measuring the eating quality of rice

7/27/2019 Measuring the Eating Quality of Rice

1/4

Presented at the 28th International Rice Research Conference, 8-12 November 2010, Hanoi, Vietnam

OP10: Quality Grain, Health, and Nutrition

Measuring the eating quality of rice

Harold CorkeSchool of Biological Sciences, The University of Hong Kong, Pokfulam Road, HongKong, email: [email protected].

Wide diversity exists in consumer preferences for rice eating quality. Different countries,regions, cultures, and end-uses may all demand different but very exacting qualityspecifications. P lant breeders have the difficult task of making genetic selections basedusually on a combination of physicochemical tests and genetic markers in order todevelop varieties that will satisfy the needs of the intended markets. It is important todevelop a dialogue between food scientists and plant breeders. Food scientists shouldact as the voice of the consumer, translating consumer sensory preferences intoobjective dimensions that can be measured by instrumental techniques. Plant breedersshould understand the relative importance of the various quality criteria, and work to

establish reliable genetic marker-facilitated selection protocols. I will discuss the majorquality criteria for rice, emphasizing the significance of amylose content, gelatinizationtemperature, viscoamylography, texture profile analysis, and various screening testssuch as gel consistency. The relationship of such tests to the complexities of humansensory perception will be discussed. I hope that, through open communication, plantbreeders can arrive at simplified protocols to facilitate the selection of rice cultivars thatcan better target consumer needs.

BackgroundRice products are diverse and rice preferences are diverse. Predictive tests may oftenpredict traits that do not in fact relate strongly to sensory properties. Imitative tests may

use an oversimplified and non-representative system. Sensory perception takes placenot in a simple starch-in-water system in the lab, but in a more complex environmentinvolving salt, saliva, teeth, tongue, and memories.

The question is: What do we need to know to select a genotype, based only onmolecular genetic markers, in the near certainty that it will taste exactly as a specifiedconsumer wishes it to taste? The complicating effects of environment, genotype environment interactions, and measurement errors must be minimized. The purpose ofthis paper is to comment on some commonly used quality parameter measurements inrice (focusing on starch-related traits), and raise discussion and questions about theirrelevance in a new era of molecular marker-assisted selection. Perhaps more questionswill be raised than can be answered at this stage of progress in rice technology. Many

underlying issues exist, related to the difficulties inherent in establishing clear,measurable indices for the sensory perception behind eating quality. Traditionally,breeders have required rapid screening tests for quality, such as the alkali spreadingvalue or the gel consistency test. These have to be continually retested in successivegenerations of segregation and selection, so high throughput may seem to be asimportant as the fundamental parameter being measured. However, we do not eat agel consistency test, and such a test measures gel behavior in a rather crude wayindependent of the evaluative processes of the human sensory system. Among other

1


2/4



factors, gel texture analysis by the human consumer is1. Subjectivepreferences differ among cultural groups.2. Mediated through a complex bite procedure involving the geometry of the teeth

and mouth, and the action of the tongue.3. Highly influenced by the incorporation of saliva.

4. Controlled by the brain!

Starch physical propertiesFurther advances in rapid, detailed characterization of the molecular structure of starch(amylose molecular weight and amylopectin size, branching, and organization) have thepotential to dramatically enhance selection for specific starch properties. Hopefully, thephysical properties of starch granules as processed into food systems will prove to behighly predictable from detailed starch structure information. Then, with clear moleculargenetic markers indicative of starch structure, and with knowledge of how it Isexpressed in different environments, we will be able to quickly identify lines with anydesired eating quality.

Gelatinization temperatureThe time required for cooking rice is determined by its gelatinization temperature.Gelatinization temperature is the temperature at which the rice absorbs water andstarch granules swell irreversibly. The alkali spreading test is a convenient rapid test forgelatinization temperature due to its simplicity and small sample required. It must beemphasized that we have no intrinsic need or desire to know the ASV; it is a proxy forreal gelatinization temperature, which in turn is a proxy for exact information on the timeand energy required to cook a given sample to a given degree. One approach has beento develop rapid methods, such as NIR, to predict other predictive tests, such as ASV(Bao et al 2007). This may be a reasonable intermediate approach until big enough data

sets on more fundamental properties can be developed, and genetic markers can beshown to do a satisfactory job of predicting these properties.The best fundamentally based method for determining GT is with differential

scanning calorimetry (DSC). DSC has the added advantage of providing moreinformation than a crude index of gelatinization peak temperature. It provides detailedinformation on onset, peak, and conclusion temperatures, and on enthalpy or energy ofmelting of the starch crystallites. Our relatively early molecular marker-based study ofthe quantitative genetic basis of these components of GT (Tan et al 2001a,b) identifiedeight QTLs mapped onto chromosome regions where starch synthesis-related genesare located, for example, granule-bound starch synthase. DSC studies can also be usedto provide data on retrogradation, which can be built into a model for eating quality.

