Development Of Phase-changing Coolant And Its Application On Preservation Of Fruit And Vegetables

With the improvement of people’s livelihood, comsumers demand for food quality increases more and more rapidly, especially for perishable fruits and vegetables. The quality of these foods is affected by temperature in the transportation and preservation process, so it is urgent to develop the cold chain transportation to guarantee the quality of perishable foods.Phase Change Materials (PCMs) develop rapidly in recent years, and they have been applied in food cold. This paper aims to develop a series of Phase change coolants with high latent heat, adjustable Phase transition temperature, non-toxic, and low cost, to be applied in fruit and vegetable storage and transportation. Our focu ses are on the main storage agent and carrier of composite Phase change coolant, and the effectiveness of preservation.According to the selection criteria for Phase change material established in the literature, we preliminarily selected L-malic acid (C4H6O5), mannitol (C6h14O6), xylitol (C5H12O5), sodium diacetate (C4H6Na2O4), lactate (C6h10Ca06), polybutylene aqueous alcohol (C4H10O2) that were relatively safe and had not been studied as experimental material. The melting point and the Phase enthalpy of the solutions of six energy storage materials were determined with the Differential Scanning Calorimetry (DSC), and several materials with good performance were selected as main storage agent of composite Phase change coolant. The latent heat of candidate materials showed significant difference with the concentration of aqueous solution. The latent heat of L-malic acid, xylitol, sodium diacetate and polybutylene aqueous alcohol decreased faster with the increasing concentration of aqueous solution, their latent heat did not meet criteria of290J/g or more in the desired Phase change temperature range, and the temperature range were wide; but the latent heat of mannitol and lactate decreased slowly, and satisfied290J/g or more in the desired Phase change temperature range, while the Onset temperature were more stable and were good monomer substance for formulating complex coolants, but they did not satisfy the Phase transition temperature of-6℃～-4℃, so they were required for compliance with other substances of wider Phase-change temperature zone to make the main storage agent that met the requirements. Experimental results showed that0.6mol/L mannitol+0.2mol/L KCl at a mix ratio of1:1v/v, and0.2mol/L lactate+0.2mol/L NH4Cl a mix ratio of1:1v/v can meet the requirements of the composite primary coolants. T-t curves revealed that the two composite primary coolants demonstrated little subcooling phenomenon.The composite Phase change coolants turned from solid into liquid during use, if the packages were broken, they would leak and contaminate food. In addition, if the coolant was used in the refrigeration vehicle, the shaking cold brine were prone to unstable and large loads. If the liquid form was turned into gel, uneasy to flow, then the range of applications of coolant would be expanded. So the suitable superabsorbent (SAP) was applied on the coolant to fixed the composite solution, SAP added amount was mainly considered from SAP absorbent could be susceptible to ion concentration of solution and the impact of the composite Phase change coolant after the addition.The study found the best formulation of mannitol/KCl/super absorbent composite was0.1mol/L mannitol,0.07mol/L KCl+1.1%super absorbent, and the Onset was-4.1℃, latent heat was299.1J/g. The best formulation of lactate/NH4Cl/super absorbent composite was0.04mol/L of calcium lactat+0.04mol/L of NH4Cl+2.1%of super absorbent, and the Onset temperature was-4.2℃, latent heat was297.8J/g. Comparison testing of a series of coolant samples in the market were made, diccovered the lowest Onset temperature was only-2.5℃that latent heat of coolant samples exceed290J/g, latent heat can’t reach290J/g that the minimum Onset temperature was-2.9℃and the results proved that the two special Phase-changing coolants developed by our research group was significantly better than existing commercial Phase-changing coolant in terms of both Phase transition temperature and latent heat.The effects of mannitol/KCl/super absorbent composite on fresh-keeping of bayberry fruit after48h storage were investigated. This study showed that the property of Phase-changing coolant significantly impacts the quality of bayberry fruit during storage. Suitable onset temperature and the highest possible latent heat were the basic guarantee to maintain the quality of bayberry fruit during transport and storage. The Phase-changing coolants developed by our research group was significantly better than existing commercial Phase-changing coolant in fresh-keeping of bayberry fruit, as demonstrated by the lowest decay index, decay rate, weight loss, alcohol content, and the maximum hardness. No over-cold spots in the box were observed, indicating that the newly developed coolants can be widely used in long-distance transport for fresh fruits and vegetables.