Whey, the liquid residue of cheese, casein and yoghurt production, is one of the biggest reservoirs of food protein available today. World whey output at approximately 180 million tonnes in 2013 contains some 1.5 million tonnes of increasingly high-value protein and 8.6 million tonnes of lactose, a very important source of carbohydrate for the world. The latest research shows that whey protein is arguably the most nutritionally valuable protein available; little wonder that nutritional markets such as sports, clinical and infant nutrition are driving an unprecedented investment level in dairy production. Packed full of ‘natural goodies’ such as high-gelling b-lactoglobulin, mother’s milk equivalent protein a-lactalbumin, lactoferrin, and immunoglobulin and as a pre-cursor to the probiotic galactooligosaccharides (GOS), whey is proving to be one of the most exciting nutrient sources available today.
Whey comprises 80 – 90 % of the total volume of milk entering the process and contains about 50 % of the nutrients in the original milk: soluble protein, lactose, vitamins and minerals.
Whey as a by-product from the manufacture of hard, semi-hard or soft cheese and rennet casein is known as sweet whey and has a pH of 5.9 – 6.6. Manufacture of mineral-acid precipitated casein yields acid whey with a pH of 4.3 – 4.6. Table 15.1 shows approximate composition figures for whey from cheese and casein manufacture.
Whey is very often diluted with water. The figures above relate to undiluted whey. As to the composition of the NPN fraction, about 30 % consists of urea. The rest is amino acids and peptides (glycomacropeptide from renneting action on casein). Table 15.2 lists some fields of application for whey and whey products.
Advances in membrane filtration and chromatography have underpinned economically viable commercial processes for the fractionation of whey into highly purified protein and lactose products that allow end users to take advantage of the various functional properties of individual whey components. This is a trend that is expected to continue as research uncovers new bioactive properties and consumers become more educated about the nutritional value of whey.
The block diagram in Figure 15.1 summarizes various processes used in the treatment of whey and its end products. The first stage is filtering the curd particles left in the whey, followed by separation of casein fines and fat (Figure 15.2), partly to increase the economic yield and partly because these constituents interfere with subsequent treatment.
Production of whey powder, delactosed whey and lactose has traditionally dominated processing of whey solids. However, the increased demand for whey proteins results in approximately 40 % of processed whey solids being directed to associated products WPC35-80, whey protein isolate (WPI), lactose and permeate. The shift in the image of whey from an unwanted by-product to a highly-valuable nutritional source is complete. Some of the products now in use are described in this chapter.
Different whey processes
Whey must be processed as soon as possible after it is drawn from the cheese curd as its temperature and composition promote the growth of bacteria that lead to protein degradation and lactic acid formation.
It is recommended that whey is drawn directly from the cheese process into short duration buffer storage then clarified, separated, pasteurized and cooled into storage to await further processing. If transporting the whey it can be concentrated by membrane filtration to reduct transport costs.
Casein fines recovery and fat separation
Casein fines are always present in whey. They have an adverse effect on fat separation and should therefore be removed first. Various types of separation devices can be utilized, such as cyclones, centrifugal separators or vibrating/rotating screens, Figure 15.2.
Fat is recovered in centrifugal separators
The collected fines are often pressed in the same way as cheese, after which they can be used in processed cheese manufacture and, after a period of ripening, also in cooking.
The whey cream, often with a fat content of 25 – 30 %, can partly be reused in cheese-making to standardize the cheese milk; this enables the corresponding quantity of fresh cream to be utilized for special cream products. Normally, this works well for short maturation cheeses such as mozzarella, but note that the risk of rancid off flavours is heightened as the maturation time is increased. It is important to break the recycle loop to avoid the build-up of free fatty acids and other undesirables that are not trapped in the curd matrix. For cheddar production, whey cream is generally not reused due to the sensitivity of the starter to bacteriophages. In some of these cases, whey cream is converted to whey butter.
Pasteurization and chilling
Whey that is to be stored before processing must be either chilled or pasteurized and chilled as soon as the fat and fines have been removed. For short-time storage (< 8 hours), chilling to < 5 °C is usually sufficient to reduce bacterial activity. Longer periods of storage and utilization of the whey in high-quality infant formula and sports nutrition applications require pasteurization of the whey directly after the removal of fat and fines; generally, this approach is recommended in order to cater to the increasingly strict demands on product quality.
