Enzyme infusion and thermal processing of strawberries: Pectin conversions related to firmness evolution

Strawberries were infused with fungal pectinmethylesterase (PME) and/or calcium chloride with the aim of minimising tissue damage during subsequent thermal processing (95 °C). Firmness measurements and micrographs provided information on the extent of tissue damage. These observations were linked to the chemical structure of pectin. When PME was infused in absence of Ca²⁺, the degree of methoxylation of pectin was lowered, but chains remained water soluble, indicating that they were not crosslinked. Thermal processing of PME-infused strawberries resulted in pectin solubilisation and depolymerisation which was reflected in pronounced firmness decrease and tissue damage, comparable to non-infused processed strawberries. On the other hand, when a combination of both PME and Ca²⁺ was infused, an important decrease in processing-related tissue damage was perceived. This can be explained by increased crosslinking of pectin chains with low degree of methoxylation, rendering them insoluble and less susceptible to thermal depolymerisation.