Mathematical modelling of microwave vacuum drying of arthichoke (Cynara scolymus L.)

Globe artichoke (Cynara scolymus L.),belonging to the family Asteraceae, is a traditionally consumed vegetable in the Mediterranean area where its commercial production contributes substantially to the agro-economy. The consumer demand for artichokes has increased because of their reputation as a health food due to their phytochemical composition. The economic use of this vegetable mainly concerns the consumption of the edible immature flower heads (capitula), commonly referred to as "artichoke heads". The fact that artichokes have a freshly limited consumption season and shelf-life, it is necesary to increase the supply diversity by using alternative methods such as canning, freezing and drying. Drying is one of the oldest methods of fruits and vegetables preservation and is done by several methods like sun, hot air, freeze, vacuum, and microwave-assisted air/vacuum drying. Microwave vacuum drying, the emerging food dehydration method, combines the advantages of the rapid volumetric heating by microwaves and the low-temperature evaporation of moisture with faster moisture removal by vacuum. This reduces the drying time without significant quality degradation compared to conventional drying methods. In this study, it is aimed to determine the effect of process parameters such as microwave power, vacuum intensity and drying time on drying kinetics and quality characteristics of artichokes during microwave vacuum drying. Two different levels of vacuum intensity (-526 mm-Hg and -667 mm-Hg) and microwave power (450 W and 800 W) were studied and the changes in drying rate, drying efficiency, total phenolics content, color values, rehydration ratio, polyphenoloxidase enzyme activity and shrinkage coefficients of artichokes were determined as a function of drying time. The obtained data were evaluated statistically by analysis of variance. Artichokes were dried until the moisture content reached below 10% (w/w) and vacuum intensity and drying time were found to be the significant factors on lowering the water activity of dried artichokes. The drying rates were increased with the increase in microwave power all vacuum levels, and drying time reduced from 25 min at 450W to 15 min at 800W. Temperature profiles were also determined and average surface temperatures were found in the range of 56.3-75.8 °C at -526 mm-Hg vacuum intensity and 46.5-60.4°C at -667 mm-Hg vacuum intensity. It was observed the activity of the polyphenoloxidase (PPO) enzyme during drying decreased gradually as the temperature reached the inactivition temperature in a short time (<3 min). Microwave power and drying time was found to be the statistically significant factors on the PPO activity. The lowest value of the polyphenol oxidase enzyme activity was found at 526 mm-Hg and at 800 W in dried artichokes (4.95 ± 0.45 U / mg fresh product), the highest value of the polyphenol oxidase enzyme activity was found at 667 mm-Hg and at 800 W in dried enginars (8,55 ± 0.45 U / mg fresh product). The total phenolic content (TPC) of dried artichokes decreased with drying time and reduced vacuum intensity, the highest content of phenolic material was found to be at -667 mm-Hg and 800 W (12.46 ±0.78 mg ga /g km) and the lowest at -526 mm-Hg and 450 W (8.82 ± 0.05 mg ga / g km). L, a, b and Hue values are close to each other, (ΔE) was at the highest 667 mm-Hg at 800 W dried artichoke (12,13 ± 0,20), at least -526 mm-Hg was seen at 450 W dried artichoke (8,35 ± 0,08). On the rehydration of artichokes, the vacuum vacuum intensity was found to be the most significant factor besides the microwave power and drying time. The lowest rehydration rate was found at 450 W at -526 mm-Hg (1,7326 ± 0,1673) and the highest rehydration rate at 800 W at -667 mm-Hg(3,3903 ± 0,0932). In terms of the rate of shrinkage, the shrinkage coefficient was found to be at least at 526 mm-Hg and 450 W (0,2280 ± 0,0127) and the highest at -667 mm-Hg and 800 W (0,3205 ± 0,0233). A high positive corelation coefficient (r=0,898) was found between the shrinkage coefficient and rehydration ratio. The lowest specific energy requirement (6,32 ± 0,42 MJ / kg water) and maximum drying efficiency (35.89 ± 2.40%) of the microwave drying system was found at drying conditions of -667 mm-Hg and 800 W.