Analysis of wheat bulk mould and temperature-humidity coupling based on temperature and humidity field cloud map

Abstract: Grain storage has very important effect on national food security and has been given increasing attentions in recent several decades. During long term storage, when grain moisture and temperature exceed critical limits of safe storage, sometimes in limited rehumidified zones of the bulk, the ever-present mould inoculum will start to develop and the deterioration process will be initiated. The mould growth may result in several kinds of food-spoilage: off-flavours, toxins, discolouration, rotting and formation of pathogenic or allergenic propagules. A lot of domestic and overseas research has been carried out on the temperature field, relative humidity field and mould in grain bulk. But grain storage system is complex, and the impact factors are both independent and coupled with each other. Therefore, it is necessary to study the multi-field coupling effect in grain bulk. To explore the coupling relationship among temperature and relative humidity fields and mould in space and time in grain bulk, wheat with different moistures (11% and 20.1%, w.b.) was stored in a self-made simulated silo at 18 ℃ for 800 h non-airtight. The internal diameter, height and thickness of the cylindrical iron simulated silo were 0.54, 0.70 and 0.01 m, respectively. The simulated silo inside was insulated with rubber (0.02 m thickness). And on the top of the silo, the air pipes (0.08 m internal diameter) were applied to exchange the gas inside and outside the silo. In the experiment, the high moisture wheat (cylinder, diameter, 0.30 m; height, 0.30 m; 20.1%, w.b.) in the silo was surrounded by low moisture wheat (11%, w.b.). A 30 ℃ heater was inserted into the center of wet wheat as the artificial hot spot to cause heat and moisture transfer. After 232 h, the heater was stopped. During the storage, changes of CO2 concentration, temperature, relative humidity and number of fungal spores were measured. And the temperature and relative humidity cloud images of the min-vertical plane were determined. In the process, changes of CO2 concentration were monitored to detect if there was mould and its degree in wheat bulk. The temperature and relative humidity cloud images indicate that under the action of micro-air flow and diffusion, water vapor from wet wheat zone gradually moves to the top layer. Then the higher relative humidity fossa district above the hot spot is formed. During the experiment, there is almost no mould in the high temperature center of the wheat bulk because of the low relative humidity. Mould is mainly located at higher relative humidity position around the center of high temperature. Under similar temperature conditions, the severe mould area coincides with the highest humidity zone. This experiment shows that in the process of temperature and relative humidity change and coupling, the grain mould is not only a function of time, but also a function of space. The study also states clearly that if the temperature is low, even if the local moisture is relatively high, the growth of the microorganism is relatively slow. At this time, the temperature and relative humidity fields coupling is not enough for the growth of microorganism, so the time needed to cause the explosive growth of microorganism is increased. Even, there is no obvious hot spot, but it will still cause the slow development of mould of the stored grain. As the heat is lost rapidly, no spontaneous hot spots are observed during the experiment. This study provides the reference for the further establishment of multi-field coupling laws and the monitoring and prediction of mould in grain storage.