Study on Korean Traditional Rice Snack ( Yukwa ) Manufacturing Process

Yukwa is a Korean traditional oil-puffed snack made of waxy rice (Oryza sativa). It is reviewed in this report outlining its traditional process as well as any significant scientific and technological steps to develop it to the status of modern processed snack. The stages of traditional process are discussed and include the steeping duration of waxy rice, the punching time and the drying and frying conditions which are necessary for obtaining the desirable quality features of Yukwa. Through controlling the steeping conditions, punching time, moisture content of Bandegi and expansion methods, we can get preference Yukwa product. Furthermore, some additives such as green tea, soy bean, rice wine and safflower were mixed with waxy rice to improve Yukwa quality and extend shelf life.


INTRODUCTION
Yukwa is a Korean traditional oil-puffed snack made of waxy rice (Oryza sativa).It has long been consumed as a popular snack due to its soft texture and unique taste and has become a factory-made product due to its large increase in consumption (Chun et al., 2004).Yukwa, is recorded in the book Gao-Zao-Li-So during the era of the Zaocen Dynasty in 1809 and this recordation indicated that Yukwa was necessary for the obsequies, wedding and banquet in Korea (Park et al., 2001).And now Yukwa has become daily and popular food for people and there are about 200 Yukwa factories in Korea (Kim et al., 2004).
Yukwa is made from only ground waxy rice of low amylose content (Sin et al., 1991).And the waxy rice must be steeped in water for a prolonged period time to produce superior texture and microscopic cell structure.On the basis of traditional manufacture, some additives such as rice wine (Park et al., 1995), soy bean (Shin et al., 1990a), green tea powder (Kim and Kim, 2001), safflower (Park, 2004) and so on are mixed with waxy rice to make more desirable properties of Yukwa.However, it is necessary to study on optimum conditions for procedure of Yukwa.
The traditional lengthy steeping process is still used in South Korea, based on years of experience.Furthermore, in recent years, there are more advanced researches on Yukwa.These researches are thus: More studies were carried out for the quality improvement and standardization of manufacturing process for Yukwa (Shin and Choi, 1993;Jeon et al., 1995;Kim, 1998;Choi et al., 2000;Kang and Ryu, 2002a).Physicochemical properties of hydroxypropylated waxy rice starches and its application to Yukwa were investigated (Park et al., 1995;Lee et al., 2001;Yu et al., 2006Yu et al., , 2007)).Mixing of non-waxy rice and waxy rice was attempted in Yukwa preparation (Shin et al., 1991) and different waxy rice cultivars were compared for quality characteristics of Yukwa (Choi and Kang, 2000;Kang and Sung, 2000).Effects of steeping and mixing time on mixing energy input influenced Yukwa quality (Kang et al., 2001).Some additives were applied to improve quality and process modifications (Shin et al., 1990a;Park et al., 2001Park et al., , 2008)).Effect of microorganism inoculation and enzyme treatment were studied on Yukwa characteristics (Park et al., 2000a;Chun et al., 2004).Changes of Korean traditional Yukwa color, texture and volatile flavor compounds and characteristics during storage were investigated and methods for extending shelf life were also studied (Park et al., 2000a;Kum et al., 2001;Yoo, 2007).
The aim of this study is to present the traditional manufacturing process of Korean traditional snack Yukwa and to discuss significant scientific or technological steps for modifications of traditional process.Furthermore, we also aim to improve or advance the development of Yukwa.

Manufacturing process:
The main procedures for Yukwa's traditional manufacture are showed in Fig. 1 (Kang and Ryu, 2002b).The quality of Yukwa depends primarily on the process variables, including the steeping duration of waxy rice, the punching time and the drying and frying conditions (Chun et al., 2004).

