Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 66

Thermoplastic starch has been blended with a variety of other polymers via extrusion to
improve moisture resistance and flexibility (Averous, 2004; Dubois and Narayan, 2003;
Janssen and Moscicki, 2009 ); Kalambur and Rizvi, 2006 ; Yu et al ., 2006 ). Polyesters, such
as polylactic acid (PLA), polyhydroxy butyrate-co-valerate (PHBV) and polycaprolactone
(PCL), have been often used as additives in these blends, since they are also biodegradable.
Intense shear forces within the extruder reduce the size of the starch domains to several
micrometers. The synthetic polymers chosen for these starch-polymer blends will usually
have compositions and rheological properties that will enable them to form the continuous
phase in these blends, so that the starch phase will be encapsulated and the resistance to
moisture will be improved. Compatibilizing agents are usually added to ensure good adhe-
sion between the starch and polymer phases. Nanofillers, such as clays, have been found to
enhance stiffness and moisture resistance (Chivrac et al ., 2009 ). Examples of applications
for starch-plastic blends include packaging films, mulch films, planting pots, food contain-
ers and hygiene articles (Bastioli, 2001).
In summary, extruders are currently used on a large scale to process starch and starch-
rich grains into ready to eat snack foods, pastas, and so on. With the current interest in
improving nutrition and decreasing chronic diseases, it seems likely that extruders will be
used in the future to create healthy, convenient foods incorporating resistant starch along
with fruits, vegetables and whole grains. Extruders are also currently used on a small com-
mercial scale to make starch-based biodegradable foam peanuts, foam sheet cushioning, and
compost bags. Interest in these areas will likely continue, due to depletion of petroleum as a
resource for plastic materials. Since extruders can heat and process starch with low moisture
contents, use of extruders to chemically or physically modify starch may expand as interest
in water and energy conservation grows.
2.8 PROCESSING BY STEAM JET COOKING
Many applications of starch require heating (i.e., cooking) the starch granules in water to
form aqueous dispersions or solutions, which can be used as liquids or gels, or dried by
various techniques. Steam is widely used to provide the heat for cooking and the
technology for cooking starch on an industrial scale has changed very little for much of
the twentieth century. A continuous steam cooking apparatus was described in 1940
(Coppock, 1940) and a significant engineering improvement, described later by Winfrey
and Black (1964), introduced the possibility of applying excess steam flow to improve the
properties of the cooked starch dispersion. Modern devices, commonly known as steam
jet cookers, can be used in the thermal mode to apply just enough steam to gelatinize or
paste the starch granules (Kasica and Eden, 1992), or in the excess steam mode, in which
the additional mechanical shear resulting from the passage of excess steam through the
cooker reduces the viscosity and molecular weight of the starch (Klem and Brogly, 1981).
It is widely known that the molecular and bulk properties of starch processed by excess
steam jet cooking vary significantly from those of starch heated with less mechanical
shear, and this fact has enabled the exploration of new applications and post-processing
methods based on the technology of steam jet cooking. It has been found that factors such
as initial slurry pH and steam pressure significantly affect the intrinsic viscosity of jet-
cooked starch dispersions (Dintzis and Fanta, 1996). Other properties of excess steam jet
cooked starch have also been investigated, including the mechanical and thermal
degradation of starch (Byars, 2003), the effect of cooling rate and stirring conditions on