Concentration of total solids
The first step in whey concentration typically involves increasing the dry matter from around 6 % to 18 – 25 % using RO (reverse osmosis) or a combination of RO-NF (nanofiltration). Then whey can either be transported to another site for further processing (e.g. evaporation and drying) or dried directly on site.
With dry matter above 25-30 %, it is more economical to use mechanical vapour recompression (MVR) evaporation to concentrate whey. Utilizing MVR in this second step to whey concentration can increase dry matter from as low as 20 % to 45 – 65 %.
After evaporation, the concentrate is flash cooled rapidly to 30 – 40 °C thus initiating nucleation of lactose crystals before being further cooled and stirred in specially-designed crystallization tanks. The product is held in the crystallizers for 4 – 8 hours to obtain a uniform distribution of small lactose crystals, which will give a non-hygroscopic product when spray-dried.
Concentrated whey is a supersaturated lactose solution and, under certain temperature and concentration conditions, the lactose can sometimes crystallize spontaneously before the whey leaves the evaporator. At concentrations above a dry matter content of 65 %, the product can become so viscous that it no longer flows. For more information on RO and Evaporators see Chapter 6, Sections 6.4 and 6.5.
Basically, whey is dried in the same way as milk, i.e. in drum or spray dryers, see Chapter 17, Milk powder.
The use of drum dryers involves a problem: it is difficult to scrape the layer of dried whey from the drum surface. A filler, such as wheat or rye bran, is therefore mixed into the whey before drying, to make the dried product easier to scrape off.
Spray drying of whey, is at present, the most widely used method of drying. Before being dried, the whey concentrate is usually treated as mentioned above to form small lactose crystals, as this results in a non-hygroscopic product which does not go lumpy when it absorbs moisture.
Acid whey from cottage cheese and casein production is difficult to dry due to its high lactic acid content. It agglomerates and forms lumps in the spray dryer. Drying can be facilitated by neutralization and additives, such as skim milk and cereal products. Increasingly it is preferred that lactic acid is removed by a combination of nanofiltration and electrodialysis improving flavour, nutritional profile, drying and handling. Salt is also removed and typically a demineralisation level of > 60% corresponds to a level of acid reduction that is acceptable.
Fractionation of total solids
Whey proteins were originally isolated through the use of various precipitation techniques, but nowadays membrane separation (fractionation) and chromatographic processes are used in addition to both precipitation and complexing techniques.
Fink and Kessler (1988) state that a maximum whey protein denaturation rate of 90 % is possible for all denaturable fractions. Proteose peptone, comprising some 10 % of the fraction, is considered undenaturable.
Whey proteins, as constituents of whey powders, can easily be produced by careful drying of whey. Isolation of whey proteins has therefore been developed. The whey proteins obtained by membrane separation or ion exchange possess good functional properties, i.e. solubility, foaming, emulsion formation and gelling, can be highly nutritional and in the case of WPI produce a very clear beverage enhancing it's healthy image.
Protein recovery by UF
Protein concentrates have a very good amino acid profile, with high proportions of available lysine and cysteine.
Whey protein concentrates (WPC) are powders made by drying the retentates from ultrafiltration of whey. They are described in terms of their protein content, (percentage protein in dry matter), ranging from 35 % to 80 %. To make a 35 % protein product, the liquid whey is concentrated about six-fold to an approximate total dry solids content of 9 %.
Example: 100 kg of whey yields approximately 17 kg of retentate and 83 kg of permeate at close to six-fold (5.88) concentration. Table 15.3 shows the compositions of the feed (whey) and the resulting retentate and permeate.
Percentage protein in dry matter according to the values in Table 15.3:
In concentration, most of the true protein, typically > 99 %, is retained, together with almost 100 % of the fat. The concentrations of lactose, NPN and ash are generally the same in the retentate serum and permeate as in the original whey, but a slight retention of these components is reported.
The overall retention figures, however, depend very much on:
- The type of membrane
- The flux
- The character of the feed (pre-diluted with water, pre-concentrated after demineralization, etc.)
To obtain a more than 80 % protein concentrate, the liquid whey is first concentrated 20- to 30-fold by direct ultrafiltration to a solids content of approximatively 25 %; this is regarded as the maximum for economic operation. It is then necessary to diafilter the concentrate to remove more of the lactose and ash and raise the concentration of protein relative to the total dry matter. Diafiltration is a procedure in which water is added to the feed as filtration proceeds, in order to wash out low molecular components which will pass through the membranes, basically lactose and minerals.
