CHEMICAL COMPOSITION, SENSORY EVALUATION, MICROBIAL LOAD OF FREEZE-DRIED AND OVEN-DRIED SOURSOP (Annona muricata) PULP POWDERS AND THEIR EFFECT ON BLOOD GLUCOSE LEVELS OF NORMO-GLYCAEMIC ADULTS

Abstract

Soursop is an underutilized and highly perishable, tropical fruit rich in several nutrients and has many biological benefits. This study determined the chemical composition, sensory evaluation, microbial load of freeze-dried and oven-dried soursop (Annona muricata) pulp powders and their effect on blood glucose levels of normo-glycaemic adults. Mature soursop fruits were purchased at a local market in Nsukka, Enugu State, Nigeria. The fruits were allowed to ripen for 5 days at room temperature (37°C). The ripe fruits were washed, peeled, seeds handpicked and the pulp spread out in drying containers. Freeze-drying was done at -40°C for 72 hours and oven-drying at 70⁰C for 48 hours. Proximate composition, mineral, vitamin, available carbohydrate, phytochemical contents and microbial load were determined using standard methods. Hundred grammes each of the freeze-dried and oven-dried soursop powders were liquefied with 500mls of water to formulate unsweetened drinks. Another 100g each of the dried powders were liquefied with 500mls of water and flavoured with four tablespoons (60g) of honey to formulate drinks. Two hundred grammes of fresh soursop pulp was liquefied with 400mls of water and flavoured with four tablespoons (60g) of honey to formulate the drink. Sensory evaluation of all the drinks was conducted using a 9-point hedonic scale. Effect of the unsweetened freeze-dried and oven-dried drinks on blood glucose levels of normo-glycaemic adults was determined by finger prick blood sample every 30 minutes at 30, 60, 90 and 120 minutes postprandially using sterile lancets and glucometer (Accu-Chek). Data obtained from the study were analyzed using Statistical Product for Service Solution (SPSS) for Windows version 21. Results were presented as means and standard deviations. Analysis of Variance (ANOVA) and Duncan's new multiple range tests were used to separate and compare means at p<0.05. The freeze-dried powder had higher moisture (12.86±0.01%), fat (6.66±0.01%) and carbohydrate (43.70±0.01%) contents while the oven-dried sample had higher protein (20.37±0.01%), ash (12.26±0.01%) and crude fibre (16.36±0.01%) contents. Available carbohydrate was 47.73±3.04g and 54.55±0.25g for the freeze-dried and oven-dried respectively. Vitamin C content was 2.76mg/100g (Freeze-dried soursop, FrDS) and 1.69mg/100g (Oven-dried soursop, ODS), vitamin B1 was 0.05mg/100g (FrDS) and 0.04mg/100g (ODS) and vitamin B2 was 0.09mg/100g (FrDS) and 0.05mg/100g (ODS). Ca content was 1.18mg/100g and 1.46mg/100g; Fe was 24.26mg/100g and 33.35mg/100g; Mg was 42.26mg/100g and 55.58mg/100g; Na was 20.15mg/100g and 25.58mg/100g; K was 23.35mg/100g and 36.67mg/100g; Zn was 0.87mg/100g and 1.56mg/100g while P was 29.36mg/100g and 66.57mg/100g for the FrDS and ODS soursop powders respectively. The FrDS had higher total phenolics (3.36mg/100g) and total flavonoids (3.69mg/100g) while the ODS had higher phytate (2.36mg/100g), tannin (1.87mg/100g) and oxalate (2.49mg/100g) contents. The drink from fresh soursop (FSD) had the best score for overall acceptability (7.75±0.85) at P<0.05 compared to the other drinks. Mean values of blood glucose levels of subjects at 30, 60, 90 and 120 minutes from baseline showed that the drinks did not spike blood glucose but maintained it within the normal range. Bacillus subtilis was isolated from the FrDS while Staphylococcus aureus was obtained from the ODS both in acceptable health levels. Dried soursop pulp, especially oven-dried, can be a good source of protein, fibre and minerals including K, Fe, P, Mg and Na while the freeze-dried soursop is a better source of carbohydrate, fat and vitamins than the oven-dried. Dried soursop pulp has potential in the prevention of diabetes due to its hypoglycemic effect