Steeping:
Rice is soaking in cold, warm or hot water for a substantial period of time, until the rice kernels have increased their moisture content, generally to at least above 20%; is to hydrate the kernel sufficiently to (Yukwa) (Kang and Ryu, 2002) enable it to be gelatinized on subsequent heating (Li and Luh, 1980;Juliano, 1985).The water enters the individual starch granules within the rice kernels and causes them to swell, then bursts releasing free molecular starch and yielding a very starchy, pasty feel in the mouth.But the optimum soaking condition has to be selected (Juliano, 1985).
According to Kang and Ryu (2002a), steeping of waxy rice at 15°C for 3 days was required to equilibrate moisture absorption in waxy rice kernel.However, steeping for more than 6 days was required soft texture and small air cell distribution of Yukwa.Meanwhile, Lee et al. (2002) reported that the desirable quality features of Yukwa was steeping waxy rice at 20°C for 7 days and Jeon et al. (1995) reported that it is to steep waxy rice at 15°C for 15 days.Even though, expansion rate and crispness of Yukwa prepared by steeping waxy rice for 24 h was satisfactory (Choi et al., 2000).
In a previous study, Yukwa made from waxy rice with longer steeping periods was found to have desirable volume expansion, mechanical and sensory textural properties as the steeping period of waxy rice changed from 0 to 15 days (Chun et al., 2002).Additionally, contents of sugar, protein, lipid and minerals of raw waxy rice were found to decrease and the volatile organic compounds were detected in steeping medium by increasing the steeping time (Lee et al., 2001).These facts implied that natural fermentation could be involved in Yukwa production (Chun et al., 2004).Furthermore, new methods for skipping the steeping process by addition of enzyme and microorganism inoculation (Park et al., 2000a) or phosphorous oxychloride (POCl 3 , 0.002-0.008%)(Yu et al., 2007) were investigated.Grinding: The water-saturated waxy rice was ground by roller mill two times.After then the wet rice flour was sieved and the 40 and 80 mesh of rice flour by wet milling was better in expansion rate and hardness than 100 mesh (Shin et al., 1990a).Furthermore, the moisture content is modified to 50% for next steaming.
Steaming: According to Juliano (1985), steaming for as little as 15-20 min is sufficient to modify the starch and proteins of rice that has absorbed sufficient water.It is reported that the gelatinization temperature of waxy rice and non-waxy rice starch was 64.5-67°C with no significant difference.While the viscosity (at 95°C) and viscosity (at 95°C after 15 min) of waxy rice were significantly higher than non-waxy rice because the amylopectin content in waxy rice was higher (Sin et al., 1989).The quality of Yukwa was deeply related with amylose contents ranging from 18.5% of non-waxy rice to 2-3% of waxy rice (Sin et al., 1989).Steaming methods and time were also investigated in other researches (Shin and Choi, 1993;Kim, 1998;Choi et al., 2000).
During steaming, the starch in rice grains changes to α-form and the protein is denatured, becoming resistant to enzyme action.At the same time, the structure of the rice grain, such as the endosperm cell wall, is partly destroyed and the grains are sterilized (Yoshizawa et al., 1978).
Punching: Punching is the traditional manufacturing after steaming.Choi and Kang (2000) reported that the gelatinized dough was punched 12 min for getting the desirable expansion degree, texture and sensory characteristics of Yukwa.Jeon et al. (1995) investigated the influence of beating times on Yukwa properties and included that 160 times of beating would be required.Seon (1995) also reported that the texture, flavor, crispness and overall acceptability of 160 beating times were better than 80 times.Figure 2 showed the air cell structure in Bandegi with different steeping and punching time.
The texture of Yukwa is influenced by dough viscoelasticity, cell size and distribution in dough which are affected by punching process (Kang et al., 2001).During punching process, gas cells are beaten in as the dough acquires its viscoelastic properties.Also, punching energy input influenced the air cell formation, texture and expansion rate of Yukwa.Molding: After punching, according to Kang and Ryu (2002a), the gelatinized dough is cut into desired size: 3×5×0.5 cm which is called Bandegi (waxy rice pallet).Amylopectin content of waxy rice is higher than nonwaxy rice and another grain, so the dough is sticky to the edge of knife.Also, the moisture content of the steamed dough is about 50% and it is floating and proteiform (Kang and Ryu, 2002b).The shape, size and uniform of Bandegi (waxy rice pallet) play an important role in the following drying and frying process, especially the thickness.Researchers (Kang and Sung, 2000;Lim et al., 2003;Yang et al., 2008) also have different opinions about the Bandegi (waxy rice pallet) size.

