Rice: Chemistry and Technology, Third Edition

CHAPTER 12

IMPACT OF DRYING, STORAGE, AND MILLING ON RICE QUALITY AND FUNCTIONALITY Terry J. Siebenmorgen Jean-Francois Meullenet Rice Processing Program University of Arkansas Fayetteville, Arkansas Rice “quality,” in its many definitions, can be affected at almost any point in the postharvest processing chain. The rice processing industry faces a continual challenge to prevent various forms of chemical and physical degradation in order to maintain quality at its highest level. Given the status of a rice lot delivered at harvest, overall quality can generally not be improved, short of specialized processes that improve particular quality attributes (e.g., parboiling can improve milling quality tremendously). It is more generally the case that processors are expected to maintain quality at the delivered levels. Given the volume of product typically handled by processing facilities, as well as the wide range of sources and production practices often experienced with rough rice, maintaining quality from the first point of delivery through final packaging is indeed challenging.

Chapters 9–11 in this text address the postharvest operations of rice drying, storage, and milling. They concentrate on the theory and current understanding of the processes that make up these unit operations, as well as the commercial procedures and equipment currently in use. This chapter emphasizes the effects these operations can have on milling performance and other aspects of rice quality and functionality.

Both physical and chemical indices of rice quality are included in this chapter.

The physical quality indices characterizing milling quality are heavily emphasized, due in large part to the economic importance of milling quality to the industry.

However, quality indices dictated more by chemical structure and behavior, such as pasting properties, are also given due attention, to reflect the increasing importance of these factors to end-use processors. Finally, the emerging fields of rheology and sensory science have allowed the effects of postharvest operations on consumer quality indices to be preliminarily assessed.

PRE-DRYING ISSUES Discussion of the effects of postharvest operations on quality must begin with consideration of the handling practices and the subprocesses that rice undergoes before drying and storage. The harvesting “window” for rice is relatively short, 301

302 / Rice: Chemistry and Technology, 3rd ed. particularly considering the volume of rice that must be dried within a short time.

Modern combine harvesters have greatly increased the speed at which rice can be harvested; along with larger and faster grain carts, trucks, and trailers for transporting rice from combines to driers, they have enabled a much greater delivery rate to driers. In some cases, the delivery rate can exceed the drying and handling capacity. In such instances, questions have arisen as to the effects on quality of temporary “wet holding.”

Respiration Rice, like any living organism, respires in the presence of oxygen. The equation describing this process is (Mohsenin, 1980): C6H12O6 + 6O2 6CO2 + 6H2O + 677.2 kcal

As the equation indicates, carbon dioxide, water, and energy are produced by the oxidation of carbohydrates. Along with the rice kernel itself, microbes associated with the rice also respire and greatly contribute to the overall respiratory activity in a rice bulk, particularly under the conditions of moisture content (MC), relative humidity, and temperature that promote microbial growth.

Several deleterious effects can be incurred from high respiration rates, especially if respiration is allowed to proceed over extended periods. Kernel discoloration, sometimes referred to as “yellowing” or “stackburn,” is the most commonly recognized negative effect of advanced respiration in rice and typically results from storage at high MC. Yellowing can be caused by delayed or improper drying of rice (Sahay and Gangopadhyay, 1985) or by improper handling of fresh rice after harvest or during storage such that respiration is increased (Aibara et al, 1984).

“Dry matter losses” from grain are also incurred, as indicated by the equation. Dry matter losses are sometimes reported as a function of storage MC and temperature for grains; an example is given for barley and wheat by Burrell (1982). Alternatively, allowable storage durations under various grain MCs and storage temperatures can be estimated for maintaining dry matter losses less than given levels, by using knowledge of the respiration rate. Brooker at al (1974) present an example for corn.

Experimentally, the rate of respiration is often measured by the rate of carbon dioxide production. However, in commercial practice, high respiration rates are indicated by elevated temperature zones, or “hot spots,” in a grain mass. Hot spots typically result from localized high-MC areas, since the rate of respiration is exponentially related to MC (see below). Foreign materials, such as leaf and stalk sections or weed seed or other weed plant material, also respire at high rates because of the typically high MC of this material. Siebenmorgen et al (1994) measured the MC of rough rice, as well as that of accompanying stalk and leaf material, throughout a harvest season. They reported an MC of 66.1% for a rice stalk-leaf mixture when the rice grain MC was 19.8%. Certainly, there is considerable merit in properly adjusting combines to minimize levels of material other than grain and in scalping or cleaning rough rice before drying and storage to avoid the unwanted effects of respiration from this material.

