BY-PRODUCTS TECHNOLOGY

Lesson 37. LACTOSE HYDROLYSIS

Module 3. Processing & utilization of whey

Lesson 37

LACTOSE HYDROLYSIS

37.1 Introduction

Lactose is a reducing sugar and disaccharide composed of a molecule each of glucose and galactose. Hydrolysis of lactose yields 1 equivalent of D-glucose and 1 equivalent of D-galactose. In other words, 100 g of lactose will produce 50 g each of galactose and glucose. This is shown schematically in Fig. 37.1. Lactose hydrolysis by acid and hydrogenation process are also discussed.

37.2 Advantages of Lactose Hydrolysis

Lactose hydrolysis is a promising process with the following advantages:

i. Increased solubility: Glucose and galactose have a higher solubility than lactose (Table 37.1).

Table 37.1 Solubility data for various sugars at room temperature

ii. A certain proportion of the world’s population is lactose intolerant. Hydrolysis would make it possible for them to consume dairy products and foods containing dairy products, which would not have been possible previously.
iii. A microbiologically stable syrup which does not crystallize may be produced, so that the expense of drying may be avoided
iv. This process could help to overcome problems associated with lactose crystallization in ice cream and other refrigerated dairy products.

37.3 Methods of Hydrolysis

There are a number of different methods available for hydrolysis, which may be categorized as follows:

37.3.1 Enzyme

a. Free or single-use
b. Immobilized
c. Reactor

37.3.2 Acid

a. Direct
b. Ion Exchange

37.3.3 Hydrogenation

In this chapter, enzymic hydrolysis of lactose is discussed in detail.

37.3.1 Enzyme process

The hydrolysis of lactose into glucose and galactose could be achieved by the application of lactose ß-galactosidase enzyme. The enzymic hydrolysis of lactose may be catalysed by utilizing free enzyme or by immobilized enzyme, which permits the reuse of enzyme and Enzymic process allows use of milder conditions of temperature and pH. The immobilized and reactor enzymic processes are designed to allow effective reuse of the lactase, making the process continuous and cost effective. To date, only the single-use system and the immobilized system have been used commercially. The relative costs and benefits of these three processes depend very much on the processor’s circumstances. Single-use systems are more likely to be economic in smaller operations, particularly when hydrolyzed products are not manufactured daily. Larger operations, manufacturing more regularly, would gain cost advantages by the use of an immobilized system. However, the cost of single-use enzymes is continuing to decrease, and this coupled with the comparative complexity and high costs of immobilized systems, is likely to make single-use systems even more attractive.

All the enzymic methods can be used to hydrolyse lactose solutions or permeate, but if there are significant quantities of protein present as in whey, then only enzyme methods a and b may be employed. The enzyme reactor which is used to retain the enzyme would also retain proteins limiting its use to deproteinised solutions.

37.3.1.1 Free or single use enzyme

ß-galactosidase is produced commercially from yeast, fungi and bacteria, Aspergillus and Kluyveromyces spp. being most common, as shown in Table 37.2. The source of the enzyme gives it its own unique properties and optimum growth conditions. In general, enzymes are only effective over a limited range of pH. Whilst it is possible to adjust the pH of whey or permeate to that of particular maximum activity of a given enzyme, this is costly and generally results in an increase in the ash content of end-product. Of the microorganisms listed in Table 37.2, lactase from only Aspergillus oryzae has sufficient activity to allow economic hydrolysis of whey and permeate at their normal pH values.

The source of enzyme affects its characteristics. Fungal enzymes, with acid pH and relatively high temperature optima, are especially suitable for hydrolysis of lactose in acid whey. Compared to bacterial and yeast enzymes, they do not require metal ions for their stability or activity. However, they are more strongly inhibited by D-galactose, thereby slowing the rate of conversion that can be achieved under practical conditions.

The yeast enzymes have neutral pH optima making them suitable for the hydrolysis of lactose in sweet whey. They are less stable than the fungal enzymes and can only be used at moderate temperatures. However, they are also less inhibited by the reaction products. It is likely that the bacterial enzyme from E.coli would not be permitted in food products due to possible toxicity factors associated with coliforms, unless it were available in a highly purified form.

