430 Part IV: Milk
LACTOSEDERIVATIVES
A number of more useful and more valuable prod-
ucts may be produced from lactose. The most signif-
icant are
- Lactulose (galactosyl--1,4-fructose). A sugar
not found in nature, which is produced from
lactose by heating, especially under slightly
alkaline conditions. The concentration of
lactulose in milk is a useful index of the severity
of the heat treatment to which milk is subjected,
for example, in-container sterilization indirect
UHT direct UHT HTST pasteurization.
Lactulose is not hydrolyzed by intestinal -
galactosidase and enters the large intestine,
where it promotes the growth of Bifidobacterium
spp. It also has a laxative effect; more than
20,000 mt are produced annually. - Glucose-galactose syrups.Produced by acid or
enzymatic (-galacosidase) hydrolysis (see
Chapter 20); the technology for the production of
such hydrolyzates has been developed but the
product is not cost competitive with other sugars
(sucrose, glucose, glucose-fructose). - Galactooligsaccharides.-galactosidase has
transferase as well as hydrolytic activity, and
under certain conditions, the former
predominates, leading to the formation of
galactooligosaccharides containing up to 6
monosaccharides linked by glycosidic bonds,
which are not hydrolyzed by the enzymes
secreted by the human small intestine. The
undigested oligosaccharides enter the large
intestine, where they have bifidogenic properties
and are considered to have promising food
applications. These oligosaccharides are quite
distinct from the naturally occurring
oligosaccharides referred to earlier. - Ethanol.Produced commercially by the
fermentation of lactose by Kluyveromyces lactis.
Other derivatives that have limited but potentially
important applications include lactitol, lactobionic
acid, lactic acid, acetic acid, propionic acid, lactosyl
urea, and single-cell proteins. Most of these deriva-
tives can be produced by fermentation of sucrose,
which is cheaper than lactose, or by chemical syn-
thesis. However, lactitol and lactobionic acid are
derived specifically from lactose and may have eco-
nomic potential. Lactitol is a synthetic sugar alcohol
produced by reduction of lactose; it is not metabo-
lized by higher animals but is relatively sweet, and
hence has potential for use as a noncalorific sweet-
ener. It has also been reported to reduce blood cho-
lesterol levels, to reduce sucrose absorption, and to
be anticarcinogenic. Lactobionic acid has a sweet
taste, which is unusual for an acid and therefore
should have some interesting applications.NUTRITIONALASPECTS OFLACTOSELactose is responsible for two enzyme-deficiency
syndromes: lactose intolerance and galactosemia.
The former is due to a deficiency of intestinal -
galactosidase, which is rare in infants but common
in adults, except northwestern Europeans and a few
African tribes. Since humans are unable to absorb
disaccharides, including lactose, from the small in-
testine, unhydrolyzed lactose enters the large intes-
tine, where it is fermented by bacteria, leading to
flatulence and cramps, and to the absorption of
water from the intestinal mucosa, which causes diar-
rhea. These conditions cause discomfort and per-
haps death.
The problems caused by lactose intolerance can
be avoided by- Excluding lactose-containing products from the
diet, which is inadvertently the normal practice
in regions of the world where lactose intolerance
is widespread; - Removing lactose from milk, for example, by
ultrafiltration; - Hydrolysis of the lactose by adding -
galactosidase at the factory or in the home. The
technology for the production of lactose-
hydrolyzed milk and dairy products is well
developed, but is of commercial interest mainly
for lactose-intolerant individuals in Europe or
North America. Because the consumption of
milk is very limited in Southeast Asia, the use of
-galactosidase is of little interest, although
lactose intolerance is widespread.
Galactosemia is caused by the inability to catabo-
lize galactose, owing to a deficiency of either of two
enzymes, galactokinase or galactose-1-phophate
uridyl transferase. A deficiency of galactokinase
leads to the accumulation of galactose, which is cat-
abolized via alternative routes, one of which leads to
the accumulation of galactitol in various tissues,