EFFECT OF STORING TIME AND TEMPERATURE ON MILLING QUALITY OF PARBOILED AND RAW RICE OF AN IMPROVED IVARIETY, AT 362 AND A TRADITIONAL VARIETY, KURULUTHUDA

Future rises in global surface temperature threaten for rice dependent nations. Previous research has failed to account for the detrimental impact of high temperatures on milling quality in paddy storing. Therefore, main ob-jectives of this study were to assess the milling quality of paddy stored at different temperatures in line with global warming and to identify adaptation techniques. Experiment was conducted as a split-split plot design with 3 repli-cates. Main, sub and sub-sub plot factors were variety (AT-362, Kuruluthuda), processing technique (raw, parboiled) and storage temperature (26, 30, 34, 38 o C) respectively. Head Rice Yield (HRY), Broken Rice Yield (BRY) and Degree of Milling (DOM) were estimated initially and after 6 months. Result revealed that HRY of paddy decreased by the range of 2.1-3.5% with increase in storage temperature from 26°C to 38°C after 6 months. How-ever, temperature was not significantly influenced on HRY and BRY of stored paddy (P>0.05). The DOM was significantly lower (2.07%) at 38 o C than other temperatures. The HRY is significantly higher (P<0.05) in parboiled rice than raw rice in both varieties after 6 month at each temperature level. It can be concluded that milling quality of stored paddy is degrading at high temperatures yielding high amount of broken rice and low head rice, Furthermore, Kuruluthuda variety stored at comparatively high temperatures seems to be more tolerant to breakage during the milling compared with AT-362.


INTRODUCTION
Global warming has become a serious challenge in the world today. Increasing temperature is the major culprit and most obvious effect of global warming. Under the predictions of high greenhouse gas emissions scenario, called RCP (Representative Concentration Pathways) 8.5, global mean surface temperature will be increased 2.6 to 4.8 o C from the year 2081 to 2100 (Pachauri and Meyer, 2014). Besides changes to the physical conditions of the planet, global warming and climate change will have deleterious potential on the crop yields and quality parameters (Deschenes and Greenstone, 2012). Peng et al., (2004) reported that 1 o C increase of daily maximum and minimum temperature lead to decline the paddy yield by 10%. Paddy is cultivated in over 100 countries in the world and more than 90% of the world's rice production is in tropical countries in Asia (Khush, 2005). Prevailing maximum temperature during day time in tropical rice-producing countries is around 35°C (Noomhorm, Yubai, 1991). However, in accordance with global warming predictions, the ambient air temperature in rice production areas can be raised more than 35 o C within next 100 years. The remarkable environmental temperature increment can adversely affect not only the rice yield but also the rice quality through altering the distribution of head and broken rice during the milling process (Lyman et al., 2013;Noomhorm and Yubai, 1991).
Milling quality can be introduced as a foremost determinant of the grain quality because it ultimately determines edible rice yield and its market value. Percent HRY is mainly used to evaluate milling quality due to high com-*Corresponding author: mggawanthi@gmail.com

MATERIALS AND METHOD Sample preparation
AT-362 (an improved rice variety) and Kuruluthuda (a traditional rice variety) certified by Department of Agriculture, Gonnoruwa, Sri Lanka were collected from a batch of freshly harvested paddy in 2016/17 Maha season at Mapalana, Kamburupitiya southern province, Sri Lanka. After cleaning, paddy was sun dried until it reach 14% of moisture content. About 120kg of each rice variety was divided into two portions and one portion of each was parboiled by adapting following processing steps. Firstly, two portions of each variety were soaked in cold water for 48 hr while replacing water with fresh in every 12 hr to get rid of the development of microbes which impart foul smell for the parboiled rice. The soaked water was drained off and steamed for about 4 to 6 min until few grains were split open over the surface of the paddy. The parboiled paddy was dried in an open sun drying yard as a thin layer (Thickness approximately 1cm) until moisture content reaches to 18%. Thereafter, these paddy were heaped up and kept for 2-3h for uniform moisture diffusion with a view to prevent the case-hardening of the grains. Finally, paddy was spread over the sun drying yard again to get the final moisture content for 14%. Thereafter, 5 kg of parboiled and raw paddy of each variety were packed in poly sacks (0.45m x 0.3m).

