starch gelatinization during heating of the bran. Parboiled bran

also presents problems with sticking to dryer surfaces resulting in self-ignition in

the dryer. Prior mixing with raw bran alleviates the problem.

The X-M process combines solvent extraction and milling of the rice (41).

Brown rice is pretreated with warm rice oil (0.5%) for 2–3 hours to soften the

bran. The rice is then milled in the presence of a rice oil miscella. The solvent slurry

is then removed from the rice and the rice oil is recovered. Advantages are that sta-

bilization is not required and the resultant oil had a minimum FFA level. This pro-

cess is no longer used.

Extraction of rice bran oil by supercritical fluid has been investigated (5). Minor

reductions in oil yield may occur. The oil yield with supercritical CO2 is 17.98%,

with CO2–ethanol 18.23%, and with hexane 20.21%.

7. REFINING OF THE OIL

The color of crude rice bran oil is dark greenish brown to light yellow depending on

the condition of the bran, extraction method, and composition of the bran. The pig-

ments include carotene, chlorophyll, and Maillard browning products (12, 28). Oil

from parboiled rice bran is generally darker in color than oil from raw rice bran.

The composition of crude rice bran oil has a major effect on refining. The crude

oil typically contains up to 0.5% bran fines and 0.5–5% wax. Agitated storage tanks

are required. Heated tanks and lines also are necessary to prevent crystallization of

478 RICE BRAN OIL

waxes. Refining losses may be in excess of ten times the FFA when the crude oil has

a relatively low FFA (<10%). Lower refining losses of approximately two times the

FFA have been reported (2, 6, 40).

Refining of crude rice oil involves dewaxing, degumming, neutralization of free

fatty acids, bleaching to improve color, and steam deodorization. Refined rice bran

oil is a light yellow color (Lovibond 3.0 R 30Y) with a mild background odor and

flavor reminiscent of rice. Similar to peanut oil, the flavor and odor are complemen-

tary to the flavor of many fried foods, such as fish, chicken, and chips.

8. DEWAXING

Waxes can increase refining losses (8). The wax content of crude oil depends on the

variety of rice, milling technique, method of oil extraction, and extraction tempera-

ture (2). Extraction temperature affects both the type of wax present and its quantity

(42). For example, extraction at 50�C yields two to three times more wax than

extraction at 20�C.

Initial dewaxing may simply be gravity settling followed by decanting (43). The

oil is gradually cooled to allow for wax crystallization followed by filtration or cen-

trifugation to recover the wax sludge. The foots recovered may be added back to the

defatted bran, sold as an animal feed oil, or further processed for oil recovery and

wax purification. Wax recovery involves acetone washing and fractionation with

isopropanol.

The characteristics of the wax are as follows:

Iodine value 11.1–17.6

FFA (%) 2.1–7.3

Phosphorous (%) 0.01–0.15

M.P (�C) 75.3–79.9

Attempts have been made to recover the wax using cold and hot extraction (2).

Wax yields of 1.29–1.82% of the crude oil are obtained. Continuous dewaxing of

rice bran oil by chilling the oil or miscella to less than 20�C followed by filtration

through plate and frame filters is practiced. Kinsey and Hummell (44) reported on

the use of sodium silicate as an aid for dewaxing. The characteristics and physical

properties of a purified rice bran wax are similar to carnauba wax (45).

Additional dewaxing may be used during degumming and alkali refining (8).

Dewaxing of refined, bleached oil by cooling to approximately 5�C followed by

filtration is necessary for production of a high-grade, chill-proof oil.

9. DEGUMMING AND DEACIDIFICATION

The phospholipids in rice oil are similar in composition to other oil sources. These

may be recovered as rice lecithin (5). Production of food-grade lecithin requires

DEGUMMING AND DEACIDIFICATION 479

prior removal of bran fines and waxes. Regular water degumming may be used.

Temperatures above 80�C are required to prevent crystallization and removal

of waxes with the gums. If food-grade lecithin is not being produced, filtration

of bran fines is not required. Pretreatment with phosphoric or organic acid is

necessary to remove nonhydratable phospholipids. Food-grade surfactants may

be added to improve wax removal (46). Degumming at less than 50�C actually

assists in wax removal. Wet gums may be added to defatted bran as a method

for disposal (8).

