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

^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.