L-(+)-ascorbic acid stabilization in the amorphous matrix of pectin edible films

PEREZ, C; GERSCHENSON LN; ROJAS, A

Lisboa, Portugal.

Conferencia; EFFOsT Joint Conference 2007; 2007

European Conference of Food Science EFFOsT

A biodegradable film based on pectin of high degree of methylation was elaborated to carry L-(+)-ascorbic acid (AA) for natural antioxidant protection of foods. AA retention along film casting (initial concentration) and storage (25ºC; 57.2%-relative humidity, RH) as well as the subsequent non enzymic browning (NEB) development were assessed. Film-forming solution was prepared by pectin (CP-Kelco) dissolution in distilled water under high speed shear, followed by heating to 95ºC on a hot plate. Glycerol as plasticizer, potassium sorbate (antimicrobial) and AA (0.100% w/w) were added. Total system was made to 100.00 g by addition of distilled water, and homogenized. Hot solution was poured on to leveled polystyrene plates, cooled to room temperature and dried under air convection at 60ºC/2 hours. Films were peeled after cooling and stored at 25ºC over a NaBr-saturated solution (RH 57.2%). For kinetic studies, samples were collected along storage: AA was determined through 2,6-DPIP-spectrophotometry, yellow index (YI) parameter was measured through a Minolta colorimeter (Hunter, D-65 illuminant/2º observer) to report the NEB developed. The glass transition temperature (Tg) was also determined on the equilibrated films with a DSC. Water activity and pH of films were also followed. The film-pH was 3.32±0.04 along storage. AA retention was around 100%±1% after drying and cooling. AA destruction followed a pseudo-first order kinetics in the pectin matrix, which obeys to a destruction mechanism proposed in a previous work (1), where water concentration is included into the rate constant. A half shelf-life (t1/2) of 69 days was then calculated. YI increases against storage time with a zero-order kinetics, showing a rate constant of 6.06x10-4 YI-units/min. The equilibrated edible films were rubbery at the storage temperature assayed (25ºC) due to glycerol presence for polymer plasticization (Tg » -99ºC). Pectin led to a significantly higher AA-retention along storage in this formulated film in comparison with edible films based on gellan gum (1), where AA presented a half shelf-life (t1/2) of 26.7 days.  AA stability in amorphous films is associated to restricted mobility of water molecules as previously demonstrated (1). It is suggested that pectin polymer can produce higher AA retention in its amorphous films because of the presence of ?disorder regions? in the film network, associated to rhamnose-promoted kinks, instead of the rigid framework developed when gellan is used. Reference: (1) Leon P.G. and Rojas A.M., 2007. Food Res. Int., 40, pp. 565-575.