texture attributesof glutinous rice cakes (mochi)

Mochi, a dough-like product and one kind of glutinous rice cake, exhibits unique viscoelastic property, which attracts consumers
attention in Oriental areas, such as Taiwan, China, and Japan. The product, generally consists of high moisture, is soft and slightly sticky with some elasticity and serves as a dessert. Conventionally, glutinous rice (Oryza sativa), also called waxy or sweet rice, is washed, cooked with water, and pounded right after cooking to lose the integrity of rice kernel and to form the viscoelasticity. Cooking method, moisture content (Horiuchi, 1980), and pounding affected the product texture. A product texture can be evaluated by physicochemical properties measured objectively and by sensory methods, which are generally subjective. The relationship between consumer preferences and rheological properties of foods is a key part of the science of rheology. Rheology is of considerable importance in the manufacture of various foods as it influences the machinability, processing conditions, and quality of products. Different methods, such as dynamic oscillatory measurement, creep test, stress relaxation, viscoelastograph, etc., have been used to study the viscoelasticity of food. The characterization of viscoelasticity still remains a challenge (Menjivar, 1990). Exhaustive literatures are available on the rheology of bread dough regarding the methods of measurement, optimum rheological parameters needed, and the interrelationship between rheological parameters and bread quality

 

Varavinit  Shobsngob (2000) have compared the viscosity of rice flour and wheat flour by using Rapid Visco Analyzer (RVA) to show that rice flour exhibited higher peak viscosity and setback than wheat flour. Dynamic oscillatory measurements illustrated a peak storage modulus occurred at the gelatinization temperature of rice starch. At water content of 45–55%, increasing the specific mechanical energy resulted in greater shear modulus (G0 ) of waxy rice extrudates (Chuang & Yeh, 2002). With a loading of 50 Pa, short time (60 s) creep data were used to select suitable rice flour for rice bread (Sivaramakrishnan, Senge, & Chattopadhay, 2004). However, the information on the interrelationship between various rheological characteristics and the eating quality of mochi is limited. The understanding of the rheological characteristics would be helpful for designing process and developing new products. Texture is defined as the attributes of a substance resulting from a combination of physical properties and perceived as the senses of touch, sight and hearing and the evaluation of the foods texture is driven in the cause of mastication (Jowitt, 1974). Texture profile analysis has been shown to have fair correlations with RVA measurements for cooked rice (Champagne et al., 1999).

 

Manohar and Rao (2002) pointed out the importance of the interrelationship between rheological characteristics of dough and quality of biscuits. Genc, Zorba, and Ova (2002) found that the consistency coefficient of boza could be used as a predictor of sensory scores of mouthfeel and appearance. The correlations between the sensory evaluation and rheological properties would be helpful for understanding viscoelastic characteristics of mochi and for developing new products. However, the information on the relationship between physicochemical and sensory properties of foods is limited in spite of the importance of such information in assessing food quality (Karel, 1996). The objectives of this paper were: (1) to determine relationships between the dynamic oscillatory measurements, creep test, and TPA; (2) to conduct sensory evaluation on the mochi and to investigate the correlation between sensory evaluation and rheological characteristics. 2. Materials and methods 2.1.

 

Commercial mochi Fifteen commercial mochi collected from different makers were used in this study. To our knowledge, all the commercial mochi were prepared by the traditional method using waxy rice as the major raw material. There existed some differences in the detail formulations which were unknown to us. Thus, the discussion on the formulation or processing was not addressed in this report. The mochi was purchased and immediately sealed in a double-layered PVC bag to minimize moisture loss. The water content of the commercial mochi was determined in duplicate using the AOAC (1984) method. To minimize the influence of starch retrogradation, the product properties (sensory and rheological characteristics) were evaluated in 24 h after the purchase.

 

The mochi purchased were categorized into three groups in terms of sweetness. Most of the products collected were sweet due to the addition of sugar and designated as sweet mochi. These products had the water content ranged from 18% to 39% (Table 2), which were much lower than 55% suggested by Horiuchi (1980). Three products were slightly sweet and had narrow water content range of 40–42%. The non-sweet products, designated as plain mochi, had the water content ranged from 43% to 55%. When the product was sweet, sugar could act as a plasticizer and softened the products. Thus sweet mochi did not require as high moisture as plain mochi.