Storing Paddy
Four wooden chambers were fabricated for 4 temperatures as storage structures under controlled conditions of temperature. The most appropriate distances to keep paddy bags in the chambers were observed by changing the distances of upper most shelfs and making holes in bottom shelf. Dimensions of a chamber are given in Fig 1. These chambers were equipped with an electrical heat energy source along with an Arduino micro controller system and DHT 11(Distributed temperature sensing) temperature and relative humidity sensors to maintain the stipulated temperature uniformly as well as to record the temperature and relative 46 mercial demand for whole kernels (Noomhorm and Yubai, 1991). Numerous factors affect the breakage of rice kernel during milling including kernel temperature and moisture content, environmental milling conditions (Temperature and Relative humidity) and the operating conditions of the milling machinery (Luh, 1980). Among these conditions, environmental conditions particularly temperature have a significant effect on the milling quality. Temperature affects various milling quality characteristics of the paddy kernel such as chalkiness, immature kernels, kernel dimensions, fissuring, protein content, amylose content and length of amylopectin chain (Wassmann et al., 2009). Decreasing HRY and increasing BRY are the adverse effect of high temperatures in terms of milling quality (Lyman et al., 2013).
Most of the research seems to indicate that environmental conditions exerted before harvesting have a significant effect on the rice milling quality. However, it was failed to account for the detrimental impact of high temperature on milling quality during different post-harvest activities. Besides conditions during harvesting and drying, storage and other pre-milling operations are found to be primarily influenced on rice milling quality (Noomhorm and Yubai, 1991). Sri Lankan paddy farmers store around 50% of the harvested paddy for consumption, seeds and future sale under the fluctuation of market price for a period of 6-12 months (Hafeel et al., 2008). Therefore, storing paddy for a long time is an enormous requirement. Furthermore, average ambient storage temperature used for commercial storage of paddy and other grains in Sri Lanka is 30±2 ºC (Prasantha, 2014). Estimated on-farm storage loss is about 8% (approximately 4 million metric tons) (Adhikarinayake et al., 2006). Therefore, the main objective of this study was to assess the milling quality of paddy stored at different temperatures in line with global warming and to identify adaptation technique to reduce the losses in milling process. humidity inside the chambers. Packed paddy were stored under 4 different temperatures namely 26, 30, 34 and 38 0 C and Relative humidity level were found 75±5%, 65 ±5%, 55±5% and 45±5% respectively for 6 months. Generally, parboiled paddy are not stored and milled after drying. However, in this study dried parboiled paddy was stored for 6 months only for detecting the quality changes during storage period.

Experimental design
The study was conducted according to splitsplit plot design, while replicating all treatments in triplicate levels. The variety is the main plot factor with 2 levels (AT-362, Kuruluthuda), processing technique is the sub plot factor with 2 levels (raw, parboiled) and temperature is the sub-sub plot factor with 4 levels (26 o C, 30 o C, 34 o C and 38 o C).

Data Collection
The milling quality tests of rough rice samples were evaluated in terms of HRY and BRY and DOM. For each test 200g of paddy samples were drawn after 6 months and passed through a laboratory huller, (Model 47 THU-35B Zatake) for two times. The resulting brown rice were poured into the laboratory polishing machine (Model TM-050) for 12 seconds. Milled rice sample was mixed and divided using sample divider for four times. Nearly 1/5 th of the milled rice sample were used to determine the weight of whole grains and broken grains as a representing unit. Separation of head rice from the broken was carried out manually. The HRY consists of rice kernels of three-quarters the size of whole kernels or larger. Weights of the paddy, brown rice, milled rice, broken rice and head rice were recorded in every sample. Thereafter, HRY and BRY and DOM were measured for each treatment according to the equations 1, 2 and 3 (Wimberly, 1983). (1) (3)

Data Analysis
All the analysis were performed in triplicates using SAS software package. Statistical significance of the data obtained from the study were analysed by three way analysing of the variance (ANOVA) using the PROC GLM procedure to understand the main and interaction effect of factors. Means were separated using Duncan multiple range test at P<0.05 probability level.