Table 15.4 shows the compositions of some typical whey protein concentrate (WPC) powders.
A process line for the production of drier whey protein concentrate using UF is shown in Figure 15.3. Up to 95 % of the whey is collected as permeate and protein concentrations as high as 80-85 % (calculated on the dry matter content) can be obtained in the dried product. Typically, the evaporator is not used for protein concentrations above 60 % dry matter so as to minimize heat damage to the proteins. Advances in high-concentration nanofiltration allows these products to be concentrated to > 35 % dry matter prior to drying. For further details about UF, see Chapter 6.4, Membrane filters.
Whey protein isolate
Whey protein isolate (WPI) containing > 92 % protein in dry matter is growing rapidly in applications such as body-building supplements – where the fat and other non-protein constituents are undesirable as well as in egg white replacements for whipped products such as meringues or as a valuable ingredient in foods and acidic fruit beverages.
Advances in microfiltration have drastically improved the quality and economics of product available, moving from a traditionally hot ceramic filter process to a cold organic spiral wound process in recent years.
Treatment of the whey retentate from a UF plant at around 35 % protein in DM can reduce the fat content of a whey protein powder from over 7% to less than 0.4 %. Microfiltration also concentrates fat globule membranes and most of the bacteria in the MF retentate, which is collected and processed separately; in some cases, this retentate is dried on the same dryer as the WPI, resulting in a high fat WPC powder. The defatted MF permeate is routed to a second UF plant for concentration; this stage also includes diafiltration.
As Figure 15.4 shows, the pre-treated whey is pumped to a UF plant (2) where it is concentrated to about 35 % protein in DM. The retentate is pumped to the MF plant (3), while the permeate goes to a collecting tank after RO concentration and cooling.
The retentate from MF treatment, which contains most of the fat and bacteria, is collected separately, and the defatted permeate is forwarded to further ultrafiltration with diafiltration (4). The resulting WPI retentate is then further concentrated using high concentration NF (35-38 % DM) and spray-dried to reduce the moisture content to a maximum of 4 % before bagging.
UF permeate from the production of WPC and WPI can be spray-dried or used for lactose production. These are explained in more detail below.
WPI and Permeate from Skim Milk
The production of ‘ideal’ or ‘native’ whey products from skim milk is of growing interest due to the resulting products’ unique properties which arise from the milk having not been subjected to the action of rennet, starter cultures or acid. Consequently, there is an absence of GMP (glycomacropeptide), lactic acid levels above those that are naturally occurring, degradation of proteins by starter culture enzymes and risks from bacteriophages.
As figure 15.5 shows, skim milk is first microfiltered (1) to separate casein (MF retentate) from the MF permeate containing whey proteins, lactose, NPN and ash. The retentate, in liquid or powder form, can be used in a variety of products where casein fortification is beneficial; this includes cheese, dairy desserts and beverages.
Ultrafiltration-diafiltration (2) is then utilized to separate the whey proteins (UF retentate) from lactose, ash and NPN (UF permeate) giving a protein rich stream with > 90 % protein in dry matter. After storage (3), the UF retentate can be further concentrated to 36 – 37 % dry matter, preheated (5) to attain specific functional properties and the dried (6) to no greater than 4 % moisture.
The resulting permeate from UF (2) is concentrated directly and then stored ready for further processing. The type of membrane used to concentrate permeate depends on whether it is being used to standardize protein in milk powders, spray-dried as permeate or used for lactose production. This is explained in more detail below. UF milk permeate is also produced when making MPC (milk protein concentrate) and in this case, skim milk is sent directly to UF-DF (2) in figure 15.5.
Recovery of denatured whey protein
In general, serum protein or whey proteins cannot be precipitated by rennet or acid. It is, however, possible to precipitate whey proteins with acid, if they are first denatured by heat. The process is divided into two stages:
- Precipitation (denaturing) of the protein by a combination of heat treatment and pH adjustment
- Concentration of proteins by centrifugal separation
Denatured whey proteins can be mixed with cheese milk prior to renneting; they are then retained in the lattice structure formed by the casein molecules during coagulation. This discovery led to intensive efforts to find a method of precipitating and separating whey proteins, as well as a technique for optimizing the yield. Adding denatured whey proteins to the cheese is not permitted by law in several countries, and also for certain types of cheese. Denatured proteins, either by adding or by pasteurization at high temperatures, affect both yield and ripening of the cheese. Figure 15.6 shows the Centri-Whey process line for manufacture of denatured whey proteins. After pH adjustment, the whey is pumped via an intermediate tank (1) to a plate heat exchanger (2) for regenerative heating. The temperature of the whey is raised to 90 – 95 °C by direct steam injection (3), before it passes through a tubular holding section (4) with a holding time of 3 – 4 minutes. Acid is introduced during this stage, to lower the pH. The acid is either organic or inorganic (e.g. lactic acid or edible hydrochloric acid) as stipulated. Those proteins that can be, and have been, modified by heat are precipitated within 60 seconds in a tubular holding section (4).
