Study On Carrot Drying By CO₂ Modified Atmosphere Heat Pump Dryer And β-carotene Degradation

At present,the main direction of research for fruit and vegetables drying technology is to raise the drying efficiency and to maintain the product quality.The quality deterioration of fruit and vegetables during drying process is mainly attributed to the catalysis of internal enzyme,high temperature and aerobic environment.The solution is to passivate oxidase,reduce drying temperature and isolate oxygen.In the production,the blanching method is often used to inactivate the enzyme.Also,antioxidant Na₂SO₃ is added to delay the oxidation of the material.However,blanching method easily causes the loss of water-soluble nutrients and energy.The residue of Na₂SO₃ affects the quality of fruit and vegetables as well as human health.Vacuum drying and vacuum freeze drying are the main drying methods for reducing the temperature and oxygen content in the drying process.However,due to the limitation of drying efficiency and cost,the application is not very widespread,and a large number of fruits and vegetables are still dried by conventional hot air drying.The CO₂ modified atmosphere heat pump drying equipment designed by our laboratory adopts inert gas CO₂ instead of conventional air for drying closed-circuit circulation,and uses heat pump system to dehumidify and heat.It not only has the characteristics of wide temperature control range and high energy utilization rate of heat pump drying,but also limit the oxygen content and fully protect the biologically active substances in dried fruits and vegetables.At the same time,carrots are rich inβ-carotene,which is the main source of vitamin A in people’s daily diet,but the highly unsaturated molecular structure makesβ-carotene highly susceptible to degradation,and carrots are perishable due to their own biochemical reaction which resulting in huge waste.Therefore,in this study,the CO₂ HPD was used to dry the carrots at different temperatures,wind speeds,oxygen contents and sample slice thicknesses.A thin layer drying kinetics model was established.The effects of various operating parameters on the drying quality of carrot slices were discussed.The degradation products ofβ-carotene were isolated and the model for predicting the degradation kinetics was determined.The effect of the drying method on the bioaccessibility ofβ-carotene was evaluated by in vitro experiments.The specific research content is as follows:（1）The drying kinetics of carrot dried by CO₂ HPD was studied.The effects of drying temperature,wind speed,oxygen concentration and sample slice thickness on the drying process of carrots were investigated.The results showed that the entire drying process was controlled by the internal moisture diffusion falling rate drying.The drying rate was significantly accelerated with increasing drying temperature and wind speed,but the effect of sample slice thickness was the opposite,and there was less influence of oxygen concentration on drying kinetics.Nine sets of test data were fitted with the seven most commonly used thin-layer drying models.It was found that the Page model can be best described the whole process of carrot drying by the CO₂ HPD.Its R² reached 0.99943⁰.99986,and theχ2 and RMSE values were also the minimum of the compared models.During the whole drying process,D_eff varied from 0.5547×10^-9m²/s to 2.0877×10^-9m²/s,especially the thickness of the sample had the most significant impact.The water diffusion activation energy Ea was 31.477kJ/mol,which was within the range of typical activation energy of food materials.（2）The effects of different operating parameters of CO₂ HPD on the drying quality of carrot slices were studied.The result showed that reducing the drying temperature,oxygen concentration and sample slice thickness or increasing the drying wind speed could improve the color of the dehydrated sample.The oxygen concentration in the drying medium had the strongest influence on the retention rate ofβ-carotene and vitamin C in the sample.5%oxygen concentration could better protect the two nutrients,and the retention rate ofβ-carotene and vitamin C reached 89.55%and 74.51%.As the gradient of drying temperature,wind speed,oxygen content and slice thickness increased,the internal structure of the dehydrated carrot collapsed,resulting in a significant decrease in volume retention,while the reduction of internal pores caused a significant decrease in the rehydration ratio.The microstructure of the sample was observed by scanning electron microscopy.It was found that the sample was loose and the surface pores were uniformly distributed in a honeycomb shape when the drying conditions were mild.When the drying conditions were severe,the deformation of the sample was intensified,the porosity was reduced,and the compactness was increased.The difference in the structure of the sample was the basis for explaining the change in volume retention rate and rehydration ratio.（3）The type of degradation products of all-trans-β-carotene standard treated by CO₂ HPD was identified,and its degradation pathway was speculated.An HPLC method was established to optimize the separation products of all-trans-β-carotene by using methanol-methyl tert-butyl ether as mobile phase and gradient elution with YMC carotenoid C30 column.Array detector and atmospheric pressure chemical ionization mass spectrometry（HPLC-DAD-APCI-MS）was used to identified degradation products ofβ-carotene standard solution dried by CO₂ HPD.The results showed that the isomers were 13-cis-β-carotene,9-cis-β-carotene,9,15-di-cis-β-carotene,9,13-di-cis-β-carotene;oxidation products wereβ-carotene-5,6-epoxide,β-carotene-5,8-epoxide,β-carotene-5,8,5’,8’-diepoxide and apocarotenones or apocarotenals,and those products are combined to infer its possible degradation pathway.Taking this as a control,the degradation products ofβ-carotene in dehydrated carrot samples were studied.It was found that the isomerization products were mainly 13-cis-β-carotene and 9-cis-β-carotene,and there was no diepoxide at low drying temperature and low oxygen.High drying temperature and long time drying led to the disappearance ofβ-carotene-5,6-epoxide.（4）The degradation kinetics ofβ-carotene in carrots dried by CO₂ HPD was studied.The results showed that the degradation reaction ofβ-carotene accorded with the first-order kinetics during the whole drying process,and the degradation rate increased significantly with the increase of drying temperature,wind speed and medium oxygen concentration,while the half-life ofβ-carotene was shortened significantly.As the thickness of the slice increased,the degradation rate decreased and theβ-carotene half-life was prolonged.Drying temperature and medium oxygen content are the most important factors affecting degradation rate.At the same time,the correlation between the content ofβ-carotene and the parameters of the sample color was analyzed dynamically.It was found that the L*,a*and△E values could be used to predict the loss ofβ-carotene.（5）The in vitro test was used to evaluate the effect of CO₂ HPD on the bioaccessibility ofβ-carotene in carrots.The results showed that the effect of drying temperature was very significant.The bioaccessibility ofβ-carotene in dehydrated samples was significantly higher than that of fresh carrots without drying,and the value of dried samples at 60°C reached the highest level.The oxygen concentration of the drying medium had great influence on theβ-carotene micelle rate in the sample,while the value of the sample dried by CO₂ gas reached the highest.Gradient addition of fat increased the bioaccessibility ofβ-carotene in all samples,especially with 10%fat addition,and they were significantly correlated.In this paper,the CO₂ HPD drying process of carrot and the changes ofβ-carotene were studied systematically.The results of this research have positive significance for the theoretical and technology development of modified atmosphere heat pump drying.