BY-PRODUCTS TECHNOLOGY

Module 3. Processing and utilization of whey

Lesson 30
DEMINERALISATION OF WHEY AND ITS UTILIZATION

30.1 Introduction

The high level of minerals (0.7-0.8%) present in whey restricts their commercial utilization in many applications. A major problem with many whey based products is their salty flavour owing to their high mineral content. Whey is demineralised to produce dry demineralised whey for specialised uses. Processes that have been utilized for the demineralisation of whey include electrodialysis, ion exchange, and loose reverse osmosis. These processes do not denature proteins, thereby preserving protein solubility in demineralised whey. The exact process used will determine the mineral profile of the final product. Demineralised whey often will have a less salty flavor because of the removal of minerals.

30.2 Electrodialysis Process

Gap between ED membrane is very small (0.4-1 mm), so whey is clarified before processing otherwise deposits will be formed varying the level of demineralisation. Other whey pretreatments such as pre-concentration, acidification and decalcification can also affects demineralisation considerably. Manufacture of demineralized whey by electrodialysis begins with the pasteurization of whey and concentration to around 28% TS. The concentrated whey is clarified to remove any insoluble protein and fines and then passed through the ED stack. The size of an electrodialysis apparatus is a function of the applied membrane surface. A typical structure with 100 pairs of cells represents a space of 50 µm and can remove 60% of salts from a 25% whey concentrate at a flow rate of 500 l/h. Each membrane has an effective area of about 0.5 m², giving a total of about 50 m²/ED stack. Production capacities of about 50 kg of whey cheese solids/m² of membrane area per day for a 75% demineralization level are typical.

The ions which are removed by electrodialysis are Na⁺ and K⁺. Calcium, magnesium phosphate and citrates are not transported as they are probably present, at the pH value of the electrodialysis, in the form of soluble complexes. Ions of different charges and sizes have quite different mobilities, and hence the removal of ions from whey by electrodialysis is not uniform, depending in particular on the ionic composition, mode of operation and contact time. In general, multivalent ions are only removed after monovalents are essentially removed. Thus the mineral profile of the product is significantly different from the feed stock. The level of demineralisation is determined by initial ash content, whey viscosity, current density and residence time. The cost of demineralisation increases very rapidly as the conductivity of feed decreases with increasing level of demineralisation. Electrodialysis is generally used for applications where low levels of demineralisation are sufficient, since power consumption at high level of demineralisation is excessive. In practice, levels of demineralisation of about 50% are viable.

Batch operation is quite convenient when a high and uniform level of demineralisation is required (i.e. 90% whey demineralisation for infant formulations). For batch operation, process times of 3.5 to 6 hours are typical, depending on operating temperature i.e. usually 35-45°C, that provides maximal electric conductivity, degree of demineralisation etc. Sometimes, operating temperature of less than 10°C is preferred for getting good microbial count. For 90% demineralisation, holding in batch system can be as long as 5-6 hours at 30-40°C. Pre-concentration of whey to 20-30% DM is desirable for better capacity utilization and lower electrical power consumption. However, lower temperatures (20°C) and feed concentrations (even 12% TS) may be used, but a higher voltage is then required to provide the corresponding current density.

In continuous operation, the whey flows through a number of electrodialysis stacks in series with partial flow recirculation through individual stacks. Operation temperature is relatively low (as low as 20°C) and the pH remains constant (about 4.7) during processing. Whey at 6% TS can also be processed, with no pre-concentration. Continuous electrodialysis has a longer period of uninterrupted operation (10-12 h) than batchwise processing, which is convenient for large-scale production. Conductivity, pH and temperature of the whey and brine streams are monitored continuously.

Losses of non mineral components occur in electrodialysis. This is obviously a loss in product yield. Loss of lactose and proteins is undesirable. These losses usually occur at higher level of demineralisation. Overall, NPN losses are about 25% and lactose losses of about 6% occur on 90% demineralisation. Protein losses of 2-3% of total protein can be expected during electrodialysis, with overall yield of 90-75% expected for 50-90% demineralisation, respectively. Physical losses may also occur due to leakage at gaskets.

After demineralisation, whey is usually further concentrated by evaporation to about 55% solids and spray dried.