After regenerative cooling to about 40 °C, the precipitated proteins are separated from the liquid phase in a solids-ejecting clarifier (6). The clarifier discharges, at intervals of about 3 minutes, the accumulated protein in the form of a 12 – 15 % concentrate, of which about 8 – 10 % is protein. This method results in 90 – 95 % recovery of the coagulable proteins.
The addition of concentrated whey protein to cheese milk – principally in the manufacture of soft and semi-hard cheeses – causes only minor changes in the coagulating properties. The structure of the curd becomes finer and more uniform than with conventional methods. The processed whey proteins are more hydrophilic than casein. In the making of Camembert cheese, for example, an increase in yield of 12 % has been reported.
UF Permeate from Whey and Skim Milk
There are currently a few options available for the further processing of UF permeate from whey or skim milk as shown below. Only milk UF permeate and lactose can be used for the standardization of protein in milk powders.
The processes for the manufacture of lactose and permeate powder are explained below in Fig. 15.7.
Lactose is the main constituent of whey. There are two basic methods of recovery, depending on the raw material:
- Crystallization of the lactose in untreated but concentrated whey
- Crystallization of lactose in whey from which the protein has been removed by UF, or some other method, before concentration
Both methods produce a mother-lye, molasses, which can be dried and used as fodder. The feed value can be increased considerably if the molasses is desalinated and if high-quality proteins are added.
The crystallization cycle is determined by the following factors:
- Crystal surface available for growth
- Purity of the solution
- Degree of saturation
- Agitation of the crystals in the solution
Several of these factors are mutually related to each other, for example degree of saturation and viscosity. Figure 15.8 shows a production line for manufacture of lactose. The whey is first concentrated by evaporation to 60 – 62% DM and then transferred to crystallization tanks (2), where seed crystals are added. Crystallization takes place slowly, according to a predetermined time/temperature programme. The tanks have cooling jackets and equipment for control of the cooling temperature. They are also fitted with special agitators. After crystallization, the slurry proceeds to decanter centrifuges and a sieve centrifuge (3) for separation of the crystals, which are dried (4) to a powder. Following grinding (typically in a hammer mill) and sifting, the lactose is packed (5). For efficient and simple separation of lactose crystals from the mother liquor, crystallization must be arranged so that the crystals exceed 0.2 mm in size – the larger the better for separation.
The degree of crystallization is determined in principle by the quantity of b-lactose converted to the desired a-lactose form, and the cooling of the concentrate must therefore be carefully controlled and optimized.
Various types of centrifuges can be used for harvesting lactose crystals. One is the horizontal decanter centrifuge (Figure 15.9), which operates continuously and has a screw conveyor for unloading the lactose. Two machines are installed in series. The lactose from the first is reprocessed in the second for more efficient separation. During separation, impurities are washed from the lactose so that a high degree of purity is obtained. The residual moisture content of the lactose after the second separation stage is < 9 % and pure lactose accounts for about 99 % of the dry solids.
The lactose is dried after separation to a residual moisture content of 0.1 – 0.3 %, depending on the future use of the product. The temperature during drying should not exceed 93 °C, as β-lactose is formed at higher temperatures. The drying time must also be taken into consideration. During quick drying, a thin layer of amorphous (shapeless, non-crystalline) lactose tends to form on the a-hydrate crystal, and this may later result in formation of lumps. Drying usually takes place in a fluidized bed drier. The temperature is maintained at 92 °C and the drying time is 15 – 20 minutes. The dried sugar is transported by air at a temperature of 30 °C, which also cools the sugar.
The crystals are normally ground to a powder immediately after drying and are then packed.
