The direct incorporation of bioactive agents into polymers has been commercially applied in drug and pesticide delivery, household goods, textiles, surgical implants and other biomedical devices. Few food-related applications have been commercialised (Appendini and Hotchkiss, 2002). Direct incorporation of AM/AO additives in packaging films is a convenient means by which AM/AO activity can be achieved. Han (2000) suggested an extrusion process for direct incorporation of AM/AO additives into a low density polyethylene (LDPE), as a polymeric matrix layer. A virgin plastic resin and active additives are compounded and pelletised to produce masterbatch resins. These expected amounts of the masterbatch pellets are then added to a plain plastic resin to homogeneously fabricate final active AM/AO packaging materials with a concentration distribution of active agents and a controlled release rate.
Apart from compounding by an extrusion process, melt blending could be an alternative process. The first step consists of melt blending the polymeric matrix (such as low density polyethylene (LDPE), polylactic acid (PLA) and polycaprolactone (PCL)) with one of the natural plant extracts by using an internal mixer, controlled by a measuring drive. The mixing temperature is 155° C, 140° C and 80° C for PLA, LDPE and PCL, respectively. In the second step a hot press is used to prepare slabs with a thickness of 1 mm. Materials were heated at the same temperature of mixing, pressed at 50 bar for 3 min and subsequently cooled to 30° C under pressure. The slabs were cut into short pieces in order to obtain a material suitable for extrusion. Finally, the active AM/AO films are obtained by feeding the pellets into an extruder (Del Nobile et al., 2009).
Due to the high loss of natural plant extracts during the higher temperature of the extrusion process, microencapsulation of these extracts via inclusion compounds (i.e. cyclodextrins) can minimise their loss. Cyclodextrins (CDs) are cyclic oligosaccharides that have been derived enzymatically from starch and have the ability to encapsulate other molecules within their ringed structure. Cyclodextrins are represented as shallow truncated cones composed of six, seven and eight glucose units and termed α-, β- and γ-CD, respectively. From the manufacturing point of view, the main product is β-CD. Purification of the α- and γ-CD considerably raises their production cost and puts them into the fine-chemical classification, i.e. very expensive.
The primary polar hydroxyl groups project from one outer edge and the secondary polar hydroxyl groups project from the other end. While outer surfaces (tops and bottoms) are hydrophilic, an internal cavity has a relatively high electron density and is hydrophobic in nature, due to the hydrogens and glycosidic oxygens oriented to the interior of its cavity (Shahidi and Han, 1993). Because of the hydrophobic nature of the cavity, the molecules of suitable size, shape and hydrophobility enable non-covalent interaction with CDs to form stable complexes. For instance, β-CD molecules are able to form inclusion complexes with volatile compounds of typical molecular mass ranging from 80 to 250 (Rusa et al., 2001). Microencapsulation is one of the most effective techniques for protecting them against oxidation, thermal degradation and evaporation (Szente and Szejtli, 1988; Reineccius, 1989; Hedges et al., 1995).
This protection occurs because volatile molecules are held tightly within the molecular structure of β-CD. The interaction between β-CD (host) and volatile molecules (guests) may involve total inclusion or association with only the hydrophobic part of the molecule (Shahidi and Han, 1993). Goubet et al. (1998) stated that retention of volatile compounds is a complex phenomenon in which several factors take part. With particular regard to the volatile compound, chemical function, molecular weight, steric hindrance, polarity and relative volatility have been shown to be important. For example, the higher the molecular weight, the higher the retention (Reineccius and Risch, 1986; Goubet et al., 1998). Among the chemical groups reviewed, alcohols are usually the best retained compounds by carbohydrates including β-CD. The same trend, namely a higher retention of alcohols than that of other compounds, has been observed also when encapsulating a mixture of 10 volatiles in β-CD. Linalool was the most retained compound among mixtures including five esters, two aldehydes, γ-deca-lactone and butyric acid (Fleuriot, 1991). As far as polarity is concerned, the more polar the compound the less of it is retained (Voilley, 1995). According to Saravacos and Moyer (1968) and Bangs and Reineccius (1981), the higher the relative volatility of a compound, the lower is its retention. When the carrier is considered, it has been shown that retention is influenced by its chemical functions, its molecular weight and its state. Based on literature data, Goubet et al. (1998) summarised that the amorphous state of the carrier is the most efficient for retention of volatiles, the collapsed state results in a loss of volatiles and crystallisation leads to the largest release of the encapsulated compounds.
β-CD is widely used in the food, pharmaceutical, medical, chemical and textile industries. Huang et al. (1999) proposed using an inclusion compound (IC) between an antibiotic and β-CD in biodegradable/bioabsorbable PCL film for medical applications. Antibiotic ICs have been incorporated into bandages, dressings and sutures. Later, Lu et al. (2001) reported a promising result for Irgasan DP300 (Triclosan)-β-CD-IC embedded in PCL films. These are rendered resistant to the growth of E. coli. Japanese horseradish (wasabi) extract containing allyl isothiocyanate (AIT) as a major active AM component has been encapsulated in β-CD to control its volatility. Allyl isothiocyanate becomes volatile when AIT-β-CD-IC is exposed to a high humidity environment. This AIT-β-CD-IC has been incorporated in LDPE film. After packaging of the food product, the evaporated AIT migrates to the food surface and then inhibits the growth of microorganisms (Koichiro, 1993).