Both alkali and physical refining have been used for FFA removal (5). With alkali

refining, batch or continuous methods may be used. Oil may be pretreated with

phosphoric or organic acid for phospholipid hydration. The oil is then treated

with 16–30 baume (Be0) caustic with 20– 40% excess. The soaps settle and may

be recovered as ‘‘soapstock’’ or ‘‘foots’’ (47).

Continuous refining consists of in-line mixers, heaters, and centrifuges (8). The

combined oils plus alkali are rapidly heated to 55–70�C to assist in breaking

the emulsion of hydrated soap in oil. In instances where neutralization is com-

bined with dewaxing, separation is performed at 28–32�C. Water washing or

post-neutralization treatment with silicates to remove final traces of soaps and phos-

pholipids is the same as for conventional oils. Miscella refining, or refining while

still in solvent, may also be used (47). Higher refining yields and good-quality

neutralized oil with less color are advantages of miscella refining. Losses were

near the calculated amount (48) based on titrated values. Rice oil miscella is often

variable.

Excessive losses may occur in refining of rice oil. A 5% FFA crude oil has losses

ranging from 12% to 40% by the cup method. The cause of high refining losses is

unknown. It is assumed the losses are caused by the presence of partial esters, oxi-

dized components, and waxes, as well as high FFA acidity (8). Steam refining is

practiced by various refineries in Japan and the United States (2).

In calculating the amount of caustic required for caustic neutralization, the oil is

titrated to a phenolphthalein end point. This titration endpoint includes not only the

FFA, but also the oryzanol compounds. With the higher caustic addition, the ory-

zanol is transferred to the soapstock away from the oil. The nutritional benefit of

these compounds is lost. An alternative indicator for titration uses alkali blue (8).

This indicator reflects the acidity contributed only by the free fatty acids.

10. BLEACHING, HYDROGENATION AND DEODORERIZATION

Standard methods are used for bleaching, hydrogenation, and deodorization of rice

bran oil. Bleaching uses activated carbon or bleaching earth (47). Activated carbon

is seldom used because of high cost and handling difficulties. Bleach clay dosage

depends on the characteristics of the rice oil as well as that of the bleaching earth.

Dosages range from 2% to 10%. Newer silica bleaching earths are more effective in

reaching satisfactory oil colors.

480 RICE BRAN OIL

Deodorization or steam stripping is used to remove objectionable odors resulting

from peroxides, aldehydes, and ketones as well as characteristic rice oil odors

and flavors (12). The oil is heated to 220–250�C under 3–5-mm Hg vacuum.

Semicontinuous deodorizer units are the most common types used. Other designs

have been evaluated (43). After deodorization, the oil is cooled to 60�C and filtered.

Storage of deodorized rice ban oil is the same as for other oils.

Physical refining, also called steam refining, combines deacidification with deo-

dorization. Physical refining is more efficient for high FFA oils giving better yields

of neutralized oil than alkali refining (2).

11. WINTERIZATION

In addition to wax removal, rice bran oil contains sufficient saturated and high melt-

ing glycerides to require winterization to gain a cold test of 5 hours (8, 43). Without

winterization, dewaxed rice oil is frequently cloudy or turbid even at room tempera-

ture or slightly lower.

Winterization consists of cooling the oil under defined rates and to specific tem-

peratures followed by filtration. With rice oil, winterization consists of cooling 30–

35�C oil slowly at a uniform rate to 15�C over a 12-hour period with slow agitation,

then further cooling to 4–5�C without agitation followed by holding over a 24–

48-hour period, allowing higher melting components to crystallize. The type of

crystals formed depends on the cooling rate and the temperature differentials.

Large, stable crystals are desired for filterability. Filter aids may be added to assist

separation of the crystals from the viscous oil. Cold tests of the winterized oil of

5–7 hours are near maximum.

Miscella winterization more effectively separates the high melting solids from

rice oil. Hexane, acetone, and isopropyl acetate are among the solvents used.

