Processed unhusked rice is prone to mildew during storage. In this study, the storage conditions were simulated at temperatures of 20, 30, and 35 °C and a relative humidity of 40%, 60% and 80%, respectively. The water, fatty acid, and total starch content and the peak viscosity, mold colony number, protein secondary structure, and spatial structure of rice were monitored in order to propose the critical point of mildew during storage. In the process of rice from lively to moldy, the water content, fatty acid contents and the peak viscosity were increased. The total starch content decreased and then showed a slow increasing trend, while the microstructure of the powder particles changed from smooth and complete to loosen and hollow. With the increase in storage time, the vibration of the amide Ⅰ band of the rice samples decreased slightly, indicating that the total contents of β-fold, β-turn, α-helix, and random curl of the rice protein also changed. PCA (Principal Component Analysis) analysis showed that rice mildew index was closely related to temperature and humidity during storage. In our investigation, the best and most suitable temperature and relative humidity for rice storge is 20 °C and 40%, respectively. These results suggested that temperature and environmental humidity are vital factors affecting the physicochemical properties and nutrient changes, which provides a theoretical basis for the early warning of rice mildew during storage.
Rice is refined through washing, peeling, rice milling, and finishing processes. It is an indispensable source of starch in the Chinese diet. According to statistics, more than 800 million people nationwide use rice as a staple food, and more than 85% of rice is consumed as a ration [1]. China’s total annual rice consumption is approximately 135 million tons [2]. At present, China’s grain reserves are mainly rice. In some rice-based distribution markets, ensuring the quality of rice during storage and delaying mildew are of great concern [3]. Compared to paddy, without the protection of rice husks, the storage conditions of rice must be more strictly controlled to avoid affecting the taste or the cooking color, or even mildew and moths [4].
Mildew is one of the biggest contributors to the loss of quality of paddy rice. It is estimated that the annual loss of rice caused by mildew in China is as high as tens of millions of tons. In the process of rice storage, the microorganisms carried by the air and the rice itself use the nutrients in rice to create conditions for its growth and reproduction, thus accelerate the aging and mildew of rice, resulting in mold [5]. In terms of the mildew of rice, some studies have reported that with the increase in storage time, the starch granules in rice change, resulting in a decrease in the viscosity of rice, a decrease in the iodine blue value, and an increase in water absorption [6]. The content of amylose soluble in hot water in rice decreases with increasing storage time, while the content of amylose insoluble in hot water gradually increases. Studies have pointed out that in rice stored for one year (20–25 °C), the amylose content increases by 4.94%, the insoluble amylose content increases by 4.26%, and the branched fixed flour content decreases by 6.05% [7].
During the storage of rice, fatty acids change via two ways: oxidation and hydrolysis. During oxidation, fatty acids are oxidized to produce peroxides, mainly aldehydes and ketones; during hydrolysis, fats are hydrolyzed by the lipolytic enzymes contained in the grain itself, and then glycerol and fatty acids are produced by molds [8]. During storage, fatty acids are difficult for microorganisms to utilize, resulting in an increase in the free fatty acid content. Therefore, the content of free fatty acids in grain has a good correlation with grain storage quality, which is one of the sensitive indexes to evaluate the deterioration degree of the grain quality [9,10].
The two fat changes of normal moisture grains can occur alternately or simultaneously [11]. Low moisture grains are mainly oxidized, and high moisture grains are mainly hydrolyzed [12]. In terms of proteins, during the storage period, grains will breathe weakly, and the proteins slowly hydrolyze under the action of hydrolytic enzymes, which will lead to a decrease in the protein content [11]. It is relatively easy to cause protein denaturation during grain aging. After protein denaturation, the spatial structure becomes loose, the peptide bond is extended, the non-polar group is exposed, and the hydrophilic group is embedded [13]. Under poor storage conditions, food is susceptible to microbial attack, and proteins are broken down into small molecules such as amino acids under the action of a series of enzymes secreted by the microorganisms, which reduces the protein nitrogen in the food, while non-proteins such as ammonia nitrogen and amine nitrogen increase the nitrogen level [7,14]. In the storage process, rice is susceptible to microbial contamination, increases in microbial substances secreted to decompose proteins into small molecular amino acids and other enzymes, decreases in protein nitrogen and ammonia nitrogen in rice, and increases in nitrogen amine nitrogen [12].
At present, the research on rice mainly focuses on its storage process from fresh to aged. However, aged rice enters the mildew stage, and the research on the changes in its nutritional composition is not yet mature. In this study, when rice was stored at temperatures of 20, 30, and 35 °C and a humidity of 40%, 60%, and 80%, the water, fatty acid, and total starch contents and the peak viscosity, mold colony number, protein secondary structure content, and spatial structure were monitored. Based on the changes in the mold colony quantity during rice storage, this study proposes the mildew critical point of rice storage, which plays a positive role in the early warning of mildew during rice storage. As a grain susceptible to mildew, it is vital to explore the change trend and mechanism of nutritional components such as starch, proteins, and lipids in the process of aging, which play a positive role in promoting the early warning of mildew in the process of rice storage.
