Enzyme Technology
The use of enzymes in detergents
The use of enzymes in detergent formulations is now common
in developed countries, with over half of all detergents presently available
containing enzymes. In spite of the fact that the detergent industry is the
largest single market for enzymes at 25 - 30% of total sales. details of the
enzymes used and the ways in which they are used, have rarely been published.
Dirt comes in many forms and includes proteins, starches
and lipids. In addition, clothes that have been starched must be freed of the
starch. Using detergents in water at high temperatures and with vigorous mixing,
it is possible to remove most types of dirt but the cost of heating the water is
high and lengthy mixing or beating will shorten the life of clothing and other
materials. The use of enzymes allows lower temperatures to be employed and
shorter periods of agitation are needed, often after a preliminary period of
soaking. In general, enzyme detergents remove protein from clothes soiled with
blood, milk, sweat, grass, etc. far more effectively than non-enzyme detergents.
However, using modern bleaching and brightening agents, the difference between
looking clean and being clean may be difficult to discern. At present only
proteases and amylases are commonly used. Although a wide range of lipases is
known, it is only very recently that lipases suitable for use in detergent
preparations have been described.
Detergent enzymes must be cost-effective and safe to use.
Early attempts to use proteases foundered because of producers and users
developing hypersensitivity. This was combatted by developing dust-free
granulates (about 0.5 mm in diameter) in which the enzyme is incorporated into
an inner core, containing inorganic salts (e.g., NaCI) and sugars as
preservative, bound with reinforcing, fibres of carboxymethyl cellulose or
similar protective colloid. This core is coated with inert waxy materials made
from paraffin oil or polyethylene glycol plus various hydrophilic binders, which
later disperse in the wash. This combination of materials both prevents dust
formation and protects the enzymes against damage by other detergent components
during storage.
Enzymes are used in surprisingly small amounts in most
detergent preparations, only 0.4 - 0.8% crude enzyme by weight (about 1% by cost).
It follows that the ability to withstand the conditions of use is a more
important criterion than extreme cheapness. Once released from its granulated
form the enzyme must withstand anionic and non-ionic detergents, soaps, oxidants
such as sodium perborate which generate hydrogen peroxide, optical brighteners
and various less-reactive materials (Table 4.1), all at pH values between 8.0
and 10.5. Although one effect of incorporating enzymes is that lower washing
temperatures may be employed with consequent savings in energy consumption, the
enzymes must retain activity up to 60°C.
Table 4.1 Compositions of an enzyme detergent
Constituent
|
Composition (%)
|
Sodium tripolyphosphate (water softener, loosens dirt) a
|
38.0
|
Sodium alkane sulphonate (surfactant)
|
25.0
|
Sodium perborate tetrahydrate
(oxidising agent)
|
25.0
|
Soap (sodium alkane carboxylates)
|
3.0
|
Sodium sulphate (filler,
water softener)
|
2.5
|
Sodium carboxymethyl cellulose (dirt-suspending agent)
|
1.6
|
Sodium
metasilicate (binder, loosens dirt)
|
1.0
|
Bacillus protease (3% active)
|
0.8
|
Fluorescent
brighteners
|
0.3
|
Foam-controlling agents
|
Trace
|
Perfume
|
Trace
|
Water
|
to 100%
|
a A recent trend is to reduce this phosphate content for
environmental reasons. It may be replaced by sodium carbonate plus extra
protease.
The enzymes used are all produced using species of Bacillus, mainly by just two companies. Novo Industri A/S produce and supply
three proteases, Alcalase, from B. licheniformis, Esperase, from an alkalophilic
strain of a B. licheniformis and Savinase, from an alkalophilic strain of
B.
amyloliquefaciens (often mistakenly attributed to B. subtilis). GistBrocades
produce and supply Maxatase, from B. licheniformis. Alcalase and Maxatase (both
mainly subtilisin) are recommended for use at 10-65°C and pH 7-10.5. Savinase
and Esperase may be used at up to pH 11 and 12, respectively. The a-amylase
supplied for detergent use is Termamyl, the enzyme from B. licheniformis which
is also used in the production of glucose syrups. a-Amylase is particularly
useful in dish-washing and de-starching detergents.