Drying:
The texture, taste, appearance, stability and also the shelf life of the final product depend on amount of water it contains.Excess moisture in mixes can cause clumping and the moisture content will continue to increase during storage, causing the product to deteriorate.For higher expansion and softer texture of Yukwa, the optimum moisture content of dried and conditioned Bandegi (waxy rice pallet) is 14-17% (Kang and Ryu, 2002a).The drying time is affected by the dying speed, drying temperature and the moisture content of Bandegi.Kim (1982) reported that the molding Yukwa was placed at 45°C for 5 min and then transferred it to 23°C for 15 min.This process was repeated for 5 h.Otherwise, Bandegi was dried at room temperature using 1 h as one unit until the moisture content is 11-13% (Jeon et al., 1995).Shin et al. (1990b) reported that it was dried at 40°C for 24 h and the lowest moisture content of Bandegi was 11-15%.
Moisture conditioning: Moisture conditioning is the process used to equalize the moisture.The Bandegi (waxy rice pallet) with moisture content about 14-17% is sealed in plastic bag at 4°C for 1-2 days.Air bubbles in Bandegi were distributed uniformly by moisture conditioning and RVA maximum paste viscosity of Bandegi was the highest after 2 days of moisture conditioning process and decreased 2 days after (Kang and Ryu, 2002a).At this condition, the highest popping value of Bandegi was obtained.
Frying: Traditional frying process is performed Yukwa in oil at 120°C for 2 min first and then put it in 180°C oil for another 2 min (Kang and Ryu, 2002b).In this process, the product Yukwa will become scrip, soft and popular to consumers.Park (2004) showed that Bandegi was fried in 110°C oil for 1 min and then in 170°C for 2 min could obtain the desirable Yukwa product.Also, Lee et al. (2002) reported that Bandegi was fried in 120°C for 1 min and then in 170°C for 40 sec.
Frying process is used to puff and obtain crispy, crunchy texture of Yukwa.It is generally agreed that the oil content in a product increases with frying time.Most of the food products have an optimum cooking time and temperature (Rossell, 2001).If the frying time exceeds the optimum time, the finished product will tend to have higher oil content (Kayacier and Singh, 1999;Esturk et al., 2000), as the oil adhering to the surface of the product is drawn into its pore structure.In contrast, if the product is fried for insufficient time, it will not release the retained moisture and result in a soggy texture.It is clear that increasing frying temperature tends to decrease the oil uptake as the product spends less time in the fryer (Rossell, 2001;Pedreschi and Moyano, 2005;Moyano and Predreschi, 2006).
In fact, oil puffed Yukwa tends to develop unpleasant rancid odors of off-flavors during storage (Yang et al., 2008).Therefore, Lee et al. (2002) compared three kinds of puffing methods (convection oven, microwave oven and fryer) on the characteristics of Yukwa.Results showed that the sample puffed by frying had the highest expansion rate, whereas the convection-, microwave-puffed samples showed no significant differences.In the consumer acceptance evaluations and characteristics intensity rating tests, the samples puffed using convection and microwave methods showed higher scores for the majority of evaluated characteristics as compared to the samples puffed by frying.

