Lactose is recovered from whey permeate as edible-grade lactose in most dairy manufacturing countries. Edible-grade lactose is used in infant formula, chocolate and confectionery, baked goods and as a flavour/colour carrier (Rajah & Blenford 1988). Pharmaceutical-grade lactose is further refined and converted into a range of products: milled lactose crystals, spray-dried lactose, anhydrous P-lactose and micronised lactose. It is mainly used in pharmaceuticals as a tableting excipient, but also as the high-price, dry-powder inhaler. Pharmaceutical lactose and edible lactose are also used as a raw material for lactose derivatives such as lactulose, lactobionic acid, lactitol, galacto-oligosaccharides, or hydrolysed to liquid lactose syrup.
The process of lactose manufacture typically involves concentration of the ultrafiltration permeate to 60% solids in a multiple effect evaporator. The concentrate is transferred to large crystalliser vats and slowly cooled over 20 h. The crystallised lactose is then separated, creating a mother liquor containing 20-30% of the lactose plus 90% of the ash. The lactose crystals are washed in a refiner washer with water in a ratio of 1 : 1 (water: lactose), then dried, milled and bagged.
Pharmaceutical-grade lactose undergoes further purification. The washed edible-grade lactose crystals are redissolved to 60% solids, fining agents are added, then filtered, recrystallised, separated and washed again (Pritzwald-Stegmann 1986). The process is less economically attractive than edible-grade lactose due to the large-scale investment required for the double crystallisation. It is also less environmentally attractive due to the larger waste streams from the double crystallisation.
The efficiency of the lactose crystallisation processes has been improved. Most dairy manufacturers use nanofiltration to reduce mineral impurities, as advocated by Guu and Zall (1992). Whey pretreatments to remove calcium phosphate as 'dairy calcium', which is sold as a dietary supplement, also reduce fouling of the evaporator (Hobman 1984). However, there is much more that needs to be done. The disposal of mother liquor is a major problem, it has limited use as stockfeed as it is perishable and the animals need to live close to the factory. Animal feeding also needs infrastructure, transport and feed lots. Mother liquor can be used as fertiliser by metering into irrigation, however it has a very high BOD5, and has been found to kill fish if leakage occurs directly into streams. The best way to deal with mother liquor is to limit its production.
Chromatographic separation of mother liquor has been proposed to reduce the problem of mother liquor disposal. Patents by Harju and Heikkilae (1989) describe a process that increases lactose yield from mother liquor, similar to the chromatographic separation of beet sugar molasses applied commercially. Further developments of this technology by researchers at the University of Western Sydney (UWS)/Food Science Australia (FSA) and licensed by Applexion (Groupe Novasep 2005) have extended the use of chromatography to recover lactose from whey permeate prior to crystallisation, thus avoiding production of mother liquor in the first place.
The ion exclusion lactose (IEL) process recovers pure lactose by ion chromatography, yielding pure lactose and a range of by-products, including calcium and soluble mineral fractions and a surplus of potable water. It is based on a four-step process wherein the whey permeate is first decalcified by ion exchange; secondly it is concentrated by nanofiltration and thirdly separated into purified lactose and mineral fractions by chroma-tography. The fourth step is the recovery of the monovalent ions from the nanofilter for regeneration of the ion exchange column; thus avoiding the cost of buying salt to operate the ion exchange, and reducing disposal of salty waste (Durham et al. 1997a). The process removes the problem of the highly pollutive mother liquor associated with traditional lactose processes and increases efficiency of downstream processing of lactose creating new high-value lactose products. The liquid lactose fraction can be directly used in infant formula without the need to crystallise and redissolve, or can be used for lactose derivatives. Alternatively the purified lactose can be crystallised into pharmaceutical-grade lactose crystals (<0.01% ash) without mother liquor waste. Due to the purity of the liquor, the crystallisation can be controlled to create lactose designed to meet customer specifications for particle size and shape (Durham et al. 2004).
Lactose can be enzymically hydrolysed into glucose and galactose with P-galactosidase, resulting in a syrup that is more easily digested by those who are lactose intolerant. Hydrolysed lactose is sweeter and more soluble than lactose and can be used for sweetening syrups in ice creams, yoghurts and drinks without lactose crystallisation problems (Hourigan 1984). Galactose isolated from hydrolysed lactose is also finding a market as an endurance sports drink additive (G-Push) (Alexander 2002). However, cost and difficulty of storage limit the production and use of hydrolysed lactose syrup.
Oligosaccharides are carbohydrates of three to ten linked monomer sugars most commonly produced by enzymic transglycosylation reactions of lactose (Playne & Crittenden 1996). Oligosaccharides pass through the colon unde-graded, where they encourage the growth of bifidobacteria in the intestine. Oligosaccharides also contribute to the development of the immune system, in human milk they have a protective effect against viral and bacterial infections, stimulate the immune response and enhance the bioavailability of minerals (Geisser et al. 2005). Oligosaccharides are water soluble and mildly sweet, they have a high viscosity and can contribute to the mouth-feel of the product. They can be used as a humectant, to control Maillard browning or inhibit starch retrogradation (Crittenden & Playne 1996).
Lactulose is produced by the alkaline isomerisation of lactose. It stimulates the growth of Lactobacillus bifidus in the large intestine, which has the same functions as oligosaccharides, i.e. lowering the pH of the colon and repressing the growth of pathogenic bacteria (Visser et al. 1988). Lactulose is widely used in hospitals for chronic constipation (Alexander 2002). Methods developed for the production of crystalline forms of lactulose have improved the range of applications of lactulose (Carobbi & Innocenti 1991; Dendene et al. 1995). New developments in lactulose production include the use of electro-membrane isomerisation followed by demineralisation by electo-dialysis to produce high yields of lactulose from lactose (Evdokimov & Alieva 2004).
Waste management and co-product recovery in dairy processing 369 Lactitol
Lactitol is produced from the catalytic hydrogenation of lactose to produce the sugar alcohol (Visser et al. 1988). Lactitol is used as a low-calorie sweetener and also acts as dietary fibre, competing against sorbitol and maltitol. Lactitol is less cariogenic than sucrose. It is not absorbed through the small intestine, so it does not raise blood glucose levels and is thus suitable for diabetics. It is fermented in the large intestine by the natural microflora to give 50% of the energy value of lactose (Booy 1987).
Lactobionic acid is produced from the chemical oxidation of lactose or enzymically by lactose dehydrogenase (Rand & Hourigan 1975). Lactobionic acid is a complexing agent for metal ions (Visser et al. 1988). It is used as an organ preservative and more recently has been used as a skin cream additive (Alexander 2002).
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