Refining of lactose
A higher degree of purity or very white lactose is required for some applications (e.g. pharmaceutical manufacturing processes). The refining of lactose can also improve the yield of a lactose process. The traditional method for manufacturing pharmaceutical lactose involves redissolving lactose from the decanters at
60 % DM in softened water at pH 4 and close to 100 °C, followed by mixing with active carbon and filter adjuvant. After filtration, the solution is recrystallized and centrifuged before drying, milling and packing. This is an expensive process where double execution of equipment is required for continuous processing, and where active carbon and filter adjuvant are sent directly to waste.
More recently, alternative processes involving continuous decalcification and riboflavin removal (source of the yellow colour in lactose) using regenerable activated carbon columns produce refined white lactose much more economically. These processes are even capable of pharmaceutical-grade lactose with the addition of additional decanting and washing.
An alternative to making lactose from UF permeate is to convert into permeate powder as shown by the process depicted in Figure 15.10. The product Permeate powder has seen high growth in applications within animal feed and food applications when high-purity lactose is not required and the ash level in permeate is acceptable. Typically, this is a less capital-intense option, does not have the issues that can be associated with disposing the mother liquor or ‘molasses’ and has almost 100 % yield of lactose in the primary product.
Functional properties such as free flowing non-caking behaviour, colour and flavour over a long shelf life are very important for permeate powder, and production conditions must be carefully selected to achieve these.
UF permeate is first concentrated by RO-NF (1) to 20 – 25 % dry matter when it is evaporated using a single MVR evaporator and flash cooled (2) to initiate the spontaneous nucleation of lactose into small crystals and stored under controlled temperature conditions in specially-designed crystallization systems (3) to cause a high degree of lactose crystallization (> 75 %). After around 4h, the crystallized permeate is fed to the spray-dryer equipped with a timing belt (4) where it is dried under conditions that promote the post-crystallization of remaining lactose (> 95 %) to ensure a free-flowing, non-hygroscopic product. Product is then stored (5) for a minimum of 6h to allow for final crystallisation and then packed in 25 kg bags or lorry.
Alternative processes that bring about a very high concentration of permeate > 80 % DM and continuous crystallization prior to drying are also in use. High temperatures required to facilitate the handling of the highly viscous permeate concentrates should be chosen carefully because they can degrade product quality, particularly colour and flavour.
As whey has a fairly high salt content, about 8 – 12 % calculated on dry matter, its usefulness as an ingredient in human foods is limited. By having the whey demineralized, various fields of application can, however, be found for whey which is partially (25 – 30 %) or highly (90 – 95 %) demineralized.
Partially demineralized whey concentrate can, for instance, be used in the manufacture of ice-cream and bakery products or even in quarg, whereas highly demineralized whey concentrate or powder can be utilized in formulas for infants and, of course, in a very wide group of other products.
Principles of demineralization
Demineralization involves removal of inorganic salts, together with some reduction in the content of organic ions, such as lactates and citrates.
The partial demineralization is mainly based on utilization of cross-flow membranes specially designed to “leak” particle species that have radii in the nanometre (10–9 m) range. This type of filtration is called nanofiltration (NF).
The high degree desalination is based on either of two techniques:
- Ion exchange
Partial demineralization by NF
By using a specially designed 'leaky' RO membrane, small particles like certain monovalent ions, e.g. sodium, potassium, chloride and small organic molecules (like urea and lactic acid) can escape through the membrane, together with the aqueous permeate. This membrane process is known by various names such as ultraosmosis, 'leaky' RO and nanofiltration (NF).
Because of their greater compactness, spiral-wound membranes are most often used in new installations. For further information about this type of membrane, see Chapter 6.4, Membrane filters.
Examples of permeation rates of normal sweet whey constituents during nanofiltration are given in Table 15.5.
As the table shows, reduction of the chloride content in sweet whey can be as high as 70 % and that of sodium and potassium 30 – 35 %. The reason for this difference in elimination of ions is the need of maintaining an electrochemical balance between negative and positive ions.
A critical aspect of nanofiltration in whey processing is that the leakage of lactose must be kept to a minimum (<0.1 %), to avoid problems with high BOD (biological oxygen demand) in the waste water (permeate). Installation of NF equipment in whey processing can be considered in the following situations:
- As a low-cost alternative to diminish the salty taste of ordinary sweet whey powder
- As a preliminary step to more complete demineralization of whey by electrodialysis and ion exchange
- For acid removal in hydrochloric and lactic acid casein whey; note that the permeation rate is low for lactate ions but high for free lactic acid molecules
- For salt reduction in salted whey (e.g. salt drippings in Cheddar cheese production)
High degree demineralization
Electrodialysis is defined as the transport of ions through non-selective semi-permeable membranes under the driving force of a direct current (DC) and an applied potential. The membranes used have both anion and cation exchange functions, making the electrodialysis process capable of reducing the mineral content of a process liquid, e.g. seawater or whey.
