By-Products Technology

Module 3. Processing and utilization of whey

Lesson 35
REFINING OF LACTOSE

35.1 Introduction

Crude lactose is refined for high degree of purity, such as in the manufacture of pharmaceutical grades. Different grades of lactose are produced varying in lactose purity as per their specific requirements. In general, the process for the manufacture of edible- grade lactose produces a product that does not meet the ash, protein and light absorption standards for pharmaceutical- grade lactose. The impurities usually consist of riboflavin, a variety of proteins, lactose phosphate and lactic acid. Durham et al. (2007) claimed to produce USP lactose directly from whey permeates by using ion exchange, nanofiltration, chromatography, evaporation and crystallisation, without the need for a second crystallization step, with a yield approaching 95%.

35.2 Grades and Specification of Lactose

The international trade recognises several commercial forms of lactose as shown in Table 35.1. BIS standard for commercial lactose is given in Table 35.2.

Table 35.1 Grades of lactose

Pharmaceutical-Grade Lactose must meet the standards for contaminants described by Anonymous (1993) and USP-25 (2001). These two standards are almost identical and the test procedures that must be followed are prescribed. As a general guideline, heavy metals must be below 5μg per g, the microbial count must be less than 100 per g, with no Escherichia coli present and with a combined mould and yeast count below 50 per g. There are also ash, clarity and light-absorbing tests which must be passed.

Table 35.2 Lactose Commercial - Specifications (IS 1000:1989)

35.3 Refining Process

In the refining process, decolourising carbon is used to adsorb colour and probably removes other impurities to some degree. Hydrochloric acid is added to assist the action of the carbon, to solubilise salts and to aid in removal of protein. Lime is used to adjust the reaction to that most favourable for the precipitation of protein and probably aids by combining with the protein to some extent. The carbon and the precipitated impurities are removed by filtration, and the soluble impurities stay with the mother liquor and in the washings from the centrifuged crystals.

The process of producing up to pharmaceutical grade lactose is to re-dissolve the crude lactose in clean water and then to remove the impurities by a combination of adsorption and filtration processes, followed by re-crystallization. A typical refining process of lactose consists of subjecting crude lactose to treatment for removing colour, residual protein and salts followed by re-crystallisation (Fig. 35.1: Flow diagram for production of refined lactose). Less refining is required for lactose prepared from permeate than that made from whey. Crude lactose is dissolved in hot water to a 50-60% concentration depending on its purity. About 1% of decolorizing paste consisting of 3 parts bone black, 1 part activated carbon and 1 part 35% HCl is added. Quick dissolution requires heating to 105ºC. The slurry is passed through self cleaning filter at a high temperature and is introduced into crystallising tanks. The crystals are cooled to approximately 20°C in about 6 h. After cooling, crystals are separated and dried. This process is shown in (Fig. 35.2).

35.4 Drying

Edible lactose is normally dried to 0.5% moisture content and pharmaceutical lactose to 0.1% moisture content. The drying process is limited to a product temperature of 93°C, otherwise lactose will be crystallised into β-lactose anhydride at temperature above 93.5°C. Another important factor is the drying time. If the lactose solution is dried rapidly, crystals do not have an opportunity to form, and the result is the non crystalline glass, which contains the equilibrium mixture of the alpha- and beta-lactose. Flash drying can easily result in the formation of a thin layer of amorphous lactose on α-hydrate crystals, resulting in the risk of lumps formation in the bagged lactose afterwards. A fluidized bed drier with a maximum product temperature of 92°C/15-20 min would give good results. Pneumatic transport of lactose from the drier must be carried out by means of dry air at about 30°C. It gives gentle product cooling.

35.5 Milling and Bagging of Lactose

Dried lactose is milled and sifted through 100-200 mesh sieves and hermetically sealed in moisture proof bags. Pharmaceutical-grade lactose is generally sold by mesh size, the different products being milled to different degrees, possibly in conjunction with air or sieve classification. As well as the traditional α-lactose monohydrate, two other forms are sold to the pharmaceutical industry, i.e. anhydrous lactose (β-lactose) and spray-dried lactose.

35.6 Other Forms of Pharmaceutical Grade Lactose

35.6.1 Anhydrous lactose

This has the same heavy metal, microbial and colour specifications as USP-grade α-lactose monohydrate. Anhydrous lactose is usually made by roller drying, a solution of USP- grade lactose at a temperature greater than 93°C which produces a flaked-type product consisting of very fine crystals of β-lactose caked together. The flaked cake is then milled to the required size distribution.

35.6.2 Spray-dried lactose

Fine pharmaceutical-grade α-lactose monohydrate crystals are partially dissolved in clean water and the slurry is then spray dried. This produces a product that has crystals of monohydrate lactose joined together by amorphous lactose into roughly spherical agglomerates. Because most of the amorphous lactose is in the centre of the agglomerates, the resultant powder is free flowing without becoming too sticky. The amount of amorphous lactose present can be controlled by adjusting the temperature of the water or lactose solution in which the α-lactose monohydrate crystals are suspended.

Selected references

Durham, R., Hourigan, J., Sleigh, R. 2007. New approach for high purity lactose-utilising the by-product from whey. IDF Symposium “Lactose and its Derivatives”, Alisa Ltd. Stavropol, Russia: 50-55.
Sachdeva, S., Bhattacharjee, P.P. and Singh, S. 1998. Technology of lactose manufacture - A review. Indian J. Dairy Sci.,51 (1): 1-12.
USP-25. 2001. The United States Pharmacopeia, 25th edn., The United States Pharmacopeia Convention Inc., Rockville.