Study On Spray Drying Process Of Lychee Yoghurt And Application Of Trehalose As Protective Agent

Yogurt has always been the hotspot of research and development for its unique flavor,rich nutrition and a variety of physiological functions.The production of yogurt powder can effectively extend the storage period,make it easy to transport and carry,a nd reduce packaging costs.In this paper,the lychee yogurt made from lychee juice and skim milk powder was taken as research object to maintain the survival rate of lactic acid bacteria during spray drying withtrehalose added as protective agent.Based on the research of spray drying process of lychee yogurt powder,the influences of trehalose onaroma components,surface morphology and mechanismsof particle formation of yogurt powder were further investigated.And the physical properties,fermentation,hygroscopicity and storage stability of lychee yogurt powder were evaluated.The main results were shown as follows:Effects of feed concentration,inlet temperature,feeding rate and the amount of trehalose on survival rate of lactic acid bacteria,moisture content and yield of lychee yoghurt powder microencapsulated by spray drying were discussed.The results showed that the addition of trehalose could significantly increase the survival rate of lactic acid bacteria(p<0.05).The optimum conditions of spray drying were obtained by optimizing the survival rate oflactic acid bacteria:35%feed concentration,120℃inlet air temperature,10.0 mL/min feeding rate,and6%trehalose was added.Under this condition,the survival rate of lactic acid bacteria,moisture content and yield reached 62.50%(5.36×10~8 CFU/g),6.05%and50.27%respectively.The aroma components of lychee yogurt powder were analyzed by gas chromatography-mass spectrometry(GC-MS),indicating that the addition of 3%or 6%trehalose was beneficial to the maintenance of aroma components.The surface morphology of lychee yogurt powder was observed by scanning electron microscopy(SEM).It was found that the addition of trehalose could reduce the surface shrinkage and cracks of particles and maintain the integrity of litchi yoghurt powder microcapsule structure.Influences of trehalose on the mechanisms of particle formation of lychee yogurt powder were investigated by taking whey protein isolate(WPI)powder as the research object with0%,20%and 40%trehalose added.The migration of trehalose to the interior of the particles was confirmed by scanning electron microscope(SEM)and X-ray photoelectron spectroscopy(XPS),which explained the influences of trehalose on the mechanisms of particle formation and surface morphology.The dissolution time of lychee yogurt powder was 20.78 s,indicating its good instant solubility.The coagulatingtime,pH,acidity,whey separate out,water holding capacity and apparent viscosity of yoghurt taking lychee yoghurt powder and commercially available lactic acid bacteria starter as fermentation agent was compared.It was proved that the former showed higher fermentation activity.The moisture absorption isotherms of lychee yoghurt powder at 10℃,25℃and 40℃were determined.Themoisture absorption isotherms were S-shaped andcorresponded to type II,and the GAB model turned out to be the best model for describing the moisture absorption characteristics of lychee yogurt powder.The glass transition temperature(Tg)of lychee yogurt powder was 71.61℃,analyzed using differential scanning calorimetry(DSC).And the shelf life of lychee yogurt powder was predicted to be 99.72 days.The viable count of lactic acid bacteria,water content,5-hydroxymethylfurfural(HMF)content and chromatic aberration of lychee yogurt powder during the 90-day storage were studied,which proved its good storage stability.