Table 37.2 Properties of microbial lactases

The attraction of using free enzyme is its simplicity of operation and that a suitable enzyme may be found to process any lactose containing stream, whether it be whey or permeate. The main drawback is that the enzyme is lost into the product, which firstly means that it could affect the properties of the product and secondly it has to be replaced, so resulting in a high running cost.

In practice, the enzyme is added directly to the reactor in such an amount as to obtain the required degree of hydrolysis within the given holding period. When a longer time is allowed for hydrolysis, the operating temperature will have to be kept low to prevent other undesirable reactions taking place, whereas when using a shorter time for hydrolysis, a higher operating temperature could be used. Since the activity of the enzyme will increase with temperature, there may not be a significant saving in enzymes by carrying out the hydrolysis over a long period. Also as the hydrolysis time is increased, the volume of the reactors will also have to be made larger, although in practice the reactor may be nothing more complex than an agitated silo. Since the reaction is inhibited by the D-galactose formed, attempts to reduce the reactor size by processing concentrated whey are restricted by the increased running costs due to higher quantity of enzyme required to overcome the inhibition.

An alternative approach is to use a continuous stirred reactor, in which feed and enzyme are continuously added to the reactor, and the hydrolysed product (containing some of the enzyme) is continuously removed. Such a system has the advantage of allowing reductions in reactor size, but is less cost effective in enzyme use than a batch reactor. Batch processing of hydrolysis is widely used in the dairy industry for the production of hydrolysed milk, whey and permeate. A number of factories regularly utilize this technology for the manufacture of syrups from whey or permeate, or for in-house use in products such as yoghurt or ice cream. However, such applications may be limited by the ash content of the product.

The commercial attraction of the single-use process lies in its simplicity and flexibility, both in enzyme selection and in operating conditions. The major drawbacks are the cost, and the fact that the enzyme is lost in the product. Although the activity of the enzyme is normally destroyed by pasteurization of the product, its presence, even in a denatured form, may create functional, legal and marketing problems.

37.3.1.2 Immobilized enzyme

In order to overcome the problem described with the use of free enzymes, immobilization techniques have been employed to fix the enzyme to an inert support. The immobilization procedures employed include adsorption, entrapment and covalent linkage.

Some of the enzyme/support systems proposed in the literature for immobilization of lactase are shown in the Table 37.3. In general, supports which are likely to be useful in commercial operations should be of a significantly higher or lower density than the substrate, should have a very even particle size, and should be resistant to degradation by operating temperatures, pH values and cleaning regimes. They should not support microbial growth, and should be safe for use in food processing operations. Particle size in particular is important, in fixed bed operation; a too small particle size may lead to the development of high back pressures, and severe packing of the bed, resulting in inefficient contact of enzyme with substrate. In fluidized bed systems, a too small particle size reduces the terminal velocity of the particles in the system, and, therefore, limits the average upward velocity in the reactor. This in turn limits the minimum holding time possible with particular designs of fluidized bed. Clearly, the selection of support particle size and its density are key factors if a system is to be commercially successful. It should be noted that the activity/pH profile of lactases is usually modified as a result of immobilization.

Table 37.3 Supports employed for the immobilization of lactose

A number of immobilized enzyme systems are now commercially available e.g. Valio systems, Corning glass works and Amerace in USA, Rhone Poulenc in France, Sumitomo Chemical Co Ltd in Japan etc. Corning Glass works are exploiting their technology through the formation of joint companies with producers of whey, e.g. Specialist Dairy Ingredients with the Milk Marketing Board of England and Wales, Thames Ditton, Surrey, England; Nutrisearch with Kroger Foods; Corvire with U.L.N.

a) Valio system

The historical development of the Valio Hydrolysis Process for hydrolysing lactose in whey using enzyme from Aspergillus niger has been described by Heikonen et al. (1985). The system operates at a pH of about 4, and a temperature of 40^oC. It cannot be used for the treatment of products of neutral pH such as milk or sweet whey (without pH adjustment). The concentrated hydrolysed product can be used in fruit yoghurts, ice cream, whey drinks and confectionery.

b) Corning system

The Corning system employs lactase from Aspergillus oryzae, which is covalently bonded to glass beads using a silane-glutaraldehyde coupling. Details of the Corning scheme are shown in Fig. 37.2. The pH of the raw whey is adjusted to the operating level of 4.5. The whey is pasteurized and then clarified to remove any fines which could block the column. The operating temperature of the column is gradually increased as the activity of the enzyme declines so that a constant degree of hydrolysis may be achieved. In order to prevent any bacteriological problems, the column is cleared once in every 24 hours. The cleaning procedure is designed to take 4 hours and includes a cleaning and sanitation procedure.

