Application of response surface methodology (RSM) to osmotic dehydration and drying of green bell peppers

A study to investigate the effect of osmotic solution concentrations of common salt (5%(w/w), 10%(w/w), 15%(w/w), 20%(w/w) and 25%(w/w)) and osmotic process durations (60 min, 90 min, 120 min, 150 min and 180 min) at average room temperature of 31 °C on the drying rate and nutritional qualities (vitamin C, crude protein, crude fibre, fat and ash content) of green bell peppers was conducted. Response Surface Methodology (RSM) under central composite design in Design Expert 8.0.3 computer software package was used to design the experiment, analyze data, and present all results with 3-dimensional plots. The temperature of 50 °C was used to dry all the pre-treated samples to the moisture content of about 7% (wb) in a fabricated cabinet dryer. The results showed that an increase in osmotic solution concentration and osmotic process duration caused the drying rate to drop to about 18g/h, then later increase to more than 21g/h; vitamin C did not reduce below 50mg/100g; crude protein increased but later reduced, however the range of about 18-20.5% was obtained; crude fibre increased to about 4.8% but later reduced to about 4.6%; also, fat decreased but later increased to a maximum value of about 16.5%. The increase in osmotic solution concentration kept the ash content at a steady value of about 4.01%, but the increase in osmotic process duration led to a very sharp increase in ash content from 4.01% to 4.25%. All drying rate (15.53g/h) and nutrients values obtained were better than the control value, that is, an untreated dried sample (vitamin C (46.02 mg/100g), crude protein (17.40%), crude fibre (4.16%), fat (11.42%), ash content (4.01%). This further confirmed that osmotic dehydration is a quality improving pre-treatment method