Formation Mechanism Of Textural And Structural Properties Of Instant Controlled Pressure Drop (DIC) Dried Carrot Chips

Carrots(Daucus carotoa L.)contain abundant functional compounds like carotenoids,which could be used as a safe supplement of vitamin A,rich in antioxidant activity,anti-inflammatory activity and anticancer activity.DIC(Instant controlled pressure drop)drying as a new drying technology can significantly expands the volume and pores of products,result in desirable texture and attractive crispy mouth-feel of fruit and vegetable chips.While,for materials with compact and relatively firm tissue morphology,such as carrots,the expansion and pores formation of the DIC-dried chips were limited when only with pre-drying.According to the results of pre-experiments,freezing,osmotic dehydration and specific tissue zones were regarded as the main factor that affacts the texture of DIC-dried carrot chips severely.In the present study,carrot was seleted as the typical material to produce DIC-dried chips and the formation mechanism of textural and structural properties of the chips were investigated.(1)The effects of freezing pretreatment with different freezing temperature on the texture and porous characteristics of DIC dried carrot chips were studied.Freezing curves and microstructure of the frozen carrots indicated that the tissues were severely damaged at freezing temperature of-18℃ and-40℃,with freezing time of 84.2 min and 45.0 min respectively.In addition,the revelant chips also showed superior texture(hardness 38.45–39.60 N,crispness 1.18–1.95 mm)and higher volume ratio(0.59–0.71)than that of the-80 and-196℃ frozen samples.Moreover,the analysis of SEM and X-ray microtomography images demonstrated that the samples obtained more homogeneous porous microstructure with mode pore-size around 981 and 513μm,respectively.In conclusion,selected and controlled freezing pretreatment before DIC drying could yield desirable and crispy carrot chips.(2)The effects of osmotic treatments on the volume,texture,microstructure,color and sensory qualities of DIC-dried carrot chips and their correlations were investigated.The results confirmed that the expanded volume of the DIC-dried samples was highly dependent on the solute gain(R~2=0.9573),and a threshold of SG(about 0.15 g/g,wb)was found to achieve well expanded to reach a plateau volume about 21–24 cm~3.The microstructural characteristics of the samples were substantially affected by OP conditions,and the distribution range of pore size and cell wall thickness were broadened with increasing of osmotic time,while narrowed with the increasing of solid gain.When the same solid gain was obtained,broadening with the increasing temperature.Sensory evaluation indicated that the samples appropriately pretreated(e.g.40°Brix 98±2℃for 21 min,60°Brix 98±2℃for 4 min or 6 min,60°Brix 50±2℃for 50 mina and 25±2℃for 90 min)were of great overall quality(≥4.03).Furthermore,the indicators included hardness,volume and color,which were significantly correlated with the solute gain(p<0.01).Overall,the controlled osmotic processing could be used as an excellent pretreatment for engineering the volume expansion and microstructure of DIC-dried carrot chips.(3)Apple juice concentrate used to enhance the textural and nutritional qualities of DIC-dried carrot chips were investigated.The apple juice concentrate impregnation substantially enhanced the physical properties of the carrot chips,including hardness(38.28 N),crispness(2.01 mm),porosity(66.72%),and homogeneous microstructure,which were comparable to the chips obtained by using sucrose and maltiltol based osmotic solutions.Furthermore,compared with the sucrose and maltiltol impregnated chips,higher retention of carotenoids(302.81μg/g,db),higher multiplicity of phenolic compounds,stronger antioxidant activities,and superior sensory score were observed in the chips pretreated with apple juice concentrate.(4)The cells morphology,characteristics of cell wall pectic polysaccharides(CWPs)from ground tissue(GT),junction of ground and vascular tissue(JT),and vascular tissue(VT)of carrot were investigated to explore the formation mechanism of textural properities of DIC-dried chips.Larger expansion volume was obtained in the carrot chips derived from GT,which accompanied with superior textural qualities compared with the chips derived from JT and VT.Remarkable differences were obtained in the cell distribution and wall thickness,amount of pectic fractions,galacturonic acid content,degree of methoxylation,and sugar composition of CWPs that fractionated from different tissue zones of raw carrots.The characteristics of CWPs were confirmed to be a substantial factor that significantly affected the expansion ratio and textural properties of DIC-dried chips.Tissues coupled with low AIR content,high pectic content and low linearity of NEP fraction were in favor for porous structure formation during DIC,which providing a mechanistic insight of the relationship between variation in CWPs and the textural properities of DIC-dried fruit and vegetable materials.