The hydrolysis is carried out in a column filled with graphite granules upon which the lactose enzyme has been immobilized. Control of column pH as well as the enzyme activity is achieved by means of an electric charge across the column. Thus, not only is the necessity of adding chemicals to adjust the pH of the medium eliminated but also any loss in enzyme activity may be countered by adjusting the electric current flowing.

37.3.1.3 Reactor systems

Most reactor systems operate at or near the optimum temperature of the enzyme, generally in the range 30-40^oC. The use of such temperatures for hydrolysis in a commercial operation, where running times of at least 6 h are desirable, means that considerable care must be taken in reactor design to minimize opportunities for microbial growth. A second area of concern is sanitation of the system after hydrolysis. The enzyme systems currently employed are highly sensitive to the commonly used high temperature and acid/alkali cleaning treatments.

a) Reactor for immobilized enzyme systems

In general, two types of reactor may be used for immobilized enzyme systems, either fixed bed or fluidized bed. In addition to these, a system where the enzyme is ‘immobilized’ or entrapped by the pressure difference between the lumen and the cartridge of a UF system has been suggested.

b) Reactor for enzyme recovery

The comparatively high costs of single-use enzymes and their presence albeit inactivated in the end-product have led to interest in the development of a reactor system utilizing membrane processing for enzyme recovery and reuse. These systems utilize ultrafiltration membranes to allow the low molecular weight lactose, and the reaction products, glucose and galactose, to pass through the membrane, whilst retaining the comparatively high molecular weight enzyme. Such system can then be utilized in a feed/bleed mode, so that, in a steady state operation, there is no change in the composition of the permeate. It could either be operated continuously or batchwise. The Process is shown in Fig. 37.3. The product, hydrolysed lactose, passes through the membrane whilst the enzyme is retained. Unlike the two other enzyme systems previously described, the use of an ultrafiltration plant to retain the enzyme means that the system is only applicable to deproteinized streams.

Whilst in principle it would be possible to utilize this technology for treatment of whey and milk, by ultrafiltration (UF) of the substrate, hydrolysis of resulting permeate in the reactor and addition of the hydrolysed stream back to the retentate from the initial ultrafiltration (Norman et al., 1978), the complexity and cost of such operations are likely to make them commercially uneconomical.

It is difficult to imagine that the recovery of the expensive enzyme using a relatively expensive extraction system will make economic sense, although one study shows that it would be feasible. Norman et al. (1978) also describe a system using two ultrafiltration plants to produce low lactose milk. The first U.F. plant removes the milk proteins, the permeate is hydrolysed in the UF. enzyme reactor and then the hydrolysed permeate is recombined with the milk protein concentrate.

37.3.1.4 Hydrolysis of lactose in milk, whey and whey permeate

A report from the USA (Anon 1982) describes a process for the hydrolysis of cheese whey, using immobilized enzymes of fungal origin (β-galactosidase) producing products combining the functional and nutritional properties of whey proteins with the sweetening power of glucose/galactose syrup. The β-galactosidase can be used to hydrolyze lactose in milk, whey and whey permeate. The selection of process technology depends upon the type and nature of substrate, characteristics of enzyme and economics of production, storage and marketing. β-galactosidases can be used as free or single use enzyme or immobilized enzyme. An enzymatic process scheme for the manufacture of hydrolysed lactose syrup from whey has been presented in Fig. 37.4. Normally the whey obtained from cheese/paneer/casein making is desalted by employing electrodialysis technique and then the pH is brought down to normally 3.6 with hydrochloric acid. The acidified whey is centrifuged and pasteurized. Whey hydrolysis of lactose is performed by application of β-galactosidase enzymes obtained from any microbial source. The content is partially concentrated (about 67.5% TS) after neutralization to pH 6.5. After lactose seeding, the content is cooled and packaged which is ready for marketing.

