preparation of rice analogues using extrusion technology

Review

Preparation of rice analogues using extrusion technology

Abhinav Mishra,* Hari Niwas Mishra & Pavuluri Srinivasa Rao

Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur 721302, India

(Received 16 October 2011; Accepted in revised form 3 March 2012)

Summary Rice is considered as staple food in many parts of the world. An issue of concern is the breakage of rice

kernels in milling processes, and these broken kernels are not generally accepted by consumers. These broken

kernels can be mixed with some desired additives to improve their quality and extruded for the preparation

of reconstituted rice kernels or rice analogues. Various studies have been conducted for the preparation of

the rice analogues in the past few decades, and recently attempts have been made to fortify these analogues

with protein, certain vitamins and minerals. The main features such as colour, shape, size, texture, and

cooking characteristics and cooking time of these rice-like grains can be tailored to the requirements of

specific applications by modification of the extrusion parameters. Various organisations, such as Wuxi

NutriRice Co. (DSM ⁄Buhler) and China National Cereals, Oils and Foodstuffs Corporate (COFCO),

Superlative Snacks Inc., Vigui and PATH, have utilised this technique to prepare fortified and reconstituted

rice. Studies have shown that it is possible to improve the nutritional quality of rice by fortified rice

analogues. This article reviews research results of the many approaches to the formation of fortified rice

analogues by extrusion-based technologies.

Keywords Broken rice, food extrusion, protein denaturation, rice analogues, starch gelatinisation.

Introduction

Rice is one of the leading food crops and sustains two-third of the world’s population providing 20% of theworld’s dietary energy supply (Choi et al., 2010).Despite being a primary food, rice is low in proteinand high in starch. The low protein levels in rice causedeficiencies of protein and some essential amino acids inpeople who take it as their primary diet. For example,lysine, which is responsible for proper growth of thehuman body, is an essential amino acid found in thelowest quantity in rice (Kato, 2006).One approach to overcome these problems is to

prepare rice analogues by extrusion, in which thevitamins are embedded and consequently do not sepa-rate from the rice grains (Steiger, 2010). This will utilisemainly the broken rice, which is either used to make riceflour or sold at subsidised prices. The fortification of riceanalogues will allow its consumers to benefit withoutmaking major changes in their dietary habits (Kunz,2009).This article reviews research results of the many

approaches to the formation of fortified rice analoguesby extrusion-based technologies. Food extrusion is aprocess in which a food material is forced to flow, under

one or more varieties of conditions of mixing, heatingand shear, through a die that is designed to form and ⁄orpuff-dry the ingredients. Shear refers to the frictionbetween the inside part of the barrel and the foodmaterial (Riaz, 2000).Extrusion can be classified into two categories: cold

extrusion and hot extrusion. Both processes pass doughmade of principal component (mostly rice flour), anadditive mix, and water through a single or twin-screwextruder. Hot extrusion involves relatively high temper-atures (above 70 �C) obtained by pre-conditioningand ⁄or heat transfer through steam-heated barrel jack-ets. It results in fully or partially pre-cooked simulatedrice kernels. At commercial level, this method iscurrently applied by Wuxi NutriRice Co. (DSM ⁄Buhler)and China National Cereals, Oils and FoodstuffsCorporate (COFCO) in China and by SuperlativeSnacks Inc. in the Philippines. On the other hand, coldextrusion, a process similar to the one used for manu-facturing pasta, does not utilise any additional thermalenergy input other than the heat generated during theprocess itself and is primarily a low temperature (below70 �C), forming process resulting in grains that areuncooked, opaque and easier to differentiate fromregular rice kernels in terms of shape. This processutilises a simple forming extruder also called a pastapress and is commercially used by Vigui (Italy) and*Correspondent: E-mail: [email protected]

International Journal of Food Science and Technology 2012 1

doi:10.1111/j.1365-2621.2012.03035.x

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PATH for the production of UltraRice (Alavi et al.,2008).