Impact of Drying, Storage, and Milling / 303 Factors Affecting Respiration Both the MC and the temperature of rice dictate the rate at which respiration occurs. Bailey (1940) reported the rate of respiration of rough rice over a limited MC range of 12–17%. Dillahunty et al (2000) measured the respiration rate of rough rice for both medium-grain (Bengal variety) and long-grain (Cypress variety) rough rice over a range of temperatures and MCs. Figure 1 presents the results of Dillahunty et al as well as those of Bailey (1940); the respiration rates of both studies were in close agreement. Figure 1 also shows the exponential response of respiration rate to MC. The rate begins to significantly increase above a MC of 14%, which indicates why rough rice must be dried to approximately this MC level quickly after harvest. The figure further shows that, in order to minimize respiration rates and resultant dry matter losses over long storage durations, the MC is typically lowered to 12–13%.

Figure 2 indicates the effect that temperature has on respiration rates. As the storage temperature of rough rice increases, respiration rate increases to a certain point, after which it decreases. The decrease is believed to be due to the thermal retardation of respiration of the kernel and associated microbes present on the kernels. Figure 2 shows that the temperature at which respiration peaks decreases as the MC increases. These trends were similar across both long- and medium-grain varieties.

Quality Effects Resulting from Respiration Postharvest respiration processes can produce yellowing of rice that affects rice quality, appearance, flavor, and yield (Singaravadivel and Raj, 1983; Phillips et al, 1988; Misra and Vir, 1991). Although yellowing occurs in paddy rice, the endosperm itself becomes discolored; thus, yellowing is not apparent until the rice is milled.

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Fig. 1. Predicted respiration rate curves as reported by Bailey (1940) and for rice cultivars Cypress and Bengal generated from experiments in which conditioned moisture content was varied and temperature was maintained at 30°C. (Reprinted, with permission, from Dillahunty et al, 2000)

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Yellowing does not necessarily produce yellow-colored kernels. Colors can range from yellow to orange to reddish. Some factors cited as being responsible for yellowing include various species of fungi or mold (Schroder, 1963); grain water activity; and surrounding air temperature, oxygen, and carbon dioxide content (Bason et al, 1990). Some of these factors may also work together.

Dillahunty et al (2001) investigated the effects of temperature and heating duration on medium-grain (Bengal variety) and long-grain (Cypress variety) rough rice at high (~21%) and low (~18%) harvest MC levels. Results, as measured by the color values chroma and hue angle, showed that, as exposure duration and temperature increased, the occurrence of yellow rice also increased (Figs. 3 and 4). Visual observations of discolored samples indicated that certain kernels were discolored more than others. The viscosity of samples measured after exposure to various temperatures (Fig. 5) showed that only rice exposed to the most severe combinations of temperature and exposure duration had reduced peak viscosity. Less severe treatments produced yellowed rice but not a reduction in peak viscosity.

Fig. 2. Respiration rates for Bengal (A) and Cypress (B) rice at different temperatures and moisture contents (MCs). Each data point is the mean of the measurement of three separate replicates. (Reprinted, with permission, from Dillahunty et al, 2000)

Impact of Drying, Storage, and Milling / 305 Wet-Holding Effects on Rice Functionality Daniels et al (1996, 1998) and Meullenet et al (1999) studied the effects on rice quality of holding rough rice at high MCs. These experiments were designed to simulate extended delays in drying, as might occur during the peak of the harvest season.

Daniels et al (1998) showed that head-rice yield (HRY) did not seem to be affected by wet-holding conditions (20.5% MC for 86 hr at 20°C) for long-grain rice. However, the delayed-drying condition, particularly for samples dried at low temperature, resulted in significantly lower water absorption and volume expansion compared to the samples that were dried immediately after harvest (Daniels et al, 1996).