30.3 Ion Exchange

Ion exchange processing of whey is based on the ability of macromolecular resins to exchange their surface-bound ions for mobile ions of the same charge from the treated whey. As the result of experience over many years, it has been found that whey processing is best carried out using strong acid cation and medium to weak base anion resins.

There are only a few options available for the application of ion-exchange resins for the practical demineralisation of whey. Column systems are the most widely employed. In such systems, either each resin is packed into its own column, or alternatively, sometimes, the resins are used in a 'mixed bed' column. The simplest system is a continuously-stirred reactor tank. Before demineralisation, whey has to be pasteurized, clarified and separated, because contamination of the resin with fat, and/or fine cheese curd, results in a shortening of the production life of the resins. Whey pasteurization should be as mild as possible (65°C, 15 sec), since any contact with fresh ion-exchange resins will shift the pH and may cause protein destabilization. Whey is first introduced into a cation exchanger, where all the positively charged ions (Na⁺, Ca^₂+etc.) are replaced with H⁺. The whey is then pumped in an anion exchanger where all the negatively charged ions (C1^-, PO₄^3-, SO₄^2-, etc.) are replaced with OH^-. The process illustrated diagrammatically in Fig.30.1. The ion exchange of whey can be carried out at higher temperatures (> 50°C), if it has previously been deproteinized. However, if the whey is not deproteinized, protein destabilization and loss during ion exchange can be avoided by using lower temperatures such as 5-12°C.

Ion exchange is actually a batch process. The transfer of whey through the resin beds continues, until the resins are saturated with cations and anions. This point is controlled by means of a conductivity meter, after which the resin beds are purged of whey, washed with water and regenerated by means of acid and alkali solutions. These solutions should be sufficiently concentrated to remove the absorbed cations and anions and replace them by H⁺ and OH^- bringing them back to their previous state. After the regeneration, the resins are washed with clean water, preferably condensate from an evaporator. After this, the process can start again. The production line consists of ion-exchange vessels, storage tanks for chemicals, feed and product storage, pumps, valves and dosing systems, a water supply, and a refrigeration system. If continuous production is required, it is necessary to erect 2 demineralisation plants. Putting one plant into operation allows the regeneration of the other. Each regeneration can process 10-15 bed volumes of whey; a figure which is based on the volume of the cation exchanger.

With a one-line configuration consisting of typical 2 hours of production and 4 hours of regeneration, four cycles can be performed per day, i.e., 8 hours of productive work. Two parallel lines provide 16 hours of production, and three parallel lines provide 24 hours of production. Usually, one or two lines are installed.

After its passage through both exchanger columns, the whey is demineralised, depending on its type, from 90 to 98%. The processing/holding time is 20 min. The processing temperature is < 10°C and therefore no bacteriological growth is experienced. Whey that has been demineralised up to 90% can be directly concentrated and dried. The obtained powder is non-hygroscopic. This advantageous phenomenon has as its probable cause the partial hydrolysis of the lactose at the cation exchanger. For some applications, though, a degree of demineralisation of only 50-60% is desirable. In such a case the demineralised whey is automatically mixed, in appropriate proportions with the untreated whey (pH control).

After completion of the operation or on exhaustion of the resin, the residual whey is flushed out. The flushing normally uses 2 bed volumes. Back washing may be employed, which results in expansion of the bed as a result of reverse flow and allows easier removal of physically entrapped material. After washing, the resins are regenerated through introduction of the regenerate solutions (generally 3 to 10 bed volumes for cation exchangers and 3 to 5 bed volumes for anion exchangers). The regenerant solution is then finally resumed. This takes place automatically and, for the cation exchanger, uses HCl. The deionized water used for the final rinse is derived from the condensation water supply of the evaporator (up to 10 bed volume). Na₂CO₃ and NH₄OH are used as regeneration solution for anion exchangers. Sanitization is possible after the regeneration stage using sodium isocyanurate solution. Counter current re-generation minimises the amount of the regenerating agent. (Fig. 30.1)

30.4 SMR Process

In SMR process, whey first enters anion column, where whey anions are exchanged for HCO₃^- ions. After this, the whey enters cation column, where cations of whey are exchanged for NH₄⁺ ions. After the process, whey salts are thus exchanged for ammonium bicarbonate. Ammonium bicarbonate is thermolytic salt which decomposes to recoverable NH₃, CO₂ and water, when heated during subsequent evaporation of whey. A plant using the SMR system at Arjang, Sweden, has achieved 70% recovery of NH₃ and 90% recovery of CO₂, with a plant yield of 85%. Of the 15% yield loss of more than two thirds is due to demineralization. The other losses are less than 5% and protein losses were less than 1%. The choice of resins is very important- some results in much higher protein losses. The details of the process have been described by Batchelder (1987) and Hoppe and Higgins (1992).