Materials used in rice analogues

Additives (in small amounts) can be used with rice-watermixture (or some other principal ingredients). Theadditives can be categorised as colouring agents, fla-vouring agents, binders, binder setting agents, lubri-cants, fortificants and antioxidants. It is important tonote that while the matrix components (generally rice)and water are essential for the preparation of riceanalogues, other additives can optionally be used indifferent combinations.If the starting matter is comminuted (made finer) in

the beginning, it is easy to add any fortifying agent tothe ingredients that form the dough. Therefore, theproduct is nutritionally rich in terms of desired nutri-ents ⁄micronutrients and is not only capable of beingconsumed by the addition of hot or cold water, but itcan also provide with recommended nutrition to theconsumers (Cox & Cox, 1993).The following discussion explains the category-wise

considerations followed so far by the researchers for thematerials used in rice analogues:

Matrix composition

The principal component of the dough for the prepa-ration of rice analogues is rice flour in most of the cases,which can utilise broken, cracked or otherwise degradedrice grains (Steiger, 2010); however, defatted soy flourhas also replaced rice flour in some studies. The ricegrains have to be selected to represent an appropriateratio of amylose to amylopectin required for the desiredend product (Cox & Cox, 1997). Waxy or high-amylo-pectin starch extrusion is dominated by melting, which isa zero-order reaction, instead of gelatinisation, which isa first-order reaction (Qu &Wang, 1994). The higher theamylose content, the higher will be the viscosity (e.g.viscosity increased from 277 to 1254 Pa s)1 whenamylose content increased from 0% to 80% under acertain condition). The amylose-rich starches showhigher viscosity and less Newtonian behaviour, whichcan be explained by their higher gelatinisation temper-ature, greater molecular entanglements between linearpolymer chains, and less gel-balls and super-globes thatare much easier to move than long linear chains (Xieet al., 2009).The ratio of pre-gelatinised (i.e. pre-cooked) to

ungelatinised flour in the compositions is also impor-tant. If <30% of the flour is pre-gelatinised, thereformed rice products will have poor rehydratingcharacteristics. On the other hand, more than 70%proportion of pre-gelatinised starch will affect theextrusion characteristics and it will be difficult to control

the size and shape of the rice analogues (Harrow &Martin, 1982).Rice flour was replaced by soy flour in the ranges

between 60–100% and 40–100% by Kato (2006) andIchikawa & Chiharu (2007), respectively, for preparingthe dough. Soybeans are found to have very high proteinand fibre content and low carbohydrates content.Ichikawa & Chiharu (2007) developed a method ofproducing rice analogues comprising heating a startingmaterial, which contained from 15% to 35% by weightof gluten on the dry basis.

Water

Water added to the dough is one of the key factorscontrolling the properties of the final product. Lowwater content can lead to higher shear inside the barrelresulting in higher degree of gelatinisation. For prepar-ing the rice analogues, the dough can contain 12–25%water by weight (Dupart & Huber, 2003). If the amountof water is more than 25%, the mixture will stick in theextruder-cooker and if it is below 12%, there is toomuch mechanical energy during the extrusion-cookingand the mixture gelatinises totally. Preferably, themixture should contain 20% moisture just ahead ofventing zone (Wenger & Huber, 1988).

Colours

Use of the colouring agents is optional. If the desiredcolour of the final product is white, titanium whimwatercolour pigment can be used to brighten themixture. In case, the desired colour is other than white,an appropriate amount of non-toxic water colourpigment can be added to achieve the desired shade andcolour (Diehl, 1988).

Flavours

Albumin (from eggs) may be used for flavour, because itrestores a strong natural flavour to the rice. The amountof albumin should be approximately one-eighth of waterused in fluid binder. Other examples of flavouring agentsare curry, chilli powder, soya derivatives, salt, vanilla,ginger, pepper, thyme, saffron, sage, cinnamon, cloves,garlic and onion (Cox & Cox, 1993).Sodium chloride affects the taste and rehydration

properties of the reformed rice products. Higher amountwill detract from the cohesion promotion of algin as abinder and affect the taste considerations (Harrow &Martin, 1982).