RESULTS AND DISCUSSION
Effect of storage temperature on HRY and BRY of stored paddy after 6 months. Temperature was not significantly affected on HRY after 6 months of storage period (F 1,24 =3.00, P>0.05). It was observed that HRY slightly decreased and BRY increased with increment of storage temperature in both rice varieties when milled as raw rice (Table  Houston et al. (1957) indicated that total or HRY were not change significantly at temperature 21°C, 32°C and 35°C. Our findings appear to be well supported by Noomhorm and Yubai (1991) who disclosed that HRY decreased by approximately 3.51%, 1.26 and 1.65% with increase in temperature of the milling environment from 20 to 35°C at 40%, 60% and 80% RH levels. It was due to exerting high temperature stress in the kernel and creating fissures or cracks. It was found that HRYs of Wells rice cultivar stored at 35°C were significantly higher than those stored at 4 and 21°C (Pereira et al., 2008). Additionally, increasing storage temperature up to 37°C enhanced the aging of the stored paddy (Pearce and Marks, 2001) which leads to increase hardness of the starch granules and limited the granule hydration and swelling due to starch protein interaction (Tulyathan and Leeharatanaluk, 2007). Therefore, HRY may increase with increase in temperature. These negative and positive impact of high temperature on HRY and BRY may be due to different kernel structure of different paddy cultivars. Jang et al. (2009) claimed that although HRY decreased with storage temperature for Chucheongbyeo and Nampyeongbyeo cultivars, there were not any changes in Ilpumbyeo and Hwayeongbyeo cultivars.
Two factors interaction (variety*processing technique) was significant in terms of HRY after 6 months of storage period (F 1,24 =5.38, P<0.05). Similar interaction was significant (F 1,24 =4.38, P<0.05) regarding BRY during the same storage period. Kuruluthuda variety showed significantly greater (P<0.05) HRY and lower BRY than At-362 in both raw and parboiled form (Fig 2). Furthermore, HRY was significantly higher while BRY was significantly lower in parboiled paddy compared with raw paddy in both varieties. It was due to changes occur during heating up step in parboiling process. Deycke et al. (2005) highlighted the three important changes occur during parboiling result in high HRY. The changes are starch gelatinization, complexation of rice lipids with amylose fraction and formation of disulphide bonds between components of the rice protein fraction.

Effect of storage temperature on DOM of stored paddy after 6 months
Based on statistical analyses pertaining to DOM, temperature found to be significant as a main factor (F 3,24 =3.29, P<0.05). Although most of the rice consumers prefer to eat rice with high DOM because of the better appearance and taste, nutritional losses are high in rice with high DOM. However, rice with low DOM has started gaining importance since its health benefits (Puri et al., 2014). Degree of milling was significantly lower (2.07%) in paddy stored at 38 o C than other storage temperatures. It implies that high temperature decrease the DOM of stored paddy after 6 months. It may be due to increasing the hardness of paddy with storage time before its ageing process (Pereira et al., 2008). Therefore, it was more difficult to remove the rice bran from the kernel during milling process. On the other hand, Archer and Siebenmorgen (1995) have explained that there was an effect of high temperature on DOM. According to their study, low DOM were obtained at low milling temperatures due to high viscosity of the oil in rice bran. Furthermore, Kanlayakrit and Maweang (2013) was found that rice bran was more susceptible to different reactions at high temperature than the endosperm of the rice kernel.

Effect of rice variety and processing technique on DOM of stored paddy after 6 months
Results revealed that (variety*processing) 2 factors interaction was significant affected on DOM (F 1,24 =19.84, P<0.05). It was highlighted that DOM in parboiled rice was significantly lower than that of raw paddy (Fig 4). It is generally accepted that parboiling leads to increase in hardness of the grain resulting in reduced DOM (Gujral et al., 2002). The highest DOM was observed in raw AT-362 (5.36%) and it was significantly higher than raw Kuruluthuda. It might be, as a result of the diverse varietal characteristics of paddy which plays vital role in milling quality (Patel, 2013).  In the present study, Kuruluthuda variety shows high HRY, low BRY and low DOM even at high temperatures. Yadav and Jindal (2008) disclosed that high HRY is given by some paddy varieties which has a high amylose (>20%) content, low alkali spreading value and high protein (>7%). Kuruluthuda rice variety contains high protein, fibre, necessary fatty acids and lower glycemic index compared to other common rice varieties in Sri Lanka (Withanawasam, 2017). Additionally, Gelatinization temperature of Kuruluthuda was recorded as high compared to other traditional rice varieties (Rebeira, 2014). Therefore, further studies have to be carried out to determine the heat resistance behaviour of Kuruluthuda paddy.

CONCLUSION
The head rice yield, broken rice yield and degree of milling are the foremost determinant in terms of rice quality. It can be concluded that milling quality of stored paddy is degrading at high temperatures result in high broken rice yield, low head rice yield and low degree of milling. In addition, combination of variety and processing technology was significantly influenced on head rice yield and degree of milling of stored paddy after 6 months. Milling quality of parboiled paddy stored for 6 months was higher than raw paddy. Kuruluthuda rice variety shows higher head rice yield, lower broken rice yield and lower degree of milling even at high temperatures than At-362. According to its investigated features, Kuruluthuda shows a natural resistance to withstand at high temperatures during storage and further experiments needs to be conducted to explore it.