The Effects Of Different Treatments On Quality Properties Of Microwave Vacuum Dried Lotus Seeds

Lotus(Nelumbo nucifera Gaertn.),as a kind of advanced nourishing food,is claimed to fight tissue inflammation,resisting cancer,detoxify the spleen,nourish the blood,ease the symptoms of diarrhea,and to reinforce the kidneys and it is widely consumed in the folk..Since the moisture content of fresh seeds is between 65%and 70%and polyphenol oxidase activity of tissue is high.Fresh seeds are easy to infected by the development of microorganisms or occur enzymatic browning,which might cause a short shelf life and quality decay.Therefore,lotus seeds are commonly dried to increase their shelf life at present.Compared to traditional air dried lotus seeds associated with unfavorable physical and chemical changes during drying period,microwave vacuum drying(MVD)can reduce drying time significantly,retain heat sensitivity and oxidizing easily nutrient component to produce high-quality dried lotus seeds whose quality is similar to freeze dried lotus seeds but much less processing cost than freeze dried lotus seeds.Therefore,MVD has become a hot topic in the field of food drying in recent years due to the fact that it has the quick advantages of microwave drying and low temperature advantages of vacuum drying.However,bound water fails to dry in later period,and drying rate decreases significantly during later period of drying.In addition,qualities are destroyed heavily in later period,which restricts further development of lotus seed industry.Based on this,fresh lotus seeds are regarded as raw materials.The effects of different treatments include MVD,tempering treatment and ultrasonic pretreatment on quality properties of microwave vacuum dried lotus seeds are investigated.The fractal theory is used to quantify the microscopic pore structure of lotus seeds,with the aid of box-counting method to build the relationship between the microstructure and macro-properties.This study will promote a theoretical foundation of microwave vacuum drying for the starch-based food,like lotus seeds;it also provides a method that can be used to optimize the technology and processes used in foodstuffs drying and provide a new quality control technique,further to promote the development of lotus seeds processing industry.There are some following main research contents:(1)The effects of MVD on quality properties of dried lotus seeds.Drying rate of lotus seeds increased with the increase of microwave power densities.Logarithmic model,Wang and Singh model,Page model,and new model were used to fit drying process of microwave vacuum dried lotus seeds and drying kinetics of microwave vacuum dried lotus seeds were established.After comparing fitting coefficients and fitting curves,Page model and new model had higher regression coefficient.MVD increased glass transition temperature and gelatinization temperature of lotus seeds.The drying process was accompanied by intermigration of water molecules in three states(bound water,non-flowing water,free water).Non-flowing water and free water were the main population groups of microwave vacuum dried lotus seeds.With the increase of microwave power densities,the color change of dried lotus seeds decreased and free amino acids contents increased.(2)The effects of tempering treatment on quality properties of microwave vacuum dried lotus seeds.Tempering treatment can significantly increase drying rate of microwave vacuum dried lotus seeds in later period,which reduce drying time from 16 min to 11 min.With the increase of intermediate moisture content,the effect of tempering treatment on drying rate was more significant.According to Fickian diffusion law,the Deff of microwave vacuum dried lotus seeds with tempering treatment ranged from 1.05 ×10-5 m2/s to 1.45 ×10-5 m2/s.After comparing fitting coefficients and fitting curves of different drying kinetics,a newly proposed model(MR=exp(-(t/b)a)+c exp(-kt))had the highest regression coefficient and it can fit well with drying kinetics of microwave vacuum dried lotus seeds with tempering treatment.Tempering treatment had an insignificant impact on glass transition temperature and gelatinization temperature of microwave vacuum dried lotus seeds.Bound water and non-flowing water were the main population groups of microwave vacuum dried lotus seeds with tempering treatment.The color change of dried lotus seeds dcreased with the decrease of intermediate moisture content,reaching minimum of △E=2.69.Tempering treatment further increase free amino acids contents of microwave vacuum dried lotus seeds,reaching maximum of free amino acids contents=648.63(dry basis,mg/100 g).(3)The effects of ultrasonic pretreatment on quality properties of microwave vacuum dried lotus seeds.Ultrasonic pretreatment can significantly enhance drying kinetics of microwave vacuum dried lotus seeds and improve drying rate in later period.With the increase of ultrasonic intensities,the effect of ultrasonic pretreatment on drying rate was more significant.The microwave vacuum drying process of lotus seeds was simulated by using different dynamic models proposed by previous researchers and drying kinetics of microwave vacuum dried lotus seeds were established.After comparing fitting coefficients and fitting curves of different drying kinetics,a newly proposed model(MR=exp(-(t/b)a)+c exp(-kt))had the highest regression coefficient and it can fit well with drying kinetics of microwave vacuum dried lotus seeds with ultrasonic pretreatment.Ultrasonic pretreatment further enhanced glass transition temperature and decreased gelatinization temperature of microwave vacuum dried lotus seeds.Bound water and non-flowing water were the main population groups of microwave vacuum dried lotus seeds with ultrasonic pretreatment.The color change of dried lotus seeds decreased with the decrease of ultrasonic intensities,reaching minimum of △E=2.17～3.69.Ultrasonic pretreatment further increase free amino acids contents of microwave vacuum dried lotus seeds,reaching maximum of free amino acids contents=666.13(dry basis,mg/100 g).(4)The effects of different treatments on relationships between microstructure of lotus seeds and rehydration capacity.By establishing relationships between microstructure of dried lotus seeds and rehydration capacity,the results showed that there was a good linear relationship between fractal dimension and rehydration ratio.That is to say the larger fractal dimension of microstructure of microwave vacuum dried lotus seeds,the greater the rehydration ratio.By establishing relationships between fractal dimension of microstructure and more macroscopic properties under different drying methods,the drying methods of dried lotus seeds can be identified.This was because although ultrasonic pretreatment and tempering treatment changed the fractal dimension of the microstructure of dried lotus seeds to some extent,the effect was not significant.However,fractal dimension of microstructure of dried lotus seeds can be significantly changed by drying methods which played a dominant role in structural change.