| Green asparagus, which trender stalk has nice flavor and crisp texture, is an abundant source of various nutrients which are the higher nutritional value and healthful efficiency. But green asparagus is difficult to storage and the harvested asparagus is liable to decay. The storage life can be extended and the nutrients can be retained better by vacuum freeze drying. So the freeze drying technology of fresh green asparagus was systematically studied in the paper. Optimal picked time, technology of pretreatment and technology of prefreezing were studied and determined through experiments. Finally,the parameters of vacuum freeze drying of green asparagus,the way increasing the rehydration and the quality stability of dehydrated green asparagus were also studied.The main aspects and the results were represented as follows:(1)The optimal picked time:The green asparagus could be divided four growing grades on the basis of growing time and condition. Contents of water, sugar, vitamin C, protein and chlorophyl were mensurated,then making a research on the deciding weight theory of principal components analysis with the five factors.It was sure that the grade 3 was determined the optimal picking time.(2)The technology of scalding:There is a close relationship between cutting length and scalding technology.By U12(122×6) uniform design with the factors which were cutting length.scalding time and scalding temperatrue, the parameters could be determined. Finally,the technology were optimized combined with the prefreezing xutting length 56 cm,scalding temperatrue 9095 ℃.scalding time 3.5—4 min.(3)The technology of green retainning:The cutting green asparagus would be soaked in concentration of Na2CO3 in order to destroy the wax covering green asparagus,then Zn2+ and Cu2+ were used to rentain green.Through L25 (56) orthogonal design,the technology of green retainning were determined:soaking in 30 g/kg concentration of Na2CO3 for 40 s,pH 7.5,the concentration of Zn(CH3COO)2 was 200 mg/kg, the concentration of Cu(CH3COO)2 was 50 mg/kg,the soaking time was 8 h.(4)The technology of hardness maintaining: Concentration of CaCl2 was used to maintain hardness,through 6×6 two factors cross design which the factors were concentration of CaCl2 and soaking time.the parameters were determined: concentration of CaCl2 was 0.2%,soaking time was 50 min.(5)The eutectic temperature:The result showed the eutectic temperature was -11 ～ - 13 ℃ byresistance.(6)The technology of prefreezing:Through 3X5 two factors cross design which the factors were prefreezing temperature and prefreezing time.and the targets were dryness velocity and ratio of rehydration, parameters were determined combined with the scalding.The optimal technology of prefreezing were:prefreezing temperature was — 20"C,preefreezeing time was 6 h.(7)The technology of vacuum freeze drying:It was the main object that pressure, tempterature of sublimation and temperature of resolution in this section.The optimal vacuum freeze drying technology obtained through Lp(34) orthogonal design which were pressure 55 Pa,tempterature of sublimation 55 °C .temperature of resolution 65 °C.(8)The study on increasing rehydration of dehydrated green asparagus. This section studied the method of increasing rehydration by hydrophile (malt dextrin, sugar, salt) added. Through L9(34) orthogonal,it was sured that the better technology of increasing rehydration was : soaking with 20% malt dextrin + 20% sugar + 15% salt.(9)The study on the quality stability of dehydrated green asparagus.lt was proved that the vavuum freeze drying green asparagus packaged with vavuum can retain nutrients and color better by mensurating various nutrients and color in the storage.
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