Fig. 3. Chroma values of milled rice resulting from exposing rough rice from each cultivar-location combination to the temperatures and durations indicated. Each point represents combined data for harvested samples at high and low moisture content levels (mean value of 16 color measurements).

(Reprinted, with permission, from Dillahunty et al, 2001)

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Fig. 4. Hue angle values of milled rice resulting from exposing rough rice to the temperatures and durations indicated. Lower values indicate darker-colored rice. Each point represents combined data of Bengal and Cypress samples harvested at high and low moisture contents (mean value of 32 color measurements). (Reprinted, with permission, from Dillahunty et al, 2001)

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Fig. 5. Viscosity profiles for Cypress rice exposed to the indicated temperatures for 72 hr. (Reprinted, with permission, from Dillahunty et al, 2001)

Impact of Drying, Storage, and Milling / 307 Meullenet et al (1999) reported slight but significant effects of wet holding (using the same holding conditions described by Daniels et al, 1998) on the sensory quality of long-grain Cypress rice. Wet holding significantly affected both the clumpiness and hardness of cooked rice. Samples for which drying was delayed yielded a significantly clumpier and less firm cooked rice. Recently, the impact of holding paddy rice (Akitakomachi and M202) with moisture contents in the 17–27% range on the aroma and flavor of white rice following drying and milling was examined (E. T. Champagne, J. Thompson, K. L. Bett-Graber, R. Mutters, J. A. Miller, and E. Tan, unpublished data). The undesirable flavor note “sour/silage” significantly increased in milled rice (i.e., for both cultivars) with an increase in paddy moisture content after 48 hr of storage at a temperature of 40°C. Sour/silage and alfalfa/green bean notes significantly increased in intensity in milled rice with paddy storage time for paddy stored for up to 48 hr in the range of 24–27% MC. Astringent mouthfeel significantly increased in milled rice with time of storage of paddy at 27% MC. Other undesirable flavor attributes such as sewer-animal, haylike musty, and silverlike metallic in milled rice did not significantly change with high-moisture storage of paddy rice. These results point out the necessity of evaluating postharvest handling as a system, as each postharvest operation has the potential to influence the functionality and quality of the final product.

DRYING The previous section indicated that if high-MC rice is not dried within a given period, respiration processes could cause reductions in quality in terms of discoloration, functionality, and sensory properties. Drying in a timely fashion is thus critical to maintaining quality at its highest possible level at the start of postharvest operations.

Lowering MC to acceptable levels involves heat and mass transfer subprocesses, which are fairly well established. However, changing the MC of rice, as of most hygroscopic materials, causes corresponding changes in many physical properties, the summation of which can cause hygroscopic stress within kernels. If localized stresses reach levels greater than the kernel material’s strength, fissures can form due to material failure. The challenge associated with rice drying is thus not only to reduce MC, but to reduce it without compromising the physical integrity of the kernel. Additionally, drying should be accomplished without imposing chemical damage that would degrade rice functionality.

Chapter 9 in this text addresses the fundamentals of rice drying. Additionally, chapters in other texts address the subject; these include Kunze and Calderwood (1980) and Wang and Luh (1991). A related chapter by Siebenmorgen (1994) presents the importance of MC, from preharvest through final laboratory assessment, in affecting milling quality of rice. Finally, a recent chapter by Siebenmorgen et al (2003) focuses on milling quality effects that can occur as a result of the drying process.

Effects of Drying on Milling Quality The most notable negative quality effect of drying is HRY reduction. The maximum possible HRY associated with a harvested rice lot, dried under the most gentle conditions, is determined by several factors, including the rice variety, growth conditions, and harvest MC. Siebenmorgen et al (1992) and Pan et al (2002) have