The miscella is slowly cooled to 15�C over 12 hours with agitation, then to 4–

5�C without agitation, and held for 24–48 hours before filtering.

12. CO-PRODUCTS FROM PROCESSING

As with all oils, coproducts of refining represent a significant revenue stream.

Waxes may be concentrated and refined to compete with other organic waxes.

The hard, high melting waxes are preferred for most applications.

Soapstock contains fatty acid soaps and, for oil that is caustically refined, ory-

zanol (5–10%). The soaps may be acidulated for feed use and the oryzanol isolated

(16). Diethyl ether, alumina chromatography, and crystallization are used for pur-

ification of the oryzanol.

The deodorizer distillate, about 1% of deodorizer feed, contains tocopherols,

tocotrienols, and sterols (Table 10). The tocols are shown in Table 11 and the sterols

in Table 12. Its value is similar to other oil distillates.

CO-PRODUCTS FROM PROCESSING 481

13. COMPOSITION OF REFINED RICE BRAN OIL

A typical specification for finished rice bran oil is shown in Table 13. These are

similar to that for other oils. Rice bran oil has a characteristic nutty, earthly flavor

not unlike peanut oil.

The fatty acid composition of rice bran oil is most similar to peanut or ground

nut oil (Table 14) (8). Palmitic, oleic, and linoleic acids make up more than 90% of

the fatty acids present. The major molecular species of triacylglycerols are palmi-

tic-linolenic-oleic, oleic-linoleic-palmitic, palmitic-linoleic-linoleic, linolenic-lino-

leic-palmitic, and trioleic. As with peanut oil, rice bran oil is most suited for general

frying and cooking applications.

TABLE 11. Approximate Tocol Composition of Rice Oil

Distillate (20).

Tocol (percent)

————————

Tocopherol Tocotrienols

Alpha 67 21

Beta 3 tr.

Gamma 30 77

Delta tr. 2

TABLE 12. Sterol Composition of Rice Oil

Deodorizer Distillate (20).

Sterol Percent

Beta-sitosterol 38

Stigmasterol 18

Campesterol 13

Delta-7-stigmasterol 10

Delta-7-avenasterol 6

Delta-5-avenasterol 5

Others 10

TABLE 10. Rice Oil Deodorizer Distillate Composition (20).

Component Percent (range)

Free fatty acids 25–40

Tocopherols 1.5–3.0

Tocotrienols 4.0–6.0

Sterols 15–25

Squalene 15–25

Monoacylglycerols, diacylglycerols, etc 15–25

482 RICE BRAN OIL

14. RICE BRAN OIL NUTRITION

The initial interest in rice bran oil resulted from work with the stabilized rice bran.

Rice bran was shown to be equivalent in serum cholesterol reduction to oat bran in

hamster trials (Table 15) (1). Two clinical studies showed rice bran reduced serum

low-density lipoprotein (LDL) cholesterol in humans (49,50). Defatted bran was

less effective in lowering cholesterol than full fat bran (1). The cholesterol-lowering

activity was concentrated in the unsaponifiable fraction of rice bran oil (Table 16)

(51). Oryzanol was found to contribute to the hypocholesterolemic activity of rice

TABLE 13. Product Specification of Refined, Bleached,

and Deodorized Rice Bran Oil (8).

Characteristic Value

Iodine value (Wijs method, g/100 g sample) 99–108

Peroxide value (meq/kg) 1.0 max

Moisture (%) 0.05 max

Color (5.25-in Lovibond red) 5.0 max

Free fatty acid (% as oleic) 0.05 max

Flavor/odor 7 min

Chlorophyll (ppb) 75 max

Saponification value 180–190

Unsaponifiable matter 3–5

Smoke point 213�CRefractive index 1,470–1,473

Specific gravity 0.916

AOMa (hr) 17.5

aActive oxygen method.

TABLE 14. Chemical Composition of Rice Bran Oil (8).

