Elsevier

Biosystems Engineering

Volume 204, April 2021, Pages 212-222
Biosystems Engineering

Research Paper
Maize grain moisture content correction: From nonstandard to standard system

https://doi.org/10.1016/j.biosystemseng.2021.01.013Get rights and content

Highlights

  • Low drying temperature and short time contribute to low apparent moisture content.

  • Correction method from nonstandard to standard system is proposed.

  • Corrections to other standard system can be readily specified if required.

The accurate determination and estimation of grain moisture content is important for maize (Zea Mays L.) production, processing and drying studies. However, different drying procedures are often adopted by different operators to determine the grain moisture content, making it difficult to compare results with each other. In this study, different drying temperatures and times were used to create different drying conditions, and the differences in the estimated grain moisture content of maize were analysed. The results showed that, compared to the standard system, an insufficient drying temperature and time contributed to a lower apparent moisture content of maize grain, but the low drying temperature could be compensated by a longer drying time, and vice versa. Additionally, a correction method for maize grain moisture content was built. This method can correct the apparent moisture content (nonstandard system) under different drying procedures to the specified standard system, and the accuracy of model validation and the stability of deviation are very satisfactory. This method is very suitable for the rapid conversion of the grain moisture content of maize between different drying methods. It is especially helpful for re-evaluating the differences and controversies of some conclusions caused by different drying procedures adopted by different researchers, and provides a reference method for moisture content correction of other foods and cereals.

Introduction

The accuracy of grain moisture content (GMC) measurement is essential for maize (Zea Mays L.) production, sales and processing operations, and is also the foundation of drying research (Bradley, 2010). The methods of moisture measurement are classified into two major categories: direct and indirect methods. Direct methods, including gravimetric and chemical methods, involve the process of water removal from grain, while indirect methods, such as optical, dielectric, nuclear and hygrometric methods, measure some physical properties related to moisture (Zambrano, Dutta, Mercer, MacLean, & Touchie, 2019). Indirect methods are faster and more convenient but usually require expensive equipment. In contrast, direct methods are the standard for moisture determination, although they usually take a long time, and provide a reference for indirect methods. The oven method, a routine direct method, is universally accepted by many studies, and has the characteristics of simplicity, practicability, and accuracy.

In general, the drying temperature and time are usually determined by the type of cereal when using the oven method (Bowden, 1984). Different drying procedures have been adopted by different researchers, who have investigated the resulting differences for maize (Ameobi & Woods, 1993), rice (Chen, 2003; Noomhorm & Verma, 1982), wheat (Jittanit, Srzednicki, & Driscoll, 2010), and other cereals (Buckee & Benard, 1995; Obi, Ezeoha, & Egwu, 2016; Resio, Aguerre, & Suarez, 2004; Young et al., 1982). Many drying procedures are more or less empirical, and the same results may not be obtained with different drying methods or types of cereals. And comparable results need be obtained using the same drying procedure (Bowden, 1984). Therefore, different standard drying procedures based on cereals are regulated by some institutions. For drying standards of maize grain, ISO standard—ISO: 6540 (ISO, 1980) and ASAE Standard S352.2 (ASAE, 2017) are universally recognised. In addition, the ISO standard is also adopted as the Chinese standard—GB/T 10362-2008 (National Standards of the People’s Republic of China, 2008) and British standard—BS EN ISO 6540: 2010 (British Standard, 2018). However, different drying standards may also contribute to significantly different results (Bowden, 1984; Tillmann & Cicero, 1996). Some models have been established by researchers to modify these differences from different drying methods (Chen, 2003; Jindal & Siebenmorgen, 1987). However, there are some limitations in the application of previous models, such as the conditions of model application and the method of parameter acquisition.