In addition to the granulated forms intended for use in
detergent powders, liquid preparations in solution in water and slurries of the
enzyme in a non-ionic surfactant are available for formulating in liquid
'spotting' concentrates, used for removing stubborn stains. Preparations
containing both Termamyl and Alcalase are produced, Termamyl being sufficiently
resistant to proteolysis to retain activity for long enough to fulfil its
function.
It should be noted that all the proteolytic enzymes
described are fairly non-specific serine endoproteases, giving preferred
cleavage on the carboxyl side of hydrophobic amino acid residues but capable of
hydrolysing most peptide links. They convert their substrates into small,
readily soluble fragments which can be removed easily from fabrics. Only serine
protease; may be used in detergent formulations: thiol proteases (e.g., papain)
would be oxidised by the bleaching agents, and metalloproteases (e.g., thermolysin) would lose their metal cofactors due to complexing with the water
softening agents or hydroxyl ions.
The enzymes are supplied in forms (as described above)
suitable for formulation by detergent manufacturers. Domestic users are familiar
with powdered preparations but liquid preparations for home use are increasingly
available. Household laundering presents problems quite different from those of
industrial laundering: the household wash consists of a great variety of fabrics
soiled with a range of materials and the user requires convenience and
effectiveness with less consideration of the cost. Home detergents will probably
include both an amylase and a protease and a lengthy warm-water soaking time
will be recommended. Industrial laundering requires effectiveness at minimum cost
so heated water will be re-used if possible. Large laundries can separate their
'wash' into categories and thus minimise the usage of water and maximise the
effectiveness of the detergents. Thus white cotton uniforms from an abattoir can
be segregated for washing, only protease being required. A pre-wash soaking for
10-20 min at pH up to 11 and 30-40°C is followed by a main wash for 10-20 min
at pH 11 and 60-65°C. The water from these stages is discarded to the sewer. A
third wash includes hypochlorite as bleach which would inactivate the enzymes
rapidly. The water from this stage is used again for the pre-wash but, by then,
the hypochlorite concentration is insufficient to harm the enzyme. This is
essentially a batch process: hospital laundries may employ continuous washing
machines, which transfer less-initially-dirty linen from a pre-rinse initial
stage, at 32°C and pH 8.5, into the first wash at 60°C and pH 11, then to a
second wash, containing hydrogen peroxide, at 71°C and pH 11, then to a
bleaching stage and rinsing. Apart from the pre-soak stage, from which water is
run to waste, the process operates counter-currently. Enzymes are used in the
pre-wash and in the first wash, the levels of peroxide at this stage being
insufficient to inactivate the enzymes.
There are opportunities to extend the use of enzymes in
detergents both geographically and numerically. They have not found widespread
use in developing countries which are often hot and dusty, making frequent
washing of clothes necessary. The recent availability of a suitable lipase may
increase the quantities of enzymes employed very significantly. There are,
perhaps, opportunities for enzymes such as glucose oxidase, lipoxygenase and
glycerol oxidase as means of generating hydrogen peroxide in situ. Added
peroxidases may aid the bleaching efficacy of this peroxide.
A recent development in detergent enzymes has been the
introduction of an alkaline-stable fungal cellulase preparation for use in
washing cotton fabrics. During use, small fibres are raised from the surface of
cotton thread, resulting in a change in the 'feel' of the fabric and,
particularly, in the lowering of the brightness of colours. Treatment with
cellulase removes the small fibres without apparently damaging the major fibres
and restores the fabric to its 'as new' condition. The cellulase also aids the
removal of soil particles from the wash by hydrolysing associated cellulose
fibres.
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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