DEVELOPMENT OF A MAGNETIC FIELD PRE-TREATMENT DEVICE AND ITS EFFECT ON THE QUALITIES OF SELECTED VEGETABLES

Abstract

Exposing vegetables to several stages of thermal pre-treatment (heat addition) during processing has the tendency of reducing their qualities. One of the promising non-thermal pre-treatment methods is the use of Magnetic Field (MF). Therefore, a MF pre-treatment device was designed, fabricated and evaluated. The MF device consists of two main units: pre-treatment chamber and the step down transformer. The pre-treatment chamber unit has eight (8) electromagnets arranged in two (2) groups, each of four (4) electromagnets along the sides of a rectangular aluminium structural frame (300 mm x 350 mm x 180 mm). The step down transformer unit is the control unit of the device. Its primary voltage (about 240 V) can be stepped down to secondary voltage values (3.2 - 10.10V), this is responsible for the variation of the magnetic field strength (5 – 30 mT) values. Other components attached to it are capacitor (50 V and 2200 μF), diodes (80 and 100 A) and timer (60 min). The capacitor and diodes manipulate the alternating current from the mains for the generation of three types of magnetic fields-static magnetic field (SMF), pulse magnetic field (PMF) and alternating magnetic field (AMF). The MF pre-treatment device works on the principle of electromagnetism. Its performance evaluation gave maximum magnetic field strength of 30 mT with average capacity of 20 g/min. Also, the effect of three types of magnetic field (SMF, PMF and AMF), magnetic field strength (5 – 30 mT) and pre-treatment time (5 – 25 min) on the qualities (microstructure, colour, nutrients, microbial load) and drying rate of sweet pepper (SP) and fluted pumpkin leaf (FPL) were investigated before and after drying at 50 oC. In addition, modeling and optimization of the process was done. The total number of experimental runs with blanched and fresh samples used as controls was one hundred (100). Scanning Electron Machine (JEOL, JSM-7600F, Japan) was used to check the micro-structures. Colorimeter (CS-260, China) was used to determine three colour parameters (L* a* b*). All the nutrients considered (fibre, vitamin C, magnesium-Mg, potassium-K, carotenoids, flavonoids and alkaloids) and drying rate were determined. Microbial load count was done with digital colony counter (Model LT-37, India). Results of the quality analyses showed that the microstructures and colour indices (hue angle and chroma) of MF pre-treated SP and FPL vary at different combinations of MF pre-treatment factors with not less than 100 o and maximum of about 34 for hue angle and chroma respectively. Some MF pre-treatment combinations reduced the microbial load of SP and FPL to 100,000 – 150,000 CFU/ml from about 200,000 CFU/ml for the controls (blanched and fresh samples). Most MF pre-treatments caused Mg, K, carotenoids, flavonoids and alkaloids to have about 100 mg/100g, 800 mg/100g, 30 mg/l, 10 mg/l - 15 mg/l and 0.1- 0.2 mg/l respectively. Vitamin C, fibre and drying rates of SP and FPL were (80 mg/100g and 61.5 mg/100g) and (2.6% and 10%), (20 g/h and 12 g/h) respectively. Furthermore, developed one hundred and fourteen (114) model equations adjudged to reliably describe the behaviours of the process were selected. Lastly, the selected optimized conditions were AMF (13.53 mT and 24.83 min) and SMF (14.31 mT and 16.40 min) for SP; and SMF (26.74 mT and 14.08 min) and AMF (10.42 mT and 9.96 min) for FPL before and after drying respectively. Generally, not less than 85% of MF pre-treatment combinations caused better retention of nutrients of SP and FPL than blanching- a thermal pre-treatment method. Hence, MF pre-treatment is a possible non-thermal pre-treatment method that can be explored for the replacement of blanching. Shelf life studies of the MF pre-treated samples under different packaging materials and storage conditions should be considered in future research