Delayed drying interval of red rice: Effects on cooking properties, in vitro starch digestibility and phenolics content
Introduction
Consumption of red rice varieties that possess different gastronomic characteristics compared to brown rice is a trend among people seeking healthier diets. The beneficial health effects of such varieties of rice are attributed to the antioxidant activity of bioactive compounds such as phenolic compounds, anthocyanins, and proanthocyanidins, which are associated with protection against chronic diseases (Masisi et al., 2016). According to Ziegler et al. (2018), red rice has phenolic and proanthocyanidin contents of 13.7 mg/g and 0.99 mg/g, respectively. However, post-harvest procedures such as storage may reduce the contents of these compounds, as well as modify the cooking properties of the rice grains (Lang et al., 2019; Zhou et al., 2007).
When harvested, rice grains usually possess a moisture content (MC) of around 19–21% (wet basis); thus, drying is required for their safe storage immediately after the harvest process (Haydon and Siebenmorgen, 2017). In this context, one of the main difficulties associated with post-harvest procedures in industries is the short harvesting period, which causes the accumulation of grains in the gathering stage (Manski et al., 2005). As the drying operation occurs seasonally during the year, introducing a system for the immediate drying of all the gathered grains is an unviable alternative. Thus, on many occasions, freshly harvested grains (with a high MC) remain stored for a short period for subsequent drying. In some cases, the grains are exposed to improper conditions such as high temperature and humidity until the drying process; these factors are associated with the presence of microorganisms, and may thus, affect the grain quality.
Other studies using brown rice have shown that long-term storage of grains with high a MC prior to drying reduces the dry matter content of the grains, and especially the rice color, representing a significant loss for growers and processors (Haydon and Siebenmorgen, 2017). Some studies have investigated the changes in the phenolic composition of some pigmented rice varieties with low MC during storage (Htwe et al., 2010; Ziegler et al., 2018). Zhou et al. (2014) have investigated the effects of storage at 4 and 37 °C for 6 months on the contents of free and bound phenolic compounds in white rice and black rice grains. The authors observed reductions in the levels of both the free and bound fractions in case of both the grain varieties following their storage at 37 °C; these reductions ranged from 15 to 20%. According to Zhou et al. (2014), rice aging is a complex process involving changes in the levels of phytochemicals and other minor components during storage, which affects rice quality. It is known that the increase in the MC may favor the metabolism of the grains by increasing their respiratory rate, thereby accelerating the aging process of rice and the degradation of its constituent compounds (Dillahunty et al., 2000). According to Dillahunty et al. (2000), when the MC of Bengal rice grains increased from 15 to 20%, their respiration rate increased by five-fold following their storage at 30 °C.
On the other hand, information regarding the effects of accelerated rice aging on starch digestibility is limited. According to a study by Zhou et al. (2016), the initial velocity of digestion and overall digestion rate of white rice grains after storage at 37 °C for 12 months were observed to be lower, compared to the case for their storage at 4 °C. These results demonstrate that storage may play a key role in modulating starch digestibility; further investigations regarding this aspect are needed. Thus, this study aims to evaluate the influence of delayed drying time interval of 3 and 6 days and temperature of 15 and 25 °C followed by storage for 12 months on the cooking properties, in vitro starch digestibility, and the phenolic contents of red rice.
Section snippets
Experimental drying and storage
Rough of red-pigmented rice produced in the municipality of Jaguarão, RS, Brazil, in the 2015/2016 harvest, were used to carry out the research. The initial moisture of 25% (dry basis) of the paddy was determined by oven drying for 3 h at 140 °C (ASAE, 2000). After the harvest, the paddy was cleaned and divided into two groups: (1) immediately dry and; (2) delayed drying time interval for 3 and 6 days at 15 and 25 °C, according experimental design show in Supplementary material 1. After drying,
Protein solubility
The protein solubility of grains subjected to delayed drying interval and subsequent long-term storage are presented in Fig. 1 and Supplementary material 4. Analysis of variance showed significant effects (P < 0.05) of waiting time, waiting temperature and storage time on the protein solubility of the grains (Supplementary material 3). Reduction trends for protein solubility were observed following the increase in the duration of the delayed drying interval (Fig. 1). Lower protein solubility
Conclusion
Changes in the cooking and digestibility properties of red rice during delayed drying time interval, such as increased cooking time and hardness of the cooked grains, and reduced rehydration capacity and glucose release rate, are associated with the reduced solubility of proteins and free phenolic contents. In addition, the proanthocyanidin contents are reduced, making the grains redder. The higher temperature (25 °C) during the pre-drying delay period promotes the degradation of phenolic
CRediT authorship contribution statement
Gustavo Heinrich Lang: Data curation, Writing - original draft. Bruno Arthur Rockenbach: Conceptualization, Methodology. Cristiano Dietrich Ferreira: Writing - review & editing. Maurício de Oliveira: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We would like to thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPERGS (Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul), SCT-RS (Secretaria da Ciência e Tecnologia do Estado do Rio Grande do Sul), Polo de Inovação Tecnológica em Alimentos da Região Sul, MPB Agroindústria, and the MPB Fine & Whole Grains.
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