Binders

The liquid binder mixed with the flour provides acohesive powder mixture forming rice analogues that

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will retain their shape well, despite wide variations inmoisture content and temperature.An algin such as sodium alginate can be used as a

binder, which may be either low viscosity type (0.1–1poise) or high viscosity type (8–20 poise). For lowviscosity type, the liquid binder may be principallywater, preferably containing 5–11% by weight of algin,and for high viscosity type of the binder, the watershould preferably contain from 0.5% to 5% of algin byweight (Cox & Cox, 1993). Hydrocolloids in an amountof 0.2–2.5% can be added to the mixture before theextrusion-cooking. This increases the quality of thetexture of the rice analogues. Alginates can be extractedfrom seaweed, such as Laminariahyperborea or Lamina-riadigita (Dupart & Huber, 2003).Other examples of binder material may be chitin

materials such as chitosan, chitosamine, chitose or otherchitin derivative from fungi and ⁄or crustacean shells,proteins, polysaccharides, acacia, casein, carrageenan,gelatins, methylcellulose, gums and low methoxypectins.The gum acts as a binding agent, which facilitatesrehydration and maintains the shape of the rehydratedproduct. Suitable gums include xanthan gum, guar gum,carrageenan, alginates and mixtures thereof (Sceliaet al., 1986).

Setting agents

The purpose of setting agents is to set or gel thebinder used in the dough. An example of the settingagent is a mixture composed of calcium lactate(62.5%) and calcium chloride (37.5%) by weight,within the range of 0.01–20% of the flour by weight.Such solution can either be sprayed onto the extrudedkernels or the kernels may be deposited in a settingbath of boiling acidic aqueous solution having a pHof 4–6, in order to keep the calcium in solution. Acidssuch as lactic acid or adipic acid can be used toproduce such acidity. To retard or prolong the effectof the setting or gelling agent, sodium carbonateNa2CO3, sodium citrate Na3C6H5O7, disodium phos-phate Na2HPO4, trisodium phosphate Na3PO4,sodium hexametaphosphate (NaPO3)6, tetrasodiumpyrophosphate Na4P2O7, sodium polyphosphateNan+2PnO3n+1 or sodium tripolyphosphate Na5P3O10

can be included in the liquid binder (Cox & Cox,1993).Cross-linking agents play a major role in the manu-

facture of foods to thicken, stabilise and provide texture(Hirsch & Kokini, 2002). Heat may be employed as across-linking agent. When heat is not the cross-linkingagent, the extrudates are wetted or dusted with cross-linking agents such as edible aldehydes, polyvalentcations, calcium sources (dicalcium phosphate, calciumchloride or calcium lactate), hydrochloric acid, volatileacids, coacervates, protein, ammonia, gelatin, albumin,

glyceraldehyde, oxazolidine and alum (Cox & Cox,1997).

Fortificants

The fortificants (mineral-vitamin mix) may be used inthe rice analogues in an amount of 0.1–5% weight of thefinal composition. The examples of fortifications arevitamin A, vitamin B1, folic acid, Niacin, vitamin B12,vitamin B2, vitamin E, vitamin C, biotin, pantothenates,vitamin K and derivatives thereof, as well as mineralsand trace elements such as iron, selenium, zinc andcalcium. The micronutrients are usually added in apowdery form, but oily vitamins like vitamin A orvitamin E may also be used as oils (Steiger, 2010).PATH (2008) has reported development of micronu-

trient concentrations in ‘Ultra Rice’. The following fourfactors are described for determining micronutrientconcentrations.• Targeted percentage of local recommended dietary

intake,

• Typical rice serving size and expected daily consump-

tion,

• Planned ratio of Ultra Rice grains to regular rice

according to the needs of the target population and

• Most practical micronutrient density in the rice ana-

logues (because there is a limit to the amount of

micronutrient that can be put into a manufactured rice

grain).For fortification of rice analogues with iron, PATH

(2008) recommended the use of ferric pyrophosphate(FePP) compounds having 24–26% iron, with anaverage particle size of three microns. FePP is manu-factured with such small particle size (and thus highsurface area) that it has better bioavailability than mostnon-soluble forms of iron. Soluble iron salts impartgreyish or brownish colouring to the rice analogues, butFePP imparts little if any colour. Certain other ironsalts also convey a slight taste that some consum-ers do not like, whereas FePP has no taste (Li et al.,2008).

Antioxidants

The purpose of antioxidants is to avoid oxidation ofvitamins in the rice analogues. Examples of antioxidantsare ascorbic acid, alpha-tocopherol, butylatedhydroxy-anisole (BHA), sodium bisulphite, potassium bisulphiteand gallate esters. A moisture barrier agent is also addedto the flour that prevents the vitamin from migrating tothe surface of the finished product where it wouldundergo oxidation. The moisture barrier may be lard,coconut fat, palm oil, saturated oil or stearates. Unsat-urated oils should be avoided because they attract freeoxygen (Cox & Cox, 1997).

