Hydrolysis of lactose
Whey is a by-product in cheese production, where mainly protein and fats are precipitated in the milk by addition of a 'coagulating' enzyme (chy-mosin or rennin, a carboxyl acid peptidase). The remaining liquid phase (whey) contains ~5% sugars, mainly lactose. Previously, whey was used as a feedstock but a more economic use involves hydrolysing the lactose to produce glucose and galactose. A high substrate content is favourable in order to reduce downstream processing costs. In milk, the lactose content cannot be changed, but in whey it can be increased by nanofiltration (Buchholz et al., 2005).
190 Handbook of waste management and co-product recovery 8.7.2 Extrusion combination processes
Extrusion cookers are often fed from pre-conditioners; this effectively increases the process time, which is desirable in matching conventional longer-duration processes. Pre-conditioners mix dry feed with water and/or steam for better distribution of moisture. Steam is also injected directly into the extruder and enables better processing of high-lipid mixtures. Supercritical fluid extrusion technology (Rizvi et al, 1995) combines two high-pressure processes - supercritical fluid processing and extrusion cooking - to extend the capabilities of conventional extrusion technology.
Casein from milk becomes soluble in water when neutralised with caustic soda. In conventional processing, agitating vessels and drum driers are required but the process has also been shown to be a candidate for the use of the extrusion cooker, followed by a continuous drier. The mixing capability of the twin-screw extruder avoids local soda concentrations (Wiedmann and Strobel, 1987).
The pulp and paper industry is applying new, ecologically sound technology in its manufacturing processes. The technologies implemented tend to change the existing industrial process as little as possible. Commercial applications include xylanases in pre-bleaching kraft pulps and the use of various enzymes in recycling paper. In the future, value-added products could be built around enzyme processes (Kenealy and Jeffries, 2003).
A related example is that of starch derivitisation for non-food use. Della Valle et al. (1990) report twin-screw extrusion of wheat starch with 3-chloro 2-hydroxypropyltrimethyl ammonium chloride, with sodium hydroxide as a catalyst, to produce cationic starches. These have affinity for cellulose and produce high-viscosity translucent pastes that are suitable for the paper industry. Wiedmann and Strobel (1987) showed that in the production of cationic potato starch the degree of reaction increased with not only caustic soda concentration but also at the lowest water contents. The latter factor argues in favour of equipment like the extrusion cooker.
Starch syrups are conventionally produced by acid or enzymic hydrolysis. The latter actually needs gelatinised starch so this could be a sequential process using unprocessed starch as the starting material. Linko et al. (1980) demonstrated the use of the twin-screw extruder as a continuous bioreactor for gelatinisation and liquefaction of starches to make syrups. A related process is actually to make bioethanol, a fuel used as a petrol substitute for road transport vehicles (Linko et al., 1984).
Linko et al. (1979) reported that starchy materials could be pretreated so that as a follow-on process glucoamylase could saccharify the extrudate.
Linko et al. (1980) showed that cereal alpha amylase was stable enough to survive extrusion cooking, although more severe conditions inactivate the amylase. This led to the addition of thermostable amylase to starches in the extrusion cooker to produce low-dextrose equivalent (DE) extrudates. If the enzyme is not inactivated immediately after extrusion, enzymic hydrolysis continues and the DE increases. Glucose syrups were produced by adding glucoamylase during or after extrusion cooking with alpha amylase. Maltose syrups were produced with beta amylase and pullulanase with a low-DE liquefied starch.
Simultaneous saccharification and fermentation were achieved by adding yeast or Zymomonas mobilis bacteria (Linko, 1992). In the case of yeast, Linko et al. (1984) give an example of a yield of 28.3 kg of 100% ethanol from 100 kg of barley. The ethanol, which is produced from the fermentation process, still contains a significant quantity of water, which must be removed using the fractional distillation process.
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