37.3.2 Acid process

Lactose hydrolysis can be carried out very effectively and economically under conditions of low pH and high temperatures. Clearly, this procedure is of value for protein-free streams such as permeates or deproteinized whey. The adjustment of pH can be made either by direct addition of acid to the system, or by treatment of the product with a cation exchange resin.

Homogenous or single-phase hydrolysis uses hydrogen ions in solution, with a defined heat treatment (ranging from about 60°C for 24 h to 140°C for 11 min at a pH of about 1.2). These hydrogen ions can be provided either by addition of mineral acids, or by treatment of the stream with a cation exchange resin. Heterogeneous or two-phase hydrolysis utilizes hydrogen ions bound to a cation exchange resin to catalyse the reaction. In such processes the demineralized product is passed at 90-98°C through a bed of cation exchange resin in the hydrogen form with a residence time of about 80 min. Significant cost benefits are claimed by this process. The products typically are brown, and may require neutralization, demineralization and decolourisation before use.

37.3.2.1 Direct acidification

In this process, significant lactose hydrolysis occurs at a temperature less than 100°C, the pH of the solution must be less than 1.0. Therefore, it would be necessary to use mineral acid such as hydrochloric acid in order to acidify the solution.

37.3.2.2 Ion exchange hydrolysis

Boer & Robbertson (1981) have described in detail a homogeneous system for hydrolysis of permeate in 3 schemes when an ion exchange system has been used to catalyse the hydrolysis reaction.

Scheme 1: Cation→ Cation Hydrolysis → Decolorizer → Anion
Scheme 2: Cation → Anion → Cation Hydrolysis → Decolorizer
Scheme 3: Cation → Heat Exchanger → Decolorizer → Anion

In scheme 1, the pH of concentrated ultrafiltration (UF) permeate of about 10% total solids was brought to 1.2 by use of a strong acid cation exchange resin.

In Scheme 2, since the solution is already at a low pH as a result of the cation column, the hydrolysis column operates more efficiently than the same column in Scheme 2. However, in Scheme 2 because the anion column has removed the nitrogen compounds the degree of coloration is much lower so that there is less colour to remove in the decolorizing column.

In Scheme 3, the heat exchanger raises the temperature of the liquor to 150°C in order for hydrolysis to occur. 80% degree of hydrolysis was reached on treatment of the liquid at 150°C for 3 min. As a result of this high operating temperature, some 50-60°C higher than that employed in Schemes 1 and 2, the necessary decolorizing is much more difficult. A brown colour was formed in the product, the level of which depended in part on the non-protein nitrogen content of the permeate. Purification of the product took place in a weak anion resin, followed by a second pass through both resins. Best results were obtained when adsorbing anion resins were used, which not only removed salts, but also the colour. Final purification took place with 0.1% activated charcoal, after concentration to 62% total solids. The final syrup contained, besides carbohydrates, 0.1% ash and 0.024% nitrogen.

Whilst both homogeneous and heterogeneous systems have been studied on a semi-commercial scale, there appears to have been no commercial adoption of these processes. Commercialization of these processes must have been inhibited by the fact they are restricted to be used with permeate, and the comparative higher cost of the product compared to its main competitors, sucrose and corn syrup product.

37.3.3 Hydrogenation

The possibility of using hydrogen gas in the presence of a nickel catalyst for hydrolysis has been mentioned in U.S. patent No. 2,642,462, but it is likely that control of product composition would be difficult. Furthermore, processing conditions may well be in excess of 100^oC, restricting the substrate to permeate or lactose solutions.

Selected references

Coton, S.G., Poynton, T.R. and Ryder, D. 1982. Utilization of lactose in the food industry. Bulletin of IDF Document 147: 23-26.
Ryder, D.N. 1988. Hydrolysis of lactose in whey products. Bulletin of IDF Document 233: 45-52.