Retention of aroma compounds by corn, sorghum and amaranth starches
Introduction
The encapsulation of aroma compounds is used to entrap volatile and labile flavorings into a carrier in order to protect them from evaporation and/or degradation as well as from off-flavor development upon storage (Baranauskiene et al., 2007, Partanen et al., 2002). The mechanism of protection and release of the encapsulated compounds would differ between the various carriers. The encapsulation wall system is often based on polymers with hydrophilic and/or hydrophobic groups that display the ability to form a network-like matrix (Baranauskiene et al., 2007).
Starch and its derivatives are widely used not only as thickeners, stabilizers, and gelling and texturing agents but also as carriers of aroma compounds. The nature of some starches, their granule size, shape, specific area, porosity, crystalline or amorphous character, etc., and the composition of starch–polysaccharide matrices were found to play a major role in binding the volatile substances (Boutboul et al., 2002, Baranauskiene et al., 2007, Savary et al., 2007, Zafeiropoulou et al., 2010). The binding of flavorings to starch is known as inclusion complex formation through the hydrophobic bonding in the amylose helix and/or polar interaction that involve hydrogen bonds between hydroxyl groups of starch and the odorant (Arvisenet et al., 2002, Boutboul et al., 2002). The binding of low molecular compounds to starch might also be based on their non-specific sorption to the starch powders or to granule agglomerates (Conde-Petit et al., 2006, Misharina, 2004).
These porous structures of corn starch treated with glucoamylase revealed a high ability to retain peppermint oil (Zhao, Madson, & Whistler, 1996). The porous starch granules were found to retain 29% of the oil after standing in the open air for two months, while only 12% of oil remained on native starches after three days of storage in an open dish. Corn and barley starches subjected to succinylation were deemed promising carriers of volatile meat flavor compounds (Jeon, Vasanthan, Temelli, & Song, 2003). Over a 4-week storage period, flavor retention by these starches was higher than by β-cyclodextrin which is commonly used for encapsulation. It was also noted that hydrophobically-modified starches were used as shell materials in spray drying to encapsulate up to 50% of flavor compounds (Baranauskiene et al., 2007). The emulsifying starch (HiCap) was found to be an effective heat-stable matrix for encapsulating caraway oil (Partanen et al., 2002). Studies on starch type effect on the retention of aroma compounds from strawberry showed that corn starch (Amilogel G) was the best carrier for the majority of the compounds analyzed (Vidrith, Zlatic, & Hribar, 2009). The retention of aroma compounds on cross-linked waxy corn starch and/or polysaccharide gel was ascribed to the physicochemical interaction between the low molecular compounds and gel components (Savary et al., 2007). Apart from the structural and physicochemical characteristics of the polymer matrix, the efficiency of aroma compound retention was also related to their structure and chemical properties (Misharina, 2004). This author studied also the storage-related changes in the retention of different aroma compounds on dry corn starch and proved that the corn starch prevented volatile component oxidation during storage.
The small pores, noted in literature as ‘pin holes’ were found on the granule surface of native sorghum starch using different microscopy techniques. It was suggested that these structures were openings to channels, with diameter from 5 to 400 nm, connecting an internal cavity at the granule hilum to the external surface (Fannon et al., 2003, Perez et al., 2009, Singh et al., 2010). It is likely that the pores and channels, naturally formed on the granule surface, allow some molecules with low molecular weight to directly access to the granule interior. However, the exact nature of the interaction between ligands (i.e. aroma compounds) and granule surface of native sorghum starch remains unknown since this topic is scarce in literature.
Amaranth starch was also found to demonstrate a unique character of granules with respect of their size and surface properties. These starch granules appeared as small (up to 2.0 μm) (Kong, Corke, & Bertoft, 2009), and they tend to form large aggregates (up to 80 μm in size) (Mariotti, Lucisano, Pagani, & Perry, 2009). The literature data showed that the specific surface area of amaranth starch was significantly higher compared to that calculated for native potato starch or maize starch (Marcone, 2001, Sujka and Jamroz, 2009, Szymonska and Wodnicka, 2005), but on the other hand the specific surface area was also affected by other factors (Boutboul et al., 2002). A stable pore structure was found to be critical to provide a high surface area including the interiors and outer regions of the microspheres (Glenn et al., 2010). For sorghum and amaranth starches demonstrate unique surface properties, they might be considered as interesting material for odorant retention (Fannon et al., 2003). Bearing this in mind, in this study the ability of corn, sorghum and amaranth starches to retain aroma compounds was studied in relation to surface characteristics, thermal properties, and microstructure of starch.
Section snippets
Plant material and starch isolation
The seeds of plant species Amaranthus cruentus L. were donated by the Metro Industrial Centre “Szarłat” s.c. (Łomża, Poland), and sorghum Sorghum grains bicolor (v. Rona 1) were purchased from the Kutno-Centre for Sugar Beet Breeding in Straszkow, Poland.
Commercial maize starch was donated by the Department of Food Concentrates in Poznan, the Institute of Agricultural and Food Biotechnology, Poland.
The starch from amaranth seeds was isolated and purified according to the method developed by
Retention of odorants by starches
The retention values of individual odorants determined for the 2-day-stored starch powders (reference sample) ranged from 0.99 to 1.0. They indicated that none of the analyzed compounds, that formed the essential oil mixture, was lost during this period of storage.
After 3 months of storage, the ability of corn, sorghum and amaranth starches to retain odorants from the essential oil mixture was observed to vary (Table 2). Amaranth starch demonstrated the highest average retention values for the
Conclusions
The corn, sorghum and amaranth starches demonstrated different abilities to retain aroma compounds from the essential oil mixture upon 3 and 7 months of storage. The retention of aroma compounds from the group of monoterpene hydrocarbons on the stored starches was the lowest, compared to such groups of odorants as: alcohols, ketone, phenols and sesquiterpene hydrocarbons. The overall exposed surface and/or pores distribution found for aggregates of amaranth starch make it the most promising for
Acknowledgments
The study was financed by a grant from the Ministry of Science and Higher Education (Grant No. N N312 101938).
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