# Compression and Fungal Heat Production in Maize Bulk Considering Kernel Breakage
## 1 Introduction  
Maize is among the major cereal crops, with a wide planting range and high harvest in China. Over 40 million hectares were sown, producing 260 million tons of maize in 2020 [1]. However, despite a massive increase in maize production, postharvest losses of maize during storage remain a significant challenge [2]. A considerable amount of maize kernel breakage is easily caused by postharvest processes such as transportation and grain loading. It reduces the quality grade of the grain and also increases the risk of grain security. The density change due to the self-weight of maize bulk further induces variation in grain properties, such as non-uniform airflow [3] and heat conductivity rate, and affects mildew on grain.  
Broken kernels are more susceptible to the development of fungi, and they produce a considerable amount of heat because the physical integrity of kernels is compromised [4; 5; 6; 7]. The distribution of fungi in broken kernels was preliminarily studied via milling and dehulling [5]. [PERSON] et al. [8] and [PERSON] et al. [9] found that fungi are usually concentrated in the bran, and the damage to kernel brans often increases the risk of fungi infection. [PERSON] et al. [7] analyzed the relationship between the physical properties of maizekernels and fungi infection and proposed that maize with a softer endosperm can present higher contamination by fungi. [PERSON] et al. [6] milled and separated breakage maize kernels into different particle sizes and concluded that aerobic plate counting (APC) was inversely proportional to the results of density and particle size.  
Fungal growth can decompose organic matter in maize, thereby developing heat energy, especially in grain bulk with suitable temperature, moisture content, and oxygen concentration [10; 11]. At a given height of grain bulk, the grain bulk will creep with storage time. The creep behavior will significantly influence the maize quality during and after storage [12]. However, the creep behavior of harvested maize grains during storage was rarely reported, especially the grain bulk deformation, considering broken kernels. In the food industry, creep behavior is mainly used to select high-quality raw materials and develop new products [13]. [PERSON] et al. [14] only established a creep model to describe the maize kernel, but there are few reports on the testing of maize bulk deformation. Thermal conductivity increased linearly with the bulk density of maize because the contact area between kernels influences heat transfer [15; 16], resulting in the redistribution of moisture content and heat in the grain bulk [17; 18]. The moisture-heat coupling model of grain bulk was established based on the principle of local heat and mass balance [19; 20], but the effect of fungal heat production on the temperature of grain bulk was not considered. Recently, [PERSON] et al. [21] proposed a solution for heat production in grain bulk, considering fungal activities induced rise in temperature, heat loss during conduction and convention, and water evaporation, but the effect of vertical pressure and kernel breakage on fungal heat production was not considered.  
In this research, the authors developed an inexpensive and easy-to-use element testing system to study the effects of vertical pressure on compression and heat production. First, vertical pressure was applied to whole kernels (WKs) and half kernels (HKs) at 50 kPa and 150 kPa, respectively. The creep behavior was then investigated and analyzed using the model of Hooke spring-Kelvin model in series. Second, the temperature changes and APC were measured to study the fungi in the grain during compression. A newly empirical-analytical-numerical method was established to analyze the heat generated by fungi in grain bulk.