Physicochemical Parameters Value

Acid value 1.2

Iodine value 100.0

Saponification value 211.8

Unsaponifiable matter 4.2

Fatty acid composition Percent

C14:0 0.6

C16:0 21.5

C18:0 2.9

C18:1 38.4

C18:2 34.4

C18:3 2.2

C20:0 —

C22:0 —

RICE BRAN OIL NUTRITION 483

oil in rats (52) and primates (53). A clinical study with 3.1 g/day of rice bran oil

unsaponifiables over a 12-month period resulted in a 14.1% reduction in total

cholesterol and a 20.5% reduction on LDL-cholesterol (Table 17) (54). HDL-

cholesterol rose, and triacylglycerols decreased significantly. Tocotrienols, also pre-

sent in rice bran oil, have been reported to reduce serum cholesterol (55).

The refining method used in rice oil production affects the oryzanol content of

finished oil (3). With alkali refining, most of the oryzanol is removed (Figure 7),

whereas with steam or physical refining, most of the oryzanol (66%) remains in

TABLE 15. Effect of Rice and Oat Brans on Serum Cholesterol

in Hamsters (1).

Bran in Diet Serum Cholesterol (mg/dL)

Cellulose (10%) 395

Rice bran (47.8%) 270

Defatted rice bran (24.7%) 347

Parboiled rice bran (31.8%) 297

Defatted parboiled rice bran (19.6%) 377

Oat bran (53.7%) 289

TABLE 16. Hypocholesterolemic Activity of Unsaponifiable Matter of Rice Bran Oil

in Rats (51).

Serum Cholesterola (mg/dL)

Diet Total HDL LDL þVLDL

Control (peanut oil) (10%) 374 43 331

Rice bran oil (10%) 228 48 240

Control þ 0.2% unsaponifiables 387 48 339

Control þ 0.4% unsaponifiables 243 48 195

aHDL ¼ high-density lipoprotein; LDL ¼ low-density lipoprotein; VLDL ¼ very low-density lipoprotein.

TABLE 17. Effect of Daily Addition of Rice Bran Unsaponifiables (RBN) on Serum Lipids

(mmol/L) in Hypercholesterolemic Subjects (54).

Serum Lipidsa Start 12 months p

RBN

Cholesterol 6.18 � 0.33 5.31 � 0.20 <0.05

LDL cholesterol 4.28 � 0.37 3.40 � 0.18 <0.05

HDL cholesterol 0.17 � 0.02 0.24 � 0.02 <0.025

Triacylglycerol/HDL 2.16 � 0.35 1.21 � 0.21 <0.05

RBN placebo

Cholesterol 5.70 � 0.21 6.06 � 0.32 ns

LDL cholesterol 3.95 � 0.18 4.05 � 0.31 ns

HDL cholesterol 0.21 � 0.06 0.22 � 0.01 ns

Triacylglycerol/HDL 1.54 � 0.31 1.55 � 0.20 ns

aLDL ¼ low-density lipoprotein, HDL ¼ high-density lipoprotein, ns ¼ not significant.

484 RICE BRAN OIL

the oil (56). Physically refined rice oil gave a serum lipid response similar to that of

crude rice bran oil. Various refining methods to preserve the oryzanol in the oil have

been attempted. Sodium carbonate instead of sodium hydroxide has been partially

successful in which two-thirds of the original oryzanol in the crude oil is preserved

in the refined oil (57). Adding back unsaponifiables to the oil has been patented

(58). Clinical trials have not been performed with high oryzanol rice bran oil. An

unsaponifiable concentrate was prepared by extracting the soapstock, with hexane

giving a deacidified concentrate with 30% unsaponifiable content.

15. RICE BRAN OIL UTILIZATION

Rice bran oil is used in foods, feed, and industrial applications. Only high-quality

oil is targeted to foods. The use of rice bran oil in Japan, where it is the largest

volume domestically produced vegetable oil, is as a frying oil where its flavor is

preferred over alternative oils. The oxidative stability of rice bran oil is equivalent

to peanut oil and cottonseed oils in deep frying applications (Table 18) (8, 59).

0

10

20O

ryza

nol (

ppm

X th

ousa

nd)

Crude Alkali refining Physical refining

Figure 7. Effect of the refining process on the oryzanol content of rice bran oil (8).

