Effect of different Pre-treatments on the physicochemical properties of Freeze-Dried Ananas comosus L

Pineapple is high in vitamins, minerals, antioxidants and it is the major source of bromelain enzyme which is a strong anti-inflammatory and anti-cancer compound. Freeze drying technique has been used to preserve the nutrients in fruits by using the principle of sublimation which dries the materials at very low temperature and pressure. However, the physicochemical properties of its product may deteriorate without suitable pre-treatment. The present study aims to study the effect of different pre-treatment techniques on the changes of physicochemical properties of pineapple slices when drying in freeze-dryer. Four pre-treatments techniques were applied such as hot blanching (HB), hot syrup (SB), soaking in ascorbic acid (AA) and citric acid (CA) to the physical properties, antioxidant activity and bromelain content of the freeze-dried pineapple slices were studied and compared to the non-treated (C) sample. The results showed that the SB pineapple has significantly higher moisture content than the untreated dried pineapple. While sample soaked in CA has the lowest pH value and all pre-treatment has higher total soluble sugar (TSS) value than the control pineapple. For colour determination, a significant (P < 0.05) differences were observed on the values b* of the samples subjected to different pre-treatment techniques. The dried untreated pineapples slices presented higher lightness (L*) and decrease yellowness (b*) of dried pineapple colour after drying. The b* values of the samples subjected to HB treatment was significantly higher and preserve the yellowish pineapple colours followed by pre-treated by CA when compared with untreated dried pineapples. For chemical analysis, the pineapple slices soaked in 2% ascorbic acid solution has strongest antioxidant activity as shown by the lowest IC50 value (17.56 mu g/mL) and the weakest activity is the SB pre-treatment (175.70 mu g/mL). However, SB treatment gives the highest bromelain activity (0.011 U/ml) followed by AA treatment at 0.0105 U/mL. Drying of fresh pineapple slices directly without pre-treated the fresh samples can cause changes to the physicochemical properties of pineapple slices when drying in freeze-dryer. Inconclusion, the physical and chemical attributes correlate with the types of pre-treatment techniques. (C) 2019 Elsevier Ltd. All rights reserved.