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Lubricants

For better results, mechanical shear needs to becontrolled during extrusion. The mixture can contain2–11% oil, with respect to the total weight of the riceflour to decrease the friction during the extrusion-cooking and to control the water absorption of thereconstituted rice grains. Coconut oil, soybean oil,cottonseed oil, palm oil or palm kernel oil can be usedfor this purpose. If too much oil is added in the mixture,the rice analogues do not absorb water very well duringrehydration (Dupart et al., 2003).

Process for preparing rice analogues

In general, all the processes developed so far for thepreparation of rice analogues follow some or all thebasic steps of formation of dough comprising theprincipal ingredients and the additives; feeding thisdough into an extruder; extruding the dough for a timeand at a temperature and pressure effective to substan-tially gelatinise the starch fraction of the rice flour anddenature the protein (for hot extrusion) or simplypassing the dough through the extruder at lowertemperature to give it a proper shape (for cold extru-sion); treating the freshly extruded rice analogues withbinder setting or cross-linking agents so that they canretain their desired properties for a prolonged time; andfinally drying these analogues to a desired moisturecontent. Various approaches have been used byresearchers to produce rice analogues, some of whichhave been mentioned in this section, which can be usedin different combinations. While dough formation andextrusion are mandatory steps, other steps may or maynot be followed, based on the requirements.

Dough formation and pre-conditioning

The rice flour or flour of some other seed is firstprepared by pulverising the grains. Better results wereobtained with the rice powder having particles smallenough to pass through a No. 10 US standard meshscreen but which would be retained on a No. 300 USstandard mesh screen (Cox & Cox, 1993). The rice flourand water were mixed and kneaded to prepare homog-enous dough. Water was added under low heat to matrixmaterial in an appropriate amount. Heat was thenturned off and pigments were added for desired colourand shade (Harrow & Martin, 1982; Scelia et al., 1986;Diehl, 1988). The rice flour may be mixed with fortifi-cations. One or more binders and vitamin antioxidantand a moisture barrier agent were added to the rice flour(Cox & Cox, 1997).While matrix material and emulsifier are mixed before

wetting to produce the dough, the micronutrients areintroduced into the dough after pre-conditioning

(Steiger, 2010). The purpose of pre-conditioning is touniformly mix or knead the liquids, steam or othervapours with the pretempered ingredient mixture (Riaz,2000).The matrix material can, either before or after being

pulverised, be heated in a suitable dryer at 80–90 �C.After cooling down, the pre-treated matrix material washydrated until a water content of 15–40% by weight isachieved. Furthermore an emulsifier and the micronu-trients were added during the hydration step (Steiger,2010).The hydrated mixture was exposed to shear force, for

example, kneaded, to form a paste-like mixture with asimultaneous heat treatment at 70–100 �C for no morethan 5 min. Heating can be accomplished by an externalheating source or by introducing steam during theprocess of producing the paste-like mixture (Steiger,2010).

Extrusion

Extrusion-cooking is not only a heating procedure but itis also a process with shear force and pressure thatresults in destruction and degradation of rice starch.Principally, two types of extruders can be used for thepreparation of rice analogues, single-screw or twin-screw extruder. In most of the cases, twin-screwextruders have been employed because they providebetter conveying capabilities than single-screw extruders(Harper, 1981). Most of the improvements that haveevolved in the development of extruders have beenincorporated into modern twin-screw extruders (Riaz,2000). Wenger & Huber (1988) used an extruder, thebarrel of which comprised of a cooking zone, a ventingzone, a forming zone and an extrusion die sequentially.In the cooking section of the extruder, the dough is

subjected to extrusion temperatures effective to substan-tially gelatinise the starch molecules and denature theprotein (Scelia et al., 1986). The reaction energy requiredfor starch gelatinisation and protein denaturation can becalculated from equations 1 & 2 (Riaz, 2000).