Cuevas et al (2010) have made a major contribution toward advancingunderstanding of GT on a suitably large germplasm collection. They reviewed thecontradictory nature of the evidence for validity of various measures that purport toindicate cooking time, and emphasized the importance of a well-established validatedtest for this trait. They also showed that chain-length distribution of amylopectin wasrelated to variation in GT within a range determined by distribution of chain lengthssynthesized by starch synthase IIa. This is an approach that must be pursued in muchmore depth.

2


3/4



Gel textureGel consistency is a crude measure of an important aspect of the eating quality of rice.There are better instrumental measures of gel behavior, such as with the Rapid ViscoAnalyser (RVA). Typically, a 10% starch-in-water mixture will be made in an aluminumcanister. The suspension is heated, maintained at high temperature, and cooled over a

programmed cycle (typically 22 minutes). During the cycle, the suspension is stirred athigh speed with a plastic paddle, and resistance to stirring (viscosity) is measured. First,with gelatinization, the viscosity rises to a peak. With continued shear but at constanttemperature, there is a breakdown of viscosity. As the sample cools, the effect oftemperature (increased viscosity) overcomes the tendency of stirring to cause shear-thinning, so the viscosity rises (setback). However, this is still a predictive test, althoughit has advantages of repeatability, ease of use (despite the initial expense of theequipment), and measurement in fundamental units. The various resultant parameters(peak viscosity, breakdown, hot paste viscosity, setback, cold paste viscosity) can beused in various ways to predict different aspects of gel texture or eating quality.

Another approach to measuring gel texture is with texture profile analysis using

an Instron-type instrument. Experimentally, this can be carried out on a gel made fromrice flour (such as by using the gel in the RVA canister after viscosity analysis). A textureprofile can also be made directly on cooked rice using a texturometer double-bitetechnique simulating chewing action. Textural attributes such as hardness,adhesiveness, stickiness, cohesiveness, and springiness are computed from the graphgenerated from the texture analyzer.

These approaches have two fundamental weaknesses: (1) the geometry of thesystem is not directly comparable to the bite/mastication action of the human mouth and(2) a rice-water system is not similar to a rice-saliva system, usually in the presence ofsalt.

Amylose contentRice is classified based on amylose content. Varieties with 12% amylose content arewaxy, those with 1020% amylose content are classified as low-amylose varieties,those with 2025% as intermediate-amylose varieties, and those with 2533% orgreater are classified as high-amylose varieties. Many of the properties of rice starchthat determine a cultivars suitability for a particular end-use are dependent on theiramylose/amylopectin ratios.

However, we all know that the measurement of amylose content is not so simple.Amylose and amylopectin are complex and variable molecules, and the degree ofbranching and molecular organization can affect measured amylose content. Basically,amylose content (usually reported as apparent amylose content, AAC) is no longer asufficient indicator of starch chemical structure. The predictive ability of AAC by itself forany given physical trait is simply not high enough.

General commentsPredictive methods have served an invaluable role for rapid screening at the breedinglevel, handling large numbers of samples. Major disadvantages are sensitivity toenvironment and measurement error, and a lack of fundamental relationship to thecomplex desired characteristics that make up eating quality.

3


4/4



4

What of the future?1. Can we establish molecular markers for fundamental structural properties of

starch, such as true amylose content and molecular weight, chain-length profilesof amylopectin, etc.?

2. Can we define sensory quality in terms of starch molecular architecture?

3. Can we further develop molecular markers for sensory-based physical testing ofeating quality traits?

4. Can we cluster sensory preferences into a limited number of discrete groups (say,about six) that will serve the majority (say, more than 95%) of consumers?

5. Can we develop rapid molecular marker-based protocols that can efficiently andquickly convert all significant new germplasm into each of these groups?

AcknowledgmentsI thank Drs. Lilia Collado, J insong Bao, and Yifang Tan for helpful discussions on ricequality.

ReferencesBao J , Sen Y, J in L. 2007. Determination of thermal and retrogradation properties of rice

starch using near-infrared spectroscopy.J . Cereal Sci. 46:75-81.Cuevas RP, Daygon VD, Corpuz HM, Reinke RF, Waters DLE, Fitzgerald MA. 2010.

Melting the secrets of gelatinisation temperature in rice. Funct. Plant Biol.37:439-447.

Tan YF, Sun M, Xing YZ, Hua J P, Sun XL, Zhang Q, Corke H. 2001a. Mappingquantitative traits for milling quality, protein content and color characteristics ofrice using a recombinant inbred line population derived from an elite rice hybrid.Theor. Appl. Genet. 103:1037-1045.

Tan YF, Xing YZ, Zhang Q, Corke H. 2001b. Quantitative genetic basis of gelatinization

temperature of rice. Cereal Chem. 78:666-674.
http://www.sciencedirect.com/science/journal/07335210http://www.sciencedirect.com/science/journal/07335210

measuring the eating quality of rice

Documents