Vacuum impregnation (VI) is a unit operation that allows the introduction of solutions into the porous structure of plant tissues. The plant tissue is immersed in the solution of interest and is then subjected to partial vacuum, causing the removal of air from the tissue. When the atmospheric pressure is restored, the solution is drawn into the tissue, replacing the air. In this way, most of the air spaces are filled by the solution, although small air pockets may still remain. VI has been extensively studied, particularly with regard to the modification of the physico-chemical properties and sensory attributes of food products. These studies include texture enhancement, enrichment of fruits and vegetables with probiotics or micronutrients, the modification of sensory attributes, and the extension of shelf life by pH reduction. VI has also been widely used as a method of pre-treatment prior to, e.g., minimal processing, freezing, or drying of fruit and vegetables. However, little is known about the metabolic consequences of impregnating plant tissue with different substances using VI. The short-term metabolic response of impregnating spinach leaves with different substances (calcium lactate, sucrose, citric acid and ascorbic acid) was investigated using isothermal calorimetry at 5 ºC, 2 h after VI treatment. The greatest increase in metabolic heat production was observed after impregnation with calcium lactate. This may be the result of metabolization of the lactate, as well as the mobilization of starch. The different impregnation solutes led to specific changes in the carbohydrate composition of the leaves during 4 days of storage, due to sugar metabolization, glucose-to-fructose interconversion and starch mobilization in the plant tissue. The effect on the nitrate content of spinach following impregnation with sucrose was also investigated. The results showed that a small amount of sucrose (5 g/100 ml) significantly decreased the nitrate concentration. Exogenously supplied sucrose reduced the nitrate concentration in the leaves by almost 70 % during 3 days of storage at 8 ºC, compared to non-impregnated leaves. Evidence showed that sucrose was metabolized during storage of the leaves, and that the reduction in nitrate was due to the use of sucrose as a substrate for the metabolization of stored nitrate and for respiration. Thus, VI could be beneficial in the food industry in reducing the nitrate content of spinach, leading to the improvement of nutritional characteristics.





The influence of impregnation with different substances on the metabolic activity of spinach leaves in modified atmosphere packaging (MAP) was investigated. The gross metabolic activity of the impregnated spinach leaves changed significantly, depending on the impregnation solute and treatment temperature. Sucrose induced the highest metabolic heat production at 21 ºC, whereas calcium lactate led to the highest metabolic activity at 5 ºC. The high metabolic activity of sucrose-impregnated leaves was reflected by the high oxygen consumption and carbon dioxide production measured in the packaged product stored at 21 ºC. However, this was not reflected by the changes in atmosphere inside the calcium lactate-impregnated, packaged products. The incongruity between calorimetric and atmospheric measurements may be the result of the different time scales of the measurements. The results obtained in this work have helped provide a better understanding of how impregnation of spinach leaves affects their metabolic activity. The findings could be of importance in the food industry as they provide a better understanding of how spinach leaves could be metabolically affected by a certain type of compound, thus influencing specific quality characteristics and their respiration upon packaging.