308 / Rice: Chemistry and Technology, 3rd ed. shown that harvesting at too high a MC, in which a high percentage of immature kernels are present, or at a very low MC, in which dried kernels may have fissured due to rapid moisture adsorption, can cause reductions in HRY relative to harvesting at the optimal MC. Given a maximum possible HRY for a lot, the goal of rice driers, particularly commercial driers, is to maximize the drying rate while maintaining a HRY as close as possible to the maximum. As such, research has been conducted to quantify the HRY reduction associated with certain drying conditions. In particular, Fan et al (2000) conducted a large drying study to elucidate the effects of several harvest and drying conditions. In this study, one medium-grain and two long-grain varieties were harvested over a range of MCs and locations and dried in thin layers under three air conditions (A, 43.5°C, 38% rh; B, 51.7°C, 25% rh; and C, 60.0°C, 17% rh) with corresponding equilibrium moisture contents (EMCs) of 9.5, 7.3, and 5.8%, respectively. After drying for a certain duration, samples were immediately cooled in a chamber held at 21°C, 50% rh, which corresponds to an EMC of approximately 12.5%. As shown by Cnossen and Siebenmorgen (2000), this cooling treatment can cause HRY reduction if a sufficient MC gradient is present in kernels at the end of drying. Fan’s study was thus indicative of a drying situation in which cooling is experienced by the rice immediately after drying.

Figure 6 represents the HRY reductions that occurred when Bengal mediumgrain rice was dried under the three indicated air conditions over a range of durations before being cooled to 21°C. The data show that under the 9.5% EMC air condition, continuous drying for periods up to 3 hr did not affect HRY. However, for more-severe, lower-EMC air conditions, HRY decreased dramatically after shorter drying durations. Figure 7 gives a comparison of the varietal responses when drying under the two more severe drying conditions; the medium-grain variety was more

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Fig. 6. Head-rice yield (each point is the average of two replications) of Bengal variety rice (harvested at 22.5% moisture content, from Stuttgart, AR) versus drying duration under the three indicated drying air conditions before being cooled to 21°C. RH = relative humidity, EMC = equilibrium moisture content. (Reprinted, with permission, from Fan et al, 2000)

Impact of Drying, Storage, and Milling / 309 susceptible to HRY reductions than the long-grain varieties. Fan et al (2000) further reported that, as the harvest MC of a rice variety is increased, more moisture can be removed before HRY reduction occurs under a given drying air condition. This point is summarized by Figure 8, which concurs with the findings of Pan et al (2002), who dried medium-grain samples of M202.

Fig. 7. Head-rice yield of three rice varieties, each at the indicated harvest moisture content (HMC), versus drying duration when dried under air conditions of 51.7°C and 25% rh (A), and 60.0°C and 17% rh (B) before being cooled to 21°C. (Reprinted, with permission, from Fan et al, 2000)

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Fig. 8. Percentage points of moisture content (MC) removed before head-rice yield reduction occurred in relation to harvest moisture content. Drying conditions: B = 51.7°C, 25% rh, 7.3 equilibrium moisture content (EMC); C = 60.0°C, 17% rh, 5.8 EMC. (Reprinted, with permission, from Fan et al, 2000)

While beyond the scope of this chapter, further work in quantifying HRY reductions was conducted by Cnossen and Siebenmorgen (2000), who showed the effects of tempering for various durations after drying. In related work, Cnossen et al (2003) determined the effect of drying and tempering treatments on kernel fissuring.

Their work indicated that tempering durations based on minimizing fissures due to the drying process may be longer than tempering durations based on minimizing HRY reduction.

Effects of Drying on Sensory Quality According to Daniels et al (1996), samples dried under high-temperature conditions (54°C, 22% rh) had a significantly higher overall sensory impact and grain flavor note. Samples dried under low-temperature conditions (33°C, 68% rh) were found to be rougher and harder and exhibited a lower cohesiveness of mass. No significant differences were found between samples for the stale grain flavor note, sulfur flavor note, clumpiness, gluiness, moisture absorption, or geometry of slurry.

Champagne et al (1997) reported no significant changes in the flavor of rice (Bengal, M-401, and Koshihikari) dried at temperatures ranging from 18 to 60°C. In the same study, Lyon et al (1999) reported no effect of these drying conditions on the sensory texture characteristics of the same three cultivars grown in Arkansas and California. After instrumental testing to assess rice texture, few effects of drying temperature were reported (Champagne et al, 1998), with the exception of cohesiveness, which was found to be lower in rice dried at lower temperature than in that dried at higher temperature.

Meullenet et al (1999) reported significant effects of drying on the sensory characteristics of long-grain Cypress rice. Although no effects of drying temperature