TABLE 18. Frying Evaluation of Rice Oil (15-day results) (8).

Days to Maximum Levelb

———————————————

Oil typea FFA FOS LY LR TPM

Rice (without additives) 3.91 3.74 6 28.0 31.9

Rice (with additives) 5.62 3.46 7 49.6 34.6

Peanut (with additives) 6.87 3.92 8 21.2 37.5

Cottonseed (with additives) 7.22 4.07 7 28.8 37.2

aSpecifications. 40 lb (18.2 kg) gas fryers, frying temperature 350�F (177�C); hourly rotation: breaded chicken,

fish, onion rings, French fries: 5-ppm dimethyl polysiloxane antifoam, 200-ppm tertiary butyl hydroguinone.bFFA ¼ free fatty acids, FOS ¼ food oil sensor; LY ¼ Lovibond yellow; LR ¼ Lovibond red; TPM ¼ total polar

material.

RICE BRAN OIL UTILIZATION 485

Blends of rice bran oil with soybean oil reduces the increase in total polar material

(TPM) depending on the amount of rice bran oil in the blend (Table 19). Potato

chips fried in rice bran oil show flavor and odor stability at elevated temperatures

between that of peanut and cottonseed oils (Table 20).

Winterized rice bran oil is an acceptable oil for salad dressing and mayonnaise.

The hard fraction of rice bran oil may be used to replace the plastic fats in margar-

ines and shortening. Hydrogenated rice bran oil is adaptable to specialty shorten-

ings and margarines.

The nonfood uses of rice bran oil are feed formulations, soaps, and glycerin.

Waxes may be used as a carnauba wax replacement in confectionery, cosmetics,

and polishing compounds products.

Use of rice bran oil grows as a specialty ingredient in the cosmetic/personal care

market. The demand is for natural, value-added healthy ingredients (60).

16. RICE OIL PRODUCTION (POTENTIAL)

World rice production is greater than 500 million metric tons. Rice oil production is

estimated at 722.2 thousand metric tons (Table 21). India, China, and Japan are the

leading producers. More than half of rice is processed in small rice mills. This

leaves approximately 20–25 million metric tons of bran available for oil production.

The rice bran oil potential is, then, 3–4 million metric tons.

In the United States, most bran is also produced in small rice mills scattered

in rice production areas with insufficient bran production to justify oil extraction.

TABLE 19. Frying Results Using Blends of Rice

and Soybean Oils (8).

Total Polar Material (%)

———————————

Oil Type 10 days 13 days

Rice 21.12 32.78

Peanut 21.07 35.53

Rice/soybean 50:50 24.11 35.80

Rice/soybean 25:75 23.25 40.42

TABLE 20. Days at 145�F (62.8�C) Before Rancid Odor

is Detected (8).

Oil Type Days to Detect Rancid Odor

Rice (without additives) 20

Rice (with additives) 25

Peanut (without additives) 14

Cottonseed (with additives) 31

486 RICE BRAN OIL

Production estimates are for less than 80 thousand metric tons. Only 15.9 to 18

thousand metric tons are produced currently in the United States at a single oil

extraction facility.

17. SUMMARY

Rice bran is an underused coproduct of rice milling. The value is partially captured

through extraction and refining of the rice bran oil. The capital costs have limited

the ability of the U.S. rice milling industry to capture this value. However, rice bran

oil has performance properties competitive to other widely used oils. An additional

advantage of rice bran oil is certainly its nutritional benefits, which include a bal-

ance of fatty acids meeting AHA recommendations. Rice oil contains a mixture of

antioxidants and promotes cholesterol reduction beyond that of more unsaturated

oils. Its taste and performance is complementary to salad, cooking, and frying

applications.

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Country Thousand Metric Tons

Bangladesh 1.5

Brazil 1.5

Cambodia 4.6

China 90.0

India 472.7

Indonesia 0.15

Japan 65.0

Korea 11.7

Republic of Korea 9.2

Laos 2.6

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Nepal 7.6

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Vietnam 7.6

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