30.5 Electrodialysis vs. Ion Exchange Process

Electrodialysis is very capital intensive, whereas ion exchange has high operating cost. Based on economic studies, the recent trend is towards installation of combined electrodialysis/ion exchange plants for high level demineralisation of whey. The cost for electrodialysis demineralization is greatly dependant on conductivity and, therefore, increases with reduced ash content. Realistic cost effective demineralization figures are 50% with electrodialysis. A consumption of 10 to 28 KWH of electrical energy must be considered for each kg of demineral-ised whey powder. The process of manufacture of demineralized whey powder is given in Fig. 30.2.

The costs for ion exchange demineralisation are essentially linear with ash removal. Lower the ash content, lower is the cost of ion exchange demineralisation. In Industrial practice, electrodialysis demineralisation of 50% is followed by further 50-95% demineralisation by ion exchange process. The typical composition of demineralised whey powder is given in Table. 30.1.

Table 30.1 Composition of two different demineralised whey powders

30.6 Nanofiltration (Loose Reverse Osmosis)

Electrodialysis and ion-exchange processes are effective, but are limited by high capital cost, high running cost and high level of effluents. Ion exchange is relatively non selective and removes both monovalent and polyvalent ions, whereas electrodialysis is more dependent on ionic mobility and tends preferentially to remove monovalent ions. Nanofiltration (also known as loose reverse osmosis or ultra-osmosis) is a membrane separation process which allows selective passage of water, salts and very low molecular weight organic molecules. The membrane pore size used is 10-50A° and the operating pressure used is about 300 psi. First installation of this process in the United States was in 1986, for the reduction of the salt content of whey. Today, there are many plants utilizing this technology for the partial demineralisation of sweet whey, hydrochloric acid casein whey and ultrafiltration permeates. About 50% demineralisation is economically feasible with this technology. An added advantage of the process is that water is also removed from the product during operation, and thus a sig-nificant degree of concentration (>20% solids) is also achieved. Diafiltration in conjunction with loose RO further increases the effectiveness and degree of demineralisation.

30.7 Utilization of Demineralised Whey

There is a high demand for demineralised whey for many applications in human and cattle feed: these include instant formula (baby food; min. 90% demin.), confectionary, baking, meats, etc.

• A most important application of demineralised whey-based products is as ingredients in infant formulae, which more closely resemble human milk. Approximately 65% of total reduced minerals whey is used to manufacture infant-formulae.

• Europe utilizes 25,000 tonnes of electrodialysed whey solids annually in the production of demineralised calf milk replacers.

• Ultrafiltration permeate may also be demineralised by electrodialysis to increase lactose yield in crystallization.

• Demineralised sweet whey (25-65% demineralisation) used in dietetic food, coffee whi-tener, soft serve icecream, milk shakes, whey drinks and caramel, citrus drinks, salad dressing, animal feeds, baker goods, confectionary coating and dry mixes.

• Although usage of reduced mineral whey in food processing products is small in com-parison to usage of whole whey powder, there are a number of applications for which the lower overall ash content or the lower level of a specific ion such as sodium is desirable.

• Demineralisation may also be used as a pretreatment of whey or permeates prior to lac-tose hydrolysis.

• Demineralised whey products have also been used in a range of beverages.

• Reduced mineral whey is used as a less expensive substitute for non fat dry milk.

• In USA, cottage cheese whey is demineralised to reduce effluent costs.

Selected references

Batchelder, B.T. 1987. Electrodilaysis in whey processing. IDF Bulletin No. 212.
Delaney, R.A.M. 1976. Demineralisation of whey. Aust. J. Dairy Technol., 30: 12.