Page 102 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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410 Carbohydrates

a. Starch-based industries. Dry milling is used to b. Chemically modified starch. The properties of
produce flours such as corn flour and meals such as oat- starch are often ameliorated by chemical modification.
meal, whereas wet milling, is used as feedstocks in the This is why today the majority of the starch produced
production of sugar syrups, used in the manufacture of for industry is chemically modified. Examples of such
foods, soft drinks, and beer. modified starches shown below.
Sweeteners From Starch: The most widely used natural Oxidized Starch: Starch is oxidized to remove some of
sweetener is the disaccharide sucrose, which is available its numerous OH groups that are responsible for hydrogen
in crystalline form as well as in the form of syrups un- bondingandforgelforming.Theresultingstarchsolutions
der various names such as molasses and invert sugar. Up after oxidation form weaker gels and are less susceptible
until recently the only serious competitor of sucrose was to autodecomposition or retrogradation. The oxidations
the cheaper glucose syrup produced since the early nine- are usually carried out in basic media using oxidants such
teenth century by the acid hydrolysis of starch and more as sodium hypochlorite or better sodium periodate. Oxi-
recently by its enzymatic hydrolysis with α-amylases and dation with the latter is usually followed by electrolysis to
glucoamylases. Because glucose syrup is significantly less regenerate periodic acid from the iodic acid formed during
sweet than sucrose, greater quantities of this syrup must the process. The product formed by periodate oxidation is
be used to yield a product of comparable sweetness to su- a 2,3-dialdehyde formed by cleavage of the 2,3- bonds of
crose. This is a serious disadvantage, duly noted by the starch (see Scheme 29). This can be made to react with
calorie-conscious public and by the food industry, which
took steps to remedy the situation. The glucose syrup was
partly converted to fructose by the enzyme glucose iso-
merase obtained from Bacillus coagulans. The resulting
product, designated fructose syrup, contains 42% fruc-
tose and possesses nearly the same sweetness as sucrose.
Recently, this sweetener was further improved upon; it
was converted into two products that are sweeter than
sucrose. They are very high fructose syrups that con-
tain about 90% fructose and crystalline fructose. Both are
obtained from fructose syrup by purification and separa-
tion on ion exchange columns. Fructose syrup and high
fructose syrup have now replaced glucose syrup in many
soft drinks, jams, confectionery, and conserves and crys-
talline fructose is slowly replacing sucrose. It is estimated
that half the sweeteners consumed in the United States
are now produced by the hydrolysis of starch in contin-
uous flow biocatalyst reactors that efficiently reuse the
enzymes.
Starch Fermentation Industries: Beer is manufactured
from barely whereas sake is obtained from rice. Ger-
minated barely is used in breweries to produce the en-
zyme β-amylase needed for the breakdown of starch,
whereas a mold, Asparagillus oryzea, is used to produce
the α-amylase needed to hydrolyze rice starch for the man-
ufacture of sake. The wort in both cases is subjected to
fermentation with the yeast, Saccharomyces serevisea.As
in many modern food-related industries, continuous flow
reactors and immobilized enzymes have been used in the
manufacture of beer, and more recently of wine, and cham-
pagne. This has resulted in a considerable reduction in cost
and residence time.
Starch and starch-rich wastes from the food industry
SCHEME 29 Oxidation of starch with periodic acid gives a dialde-
have been proposed as feedstocks for the fuel industry to
hyde, which can be grafted to natural polymers such as cellulose.
produce gasohol, and the food industry to produce single- Oxidation of starch with ceric ions gives a free radical which can
cell proteins for animal consumption. be grafted to acrilonitrile to give an absorbant graft polymer.