Sulphur dioxide was chosen for being incorporated into the
active packaging. The list of food additives within the European
Union contains 7 different chemicals that can be used to release
sulphur dioxide: numbers E221 to E228. Sodium metabisulphite
(E223) was selected because of its good solubility in glycerine and
propylene glycol, chosen as potential solvents, compatible to the manufacturing of the active paper. Sulphur dioxide had to be
generated in situ and progressively released from the packaging to
act on the foodstuff, so an acid was needed. Among the wide range
of acids that can be used to add to the packaging, citric acid was
chosen because it is commonly used as an antioxidant food additive
(E330). The whole mixture was added as a coating on paper substrate,
which was later used as active paper label.
The selection of other active agents was based on their antioxidant
capacity, previously measured using the ORAC assay. Cinnamon
essential oil (2.11 ± 0.16 g Trolox/g active agent) and green tea
extract (1.97 ± 0.02 g Trolox/g active agent) showed the highest
antioxidant capacity. The extract of green tea showed ten times
more antioxidant capacity than any extract containing anthocyanins
(purple carrot liquid extract: 0.20 ± 0.02 g Trolox/g active
agent; purple carrot powdered extract: 0.19 ± 0.02 g Trolox/g active
agent; bilberry extract: 0.15 ± 0.02 g Trolox/g active agent). The
liquid extract of purple carrot was also selected to be incorporated
into the active film.