Lastly, check the rice for mold, which often appears as green, blue, or black spots. If you notice any of those signs of spoilage, make sure to discard your rice. Cooked rice lasts for about 4 days in the fridge or 8 months in the freezer. Expired rice may have an unpleasant smell, gooey texture, or mold.
4. Changes in the Total Starch Content
Changes in the starch content during storage have been reported [14]. Our results also shown that during rice storage, a change in starch content can be used as the critical point to judge mildew. When the temperature was 20 °C, if the grain pile was in a high humidity environment, the microbial growth was slow, and the starch content was basically in a stable state. When rice is stored at 30 °C and in a high-humidity environment, although the temperature is not suitable temperature for microbial growth, samples should be tested on the 18th day and certain measures should be taken. Rice should be stored at 35 °C, and the environment and samples should be kept dry. In the experiment, the first decline in the starch content appeared at the earliest on the eighth day, and the number of microorganisms in the grain pile reached the first saturation point, which may cause grain pile deterioration and harm the edible quality of rice.
3. Changes in the Fatty Acid Content
As an important standard to measure rice quality, the fatty acid content can be used as a sensitive index in the early warning stage of rice mildew, and it is also the main indicator for judging whether rice is aging [23,24,25]. The content of free fatty acids in the fresh rice samples was very small. The initial fatty acid content of rice was the highest when stored at 20 °C, which was 3.27 mg/100 g KOH. Moreover, the initial fatty acid contents of rice at 30 °C and 35 ℃ were slightly lower. The higher the temperature and the ambient humidity of rice storage, the faster the fatty acid value rises. When the ambient temperature is constant, the moisture content of the rice after equilibrium determines the increase in the fatty acid value [21]. As shown in , at a storage temperature of 20 °C and an environmental humidity of 40% and 60%, the initial aging of rice lagged behind on the 35th and 40th days, respectively. In a high-humidity environment (80%), the fatty acid value slowly increased, and on the 35th day, the fatty acid value began to increase rapidly. When the temperature was 30 °C, the temperature at this time was the most suitable growth temperature range for fungi. At this temperature, the fatty acid content in three environments with different humidity rose rapidly. The same trend is observed at storage temperatures of 35 °C. A horizontal comparison showed that the higher the storage temperature, the faster the fatty acid content rose, and the earlier the initial stage of aging of the sample rice. These results were in agreement with previously published data on Food Chemistry [21,26]. It is inferred from the dynamic balance of water that this may be because the water content of rice had completed the first dynamic balance in the same period and had reached the maximum value, which was higher than the safe water content of rice [27].
At 20 °C and 80% of relative humidity, the fatty acid content of the rice samples showed a trend of first increasing and then decreasing. Due to the decomposition of fatty acids in rice, the water content of rice is reduced, and the value of fatty acids is also decreased accordingly [28]. When the ambient humidity was 60% and 80%, the turning point of the fatty acid value increase was at the 35th day, which was the same as the change of water content. However, when the ambient humidity was lower (40%), the growth rate remained relatively stable. This indicates that when stored at a low temperature of 20 °C, the change trend of the fatty acid value was relatively stable, while the content of fatty acid at a high temperature will increase significantly in a short time [29]. When stored at 30 °C and 35 °C, the content of fatty acids increased significantly with the increase of ambient humidity.
Why Eating Leftover Rice Can Actually Be Dangerous For You
FAQ
Is it okay to eat moldy rice?
Can uncooked white rice mold?
How quickly does rice grow mold?
What does mold look like on white rice?
Can you eat moldy rice?
While not all species of mold produce toxins, some do, and their consumption can pose significant health risks. Consuming rice with mold can lead to various symptoms, including nausea, vomiting, abdominal pain, and allergic reactions. In some cases, the ingestion of moldy rice can also cause respiratory issues and even neurological symptoms.
Is only eating rice good for you?
Not at all. Rice is a carbohydrate and does not supply the daily needs of lipids and even less of proteins. In addition, it lacks many vitamins and minerals, which are found abundantly in fruits and salads.
Can you eat green mold on rice?
One of the three most common colors of mold found on rice and food, in general, is green mold. As you can probably tell, green mold is no better to eat than other strains, as it can still cause gastrointestinal discomfort, nausea, headaches, and shortness of breath if released spores are inhaled.
Does cooking rice kill mold?
Even if the mold is only present in one small area of the rice, it may have released spores throughout the container, making the entire batch unsafe to consume. While cooking rice may kill the mold, it is still not safe to consume due to the presence of mycotoxins, which can cause vomiting and diarrhea.