Qsg ¼ me � Xce � Xsgout � Xsgin

� �� DHo

sg; ð1Þ

Qpd ¼ me � Xpe � Xpdout � Xpdin

� �� DHo

pd: ð2Þ

where, Qsgand Qpdare energy rates (kJ h)1) consumed bystarch gelatinisation and protein denaturation, respec-tively, meis total mass flow rate of material in extruder(kg h)1), Xceand Xpe are mass fractions of carbohydrateand protein, respectively, in extruder barrel, Xsgout andXsgin are mass fractions of gelatinised starch out and in,respectively, Xpdout and Xpdin are mass fractions ofdenatured protein out and in, respectively, DHo

sg andDH o

pd are heat of reaction (kJ kg)1) for starch gelatin-isation and protein denaturation, respectively.




In the case of rice analogues, gelatinisation of starch isvery important, because it helps in achieving a quick-cooking, non-expanded product. It is important tooptimise the processing conditions in the barrel formonitoring gelatinisation (Akdogan, 1999). The criticalparameters that directly influence the gelatinisation andfinal product quality are actual moisture involved in theprocess, mechanical energy input, thermal energy inputand residence time (retention time) of the material in theextruder barrel. Higher feed water content and higherbarrel temperature are two important parameters thataccount for physicochemical properties of extrudatesand increase starch gelatinisation (Eun et al., 2000),whereas extruder screw speed has a complicated effecton these properties of extrudates (Zhuang et al., 2010).The description of these parameters for twin-screwextruders is given in Table 1. The rice extrudates afterhot air dehydration (100 �C, 1 h) and rehydration areshown in Fig. 1. Table 2 gives an understanding aboutthe relationship between independent variables andcritical parameters for twin-screw extruders. Gelatinisa-tion should be preferred only when rice analogues willcompletely replace regular rice in the meal.The higher temperature will lead to higher degree of

gelatinisation of starch (Zhuang et al., 2010). Theheating temperature in the extruder was taken in thevarious ranges (30–150 �C) by various researchers(Scelia et al., 1986; Wenger & Huber, 1988; Koideet al., 1999; Dupart & Huber, 2003; Ichikawa &Chiharu, 2007; Steiger, 2010). At 80 �C, the percentageof pre-gelatinisation in the produced fabricated riceamounts to 50–60%, and at 120 �C, it goes up to 90%or more (Koide et al., 1999).

Besides temperature and pressure, the screw speed willalso affect the length of time the composition remains inthe extruder. When extruder screw speed is lower,

Table 1 Critical parameters in extrusion (reprinted with permission

from Riaz, 2000)

Parameter Description

Moisture Actual moisture in the process

Mechanical energy input

GME = Gross

Mechanical Energy

GME ¼ Power ðkWhÞMass flow rate ðkgÞ

SME = Specific

Mechanical Energy

SME ¼ Power loaded ðkWhÞ�Power empty ðkWhÞMass flow rate ðkgÞ

Thermal energy input For heating the extruder barrel

Expressed in same energy

units as mechanical energy

units = kJ ⁄ kg or kWh ⁄ kg

Thermal fluids

Steam heat

Electrical heat

For direct heating of the extrudates

Direct steam injection

Other liquid or vapour injection

Retention time Total time in each part of the process

T = (M ⁄ m)T = Average retention time

M = Amount of extrudate

in the process

m = Mass flow rate

(b)

(c)

(a)

Figure 1 Photos of (a) rice extrudates, (b) rice extrudates after dehy-

dration and (c) rice extrudates after rehydration (Zhuang et al., 2010).




relatively looser structure is formed inside extrudates.When extruder screw speed is higher, retention time isshortened, and as a result, degree of gelatinisation offeed is lower and inhomogeneous mixing exists, so thereis a relatively denser structure inside extrudates. Withextruder screw speed increasing, hardness, springinessand chewiness of rice extrudates increase, but adhesive-ness has no regular change (Zhuang et al., 2010). Thepressure inside the extruder was between about 1400 and8000 kPa (Wenger & Huber, 1988), 2000 and 5000 kPa(Dupart & Huber, 2003; Kato, 2006), depending on theingredients contained therein. Venting may be providedto release a significant amount of gaseous products fromthe mixture and cooling the mixture while passing fromcooking zone to forming zone and thus to control theamount of shear that would otherwise be imposed uponthe food material (Wenger & Huber, 1988). Steam thatexpands at the end of the die results in porosity. Thehigh porosity ensures quick rehydration characteristicsof the rice analogues (Dupart et al., 2003).In their research, Wenger & Huber (1988) and Dupart

& Huber (2003) used a venting zone equipped with adevice with a conical housing connected to a vent and ascrew was provided with flights for biasing any portionof the rice-water mixture entering the housing backtoward the extruder in a direction opposite the flow ofthe vented gases. The extruded material undergoing alow shear and low moisture content in the end ensuresbetter tolerance to overcooking.

