Prolongation of shelf life of minimally processed spinach by combining vacuum impregnation and electric field treatments

Sammanfattning: A continuous growth of the world population raises the importance of preserving food resources while maintaining their nutrient value. Leafy vegetables such as spinach are a food category which decay quickly, and a large amount ends up as food waste. Therefore, shelf life extension while keeping a good product quality is of great interest in food processing. In this study, the effect of combining vacuum impregnation (VI) and electric pulses on eco-spinach leaves was investigated aiming to prolong their shelf life. Shelf life extension was to be achieved by provoking different metabolic processes and reactions as stress response. Various pulsed electric field (PEF) and moderate electric field (MEF) electroporation parameters were studied as stressors. In the first part of the study, protocols achieving reversible electroporation to ensure cell survival after treatment were identified. Vitamin B1, B3, B5 or B9 was introduced into the plant tissue by VI to analyse their ability to delay leaf senescence. For the shelf life evaluation treated spinach leaves were packed in perforated plastic bags and stored at 5 °C until spoilage. As retail shelf life, a decay of >10% spinach leaves per bag was defined. The atmosphere in the bags was controlled by measuring O2 and CO2 concentrations with a gas analyser, L', a' and b' values were measured with a spectrophotometer for detection of colour changes and pictures were taken under controlled conditions to record the deterioration process. VI led to a weight gain of 58-65% due to volumetric expansion since air spaces in the spinach tissue were filled up with the VI solution. VI with vitamin B3 showed the longest shelf life among the different vitamins and was used for the combined treatment. Results show that the composition of O2 and CO2 did not significantly change during storage time. PEF and MEF treated samples preserved their colour. VI led to a decrease of L' value and leaves appeared darker. The different treatments resulted in a significant faster deterioration of the leaves probably due to increased metabolic activity. The combined treatment of VI with vitamin B3 and PEF or MEF showed the longest retail shelf life with 12 days. Cells might be protected better through a synergistic effect of different metabolic processes. However, the shelf life was still significant lower compared to untreated leaves with 14 days.