IMPROVEMENT OF THE KOREAN TRADITIONAL RICE SANCK-YUKWA
Steeping process and fermentation: Considering that steeping for 30-60 min in water appears sufficient to reach the equilibrium state (Bean et al., 1984;Noomhorm et al., 1997), the extraordinary lengthy steeping period of waxy rice in Yukwa preparation is required for hydration of the waxy kernel and for induction of biochemical changes via natural fermentation.It is well known that fermentation is usually a desirable process for biochemical modification of primary food products resulting from the activity of microorganisms and their enzymes.Fermentation is a commonly used food process throughout most of the world for the purpose of enhancing food properties such as taste, flavor, texture, nutritional value and shelf life.The preparation of many traditional or indigenous cereal-based foods such as Ogi (fermented maize), Kwunu (fermented corn), Fufu (fermented cassava) and Togwa (fermented sorghum,   + C -Un-interpretable profile -1) % ID: Percentage of identification; 2) GLAB: General type of lactic acid bacteria; 3) C: Coccus; 4) R: Rod; 5) Brev.: Brevibacillus; 6) Paenib.: Paenibacillus; 7) Lacto.: Lactobacillus maize and millet) is normally carried out by natural fermentation involving mixed cultures of bacteria, yeast and/or fungi (Odunfa and Adeyele, 1985;Adegoke and Babaola, 1988;Uzogara et al., 1990).Table 1 shows the characterization and identification of major strain appearing on selective media at different steeping periods for Yukwa.
New method for skipping the steeping process by addition of enzyme and microorganism was developed (Park et al., 2000b).Main microorganism in steeping liquid were Basillus spp.and Lactobacillus spp.and the optimal period of cell cultivations for making the glutinous rice powder was determined for 18 h.Results (Park et al., 2000a) showed that in the evaluation of the expansion volume of Yukwa, all microorganism inoculated groups showed similar to that of Yukwa prepared from 28-day steeping.In all microorganism inoculated groups, hardness and peak number of Yukwa showed significantly lower level than 28-day steeping group.
Quality improvement and shelf life extending of Yukwa: For improving functionality and variety of Yukwa, green tea powder and Angelica keiskei powder were added (Kim and Kim, 2001).The results showed that addition of green tea powder by 2% level and angelica keiskei powder up to 4% gave a beneficial effect in the aspects of functionality and storage life of Yukwa without any detrimental effects on the quality characteristics of it.Shin et al. (1990b) also reported that addition of soaked soybean (3%, w/w) to dough showed higher expansion rate and better physical properties with more acceptable quality by sensory evaluation of Yukwa.And the better results about tenderness, crispness, taste and overall desirability were also obtained by adding bean water (Jo and Jeon, 2001).
The shelf life of oil puffed Yukwa was less than 4 weeks at 30°C by peroxide value and negligible changes in physical texture was detected after 9 weeks storage (Shin et al., 1990a).Furthermore, this research investigated that it was possible to apply air puffing method for Yukwa making and its optimum temperature was around 250°C.Air puffed Yukwa was only a little less expansion rate, same level of hardness and high brittleness compared with oil puffed.(Kim et al., 2004) Preference of Yukwa in America: The preference of flavor, texture, packaging, design, printing and overall preference of Korean traditional snack-Yukwa was surveyed among 10's and 20's in New York, Atlanta, Chicago and Sanfrancisco in USA (Kim et al., 2004).About thirty percent of the respondents showed high score in packaging among the items.The design and color of Yukwa was 22.0 and 24.1% in "excellent" (Table 2).On the basis of this result, Yukwa would be developed for western country people's preference.

CONCLUSION
In Korea, Yukwa is one of the most important traditional snacks.It has been widely consumed by Korean people for more than 200 years.Throughout history, the Koreans seemed to follow their own ways in developing the product without foreign influences.To produce the products in mass and under standardized processes, further studies are needed to reduce the production cost and time through the analysis of traditional processes and normalization of manufacturing processes.If innovations in taste, flavor and product quality are made and the shelf life is prolonged, Yukwa may become more widely popular all over the world.

Table 1 :
Characterization and identification of major strain appearing on selective media at different steeping periods

Table 2 :
Preference test of Yukwa quality characteristics in 10's and 20's Age