Shape

The appearance of the rice analogue should be like thatof natural rice kernel. For this purpose, the importanceof a suitable cutter-die is inevitable. Two approachescan be used to cut the extrudates: length-wise (Diehl,1988) or width-wise (Harrow & Martin, 1982; Steiger,

2010). To avoid chain formation, that is, the stickingtogether of the cut extruded simulated rice grains, knifeblades coated with a synthetic polymer such as polytet-rafluoroethylene can be optionally used (Harrow &Martin, 1982).Diehl (1988) forced the extruded material through a

row of holes 2 mm in diameter. Pieces were cut by anautomated blade at 45� angle upon each 4-mm intervalof conveyer movement. Thus, a finished piece havingdimensions approximating 6 mm in length and 2 mm indiameter was prepared, with each piece having a taperedend.The strands leaving the extruder were adjusted to a

diameter similar to that of rice grains and were cut intopieces to the size of rice grains (Steiger, 2010). Harrow &Martin (1982) forced the extruded material through adie having elliptical apertures, with a long axis length of4–8 mm, and a short axis length of 0.5–3 mm. Thedough was then cut as it was extruded by a rotatingcutting knife adjacent to the outer face of the die. As thequick rehydration of the reformed rice product isdependent on the particle size, this was ensured thatthe cut thickness of the simulate rice grains as extrudedwere from 0.5 to 3 mm.

Post-extrusion treatment

If cold extrusion is used, the rice analogues can be setmainly by surface spraying with a suitable metallic saltsetting agent ⁄ cross-linking agent, such as a calcium salt.In some of the processes, rice analogues were submergedin boiling water containing salt setting agent ⁄ cross-linking agent for 3–20 min and the expanded analogueswere rinsed so as to remove excess setting agent and tocool the rice analogues. Then these expanded kernelswere retrograded by freezing to set the product in itsexpanded state (Cox & Cox, 1993).

Drying

Drying of the analogues (both cold and hot extruded) tooptimum moisture content is necessary to increase theshelf life. The rice analogues were dried to the moisturecontent of 4–15% (Harrow & Martin, 1982; Wenger &Huber, 1988; Cox & Cox, 1993; Kato, 2006; and Steiger,2010). The drying can be done in a suitable dryer, orsimply air drying can be applied (Kato, 2006). Examplesof dryer are fluidised bed dryer (Steiger, 2010), traydryer, conveyor belt-type dryer, tumble dryer androtating cylinder dryer (Kato, 2006). Diehl (1988) driedthe extrudates under conditions of naturally varyingambient humidity for 24 h before containerising. Thefinal product should be packaged in air and moisturetight bags to prevent moulding. Harrow & Martin(1982) dried the extruded rice analogues by air drying ormicrowave heating, at an elevated temperature from

Table 2 General interaction table for independent variables and

critical parameters (reprinted with permission from Riaz, 2000)

Independent

processing

variables

Critical parameters

Moisture

Mechanical

energy

Thermal

energy

Retention

time

Feed rate ) ) ++ ) )Lipid · ) · +

Moisture +++ ) ) ) + ++

Steam energy + ) ) +++ +

Extruder speed · + ) )Barrel

temperature

· ) + +

Extruder flow

resistance

· ++ * +

Die open area · ) ) )

·, no change; +, increase; ), decrease; *, not available.




about 130–150 �C, but no higher to avoid charring ofthe end product. Air drying was performed in a fluidisedbed air drier.