The physical principles of many complex phenomena in drying have not been made entirely clear; the present moisture measurement procedures operate under the assumption that the mass loss in drying is only from water, and volatile substances in grains are not included (Bala, 2016, pp. 5–29; Berk, 2018, pp. 513–566). Although some standard drying procedures have been developed, drying methods still depend on experience and experimentation, and the “true” value of moisture content cannot be acquired (Bowden, 1984). In addition, many researchers have preferred to adopt their own drying methods for moisture content measurement rather than a standard method. For example, in the determination of the GMC of maize, 80 °C, 85 °C and 105 °C have been used by different researchers, and the drying times were also different (Borrás et al., 2006; Gao et al., 2018; Martinez-Feria et al., 2019). Therefore, due to the different moisture measurement methods, it is difficult to guarantee the truth of differences in results for the same topic and obtain a comparable research conclusion. In fact, this is a weakness of much published scientific research.

The lack of comparability of the GMC results of many nonstandard drying methods of maize is a major problem. The different test environments, grain types or machines were always blamed when the differences appeared. It is difficult to identify the real reasons for differences due to the different moisture measurement methods. Therefore, it is essential to develop a moisture content correction method for different drying procedures of maize. The objective of this study was to establish a method for comparing the GMC of maize between different drying procedures. The method can correct the maize GMC from a nonstandard drying system to a specified standard system and is especially suitable for the rapid conversion of maize GMC in different drying procedures.

Section snippets

Materials and design

Samples of maize grain were collected directly from the field. Five maize commercial hybrids, Dika 517, Dika 653, Zhengdan 958, Xianyu 335, and Yudan 132, were investigated in this study and all were semi-dent maize. The range of moisture content was depended on different hybrids and sowing dates, with a variation of approximately 18%–130% (d.b.; the wet basis was 15 %–55%, and all moisture content is based on dry basis in this study, unless otherwise stated). Maize ears taken from the field

Differences in the results of different drying procedures

The GMC of maize is the ratio of the water mass to the dry mass of the grains (d.b.). The calculation of the real moisture content (M0) requires the determination of the initial mass (m) before drying and the final mass (m0) after drying of the samples. The water mass of the samples is (m-m0). The calculation formula of M0 is:Mo=mmomo×100%

Due to the limitations of the drying temperature and time, the real dry mass of a sample (m0) can hardly be measured accurately. When drying for time t (h)

Model development

A sufficient drying temperature and time (generally considered to be the drying temperature and time specified in a standard) can ensure a relatively accurate AMC. However, when drying at a lower temperature or for a shorter time, the AMC will be lower than M0, resulting in an error in the moisture measurement. Different drying temperatures and times will form four states:

  • State 1: Sufficient drying temperature and sufficient drying time.

  • State 2: Sufficient drying temperature but a short drying

Model validation and deviation

The model was used to correct the differences in AMC caused by different drying temperatures and times, and the validation results are shown in Fig. 6. All data in the validation group were converted to the standard system (all combinations of drying temperature and time except for drying at 130 °C for 72 h). The root mean square error (RMSE) in the moisture range of 18.1 %–56.7% was 0.884 (N = 201). The prediction accuracy of the model was evaluated according to different drying temperatures

Conclusion

Based on a comparative analysis of maize GMC determination results at different drying temperatures and times, a correction method was established that could correct the GMC under different drying procedures to a specified standard system. The accuracy of model validation and the stability of deviation are very satisfactory. In addition, this method can adjust to a standard system as required, and is very suitable for the rapid conversion of GMC determination results of maize between different

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.

Acknowledgments

We thank the National Key Research and Development Program of China (2018YFD0300405), the National Natural Science Foundation of China (31971849), the Earmarked Fund for Modern Agro-industry Technology Research System China (CARS-02-25), and the Science and Technology Innovation Project of the Chinese Academy of Agricultural Science for their financial support. We would also like to thank the editors and reviewers for helping us improve our original manuscript.

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  • Cited by (5)

    1

    Shang Gao and Bo Ming contributed equally to this paper.

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