The two electrodes at each end of the cell stack have separate rinse channels as shown in Figure 15.11, through which a separate acidified stream is circulated, to protect the electrodes from chemical attack.
For whey treatment, the whey feed and acidified brine pass through alternate cells in the stack, whose construction can be likened to that of a plate heat exchanger or plate sheet ultrafiltration module. Figure 15.11 is a schematic picture of an electrodialysis unit. It consists of a number of compartments separated by alternate cation and anion exchange membranes which are spaced about 1 mm or less apart. The end compartments contain electrodes. There can be as many as 200 cell pairs between each pair of electrodes.
Alternate cells in the electrodialysis stack act as concentration and dilution cells respectively. Whey is circulated through the dilution cells, and a 5 % brine carrier solution through the concentration cells.
When direct current (DC) is applied across the cells, cations attempt to migrate to the cathode and anions to the anode, as shown in Figure 15.11. However, completely free migration is not possible, because the membranes act as barriers to ions of identical charge. Anions can pass through an anion membrane, but are stopped by a cation membrane.
Conversely, cations can pass through a cation membrane but not an anion membrane. The net result is depletion of ions in the whey (dilution) cells. The whey is thus demineralized, to an extent determined by the ash content of the whey, residence time in the stack, current density and flow viscosity.
The electrodialysis plant can be run either continuously or in batches. A batch system, which is often used for demineralization rates above 70 %, can consist of one membrane stack over which the process liquid, e.g. whey, is circulated until a certain ash level is reached. This is indicated by the conductivity of the process liquid. The holding time in a batch system can be as long as 2 – 3 hours for 90 % demineralization at 10 – 15 °C. Pre-concentration of the whey to 20 – 30 % DM is desirable with regard to capacity utilization and electric power consumption. The whey concentrate should be clarified before it enters the electrodialysis unit.
The process liquid heats up during the process, so a cooling stage is needed to maintain the process temperature. In a continuous plant, consisting of five membrane stacks in series, the holding time can be reduced to 10 – 40 minutes. The maximum demineralization rate of such a plant is often limited to about 60 – 70 %. In relation to capacity, the installed membrane area is much larger in a continuous plant than in a batch plant.
An electrodialysis plant can easily be automated and furnished with a programmed CIP system. The cleaning sequence normally includes water rinse, cleaning with an alkaline solution (max. pH 9), water rinse, cleaning with hydrochloric acid (pH 1) and a final water rinse. A typical cleaning programme takes 100 minutes.
Power supply and automation
Direct current is used in the electrodialysis plant, which should have facilities for regulating current in the range of 0 – 185 A and voltage in the range of 0 – 400 V. Flow rates, temperatures, conductivity, pH of process water and product, product inlet pressure, pressure difference between the stacks and current, as well as voltage over each membrane stack, are monitored and controlled during production.
Limiting factors in electrodialysis
A major limiting factor for using electrodialysis in dairy processing is the cost of replacing membranes, spacers and electrodes, which constitutes 35 – 40 % of the total running costs in the plant. Replacement is necessary due to fouling of the membranes, which in turn is caused by:
- Precipitation of calcium phosphate on the cation exchange membrane surfaces
- Deposition of protein on the anion exchange membrane surfaces
The first problem can be handled by proper flow design over the membrane surface and regular acid cleaning. Protein deposits are the main factor in shortening the lifetime of the anion membranes. The background to this problem is as follows: at the normal pH of whey, the whey proteins can be regarded as large negative ions (anions) and move as such under the influence of the electrical field in the stack. These molecules, being too large to pass through the anion exchange membranes, are deposited as a thin protein layer on the faces of the anion exchange membranes in the whey compartments. Techniques such as polarity reversal can be used to dislodge these deposited materials from the membrane.
Although frequent high-pH cleaning removes most of the deposits, disassembly of the stack for manual cleaning is recommended at intervals of 2 – 4 weeks. The processing cost of electrodialysis depends very much on the de-mineralization rate. Increasing the capacity in steps from 50 % to 75 % to 90 % doubles the processing cost per step. This means that it is four times as expensive per kilo of product solids to demineralize to 90 % than to 50 %; the reason is that plant