Structure, nutritional and sensory characteristicsof rice analogues

Zhuang et al. (2010) found out that structure of theirrice analogues achieved at barrel temperatures 30 and50 �C was much denser than that achieved at 70 �C(Fig. 2); however, the bulk density of extrudates variedinsignificantly with the change in temperatures 30 and50 �C. It was also evident that structure of the extru-dates became denser as the feed moisture contentincreased from 28% to 36%. When extruder screwspeed was lower, relatively looser structure was formedinside extrudates. With a higher extruder screw speed,retention time was shortened, and degree of gelatinisa-tion of feed was lower and inhomogeneous mixingexisted, so there was a relatively denser structure insideextrudates. But from the overall view of their pictures,there was no significant difference on the microstructure,which indicated that bulk density of these extrudateswould also have no significant difference.Fortification of rice with iron using extrusion is an

efficacious strategy for preventing iron deficiency (Hotzet al., 2008; Li et al., 2008). In a study on multiplefortified rice analogues to develop a stable premixcontaining iron, zinc and B vitamins, Li et al. (2008)tested the performances of four iron sources underaccelerated storage conditions (40 �C, 60% RH) for32 weeks and found that formulations containing FePPshowed minimal losses of thiamin and had goodsensory ⁄physical properties; formulations containingferrous fumarate lost more than 50% thiamin, whilethose containing FeNaEDTA showed minimal loss ofthiamin, but developed the most rancidity. FeNaEDTAand ferrous fumarate gave darker-coloured grains, butresulted in much higher in vitro bioavailability than theformulations containing FePP. The choice of ironcompound had no or little effect on moisture contentand pH of the formulation during storage.Bett-Garber et al. (2004) determined the impact of

various iron sources on the flavour of rice analogues.The rice analogues fortified with one of the two ironsources (elemental iron and FeSO4), zinc, thiamin, andfolic acid was mixed with natural rice at different ratios.Their results showed that intensities of water-like, sourtaste, hay-like, musty and bean flavours were enhancedby the addition of ferrous sulphate, ferrous sulphateplus multiple fortificants and multiple fortificants with-out iron at the ratio of 1:50. Astringent mouth-feel wasaffected by ferrous sulphate and ferrous sulphate plusmultiple fortificants.Lee et al. (2000) tested natural rice fortified with

micronutrients-enriched rice analogues containing retinyl

palmitate (RP) at the ratio of 1:99 (weight basis) forvitamin A delivery. They studied the stability of RP inthe rice mixture during cooking and storage for a periodof six months. After cooking, the RP was present from75% to 87% depending upon the method of cooking. Itwas found that the stability of RP in the rice was moreaffected by temperature than relative humidity (RH).Therefore, they recommended that under tropical

(b)

(c)

(a)

Figure 2 Scanning electron microscope photos of hybrid indica rice

extrudates processed at different barrel temperature (a) 30 �C, (b)50 �C and (c) 70 �C. Magnification: ·600 (Zhuang et al., 2010).




conditions, rice fortified with RP should not be storedfor long periods at high temperatures to avoid signifi-cant RP losses.Murphy et al. (1992) used a rice analogue premix for

vitamin A fortification to test for stability after washingand cooking, and for long-term chemical stability of thevitamin A. They conducted accelerated storage studiesat various temperatures and water activities (aw) toevaluate the effect of different formulation ingredientson the stability of vitamin A. Combinations of moresaturated oils, tocopherols and ascorbate made vitaminA more stable. Washing tests showed that all of thevitamin A (100%) was retained after washing; cookingtests resulted in complete absorption of cooking waterby the rice analogues. It was also found out thatunsaturated lipids enhance the stability of vitamin A.Murphy (1996) found that the type of lipid andantioxidants affected the chemical stability of vitaminA in the rice analogues in the tropical temperature andhumidity. It was also determined in their study thatvarious antioxidants as a combination maximised vita-min A stability in such conditions.

Conclusions

Literature on development of rice analogues fromextrusion and studies based on these analogues has beenreviewed and presented. The materials and processparameters adopted so far for the preparation of riceanalogues using extrusion technology have been dis-cussed. The materials were categorised into two groups:principal components and additives. The principal com-ponents include rice and water. The additives are acombination of some or all of colouring agents, flavour-ing agents, binders, binder setting agents, lubricants,fortificants and antioxidants. The process comprises ofthe basic steps of formation of dough, extruding thedough at suitable temperature and pressure to gelatinisethe starch and denature the protein, treating the extru-dates with binder setting or cross-linking agents, andfinally drying these analogues to desired moisture con-tent. Various studies have suggested that rice analoguesare potentially very useful means of improving nutritionin rice-consuming societies.

Acknowledgments

This work was financially supported by Department ofBiotechnology, Government of India. The first authorwould like to dedicate this article to the memories of hisdear father.

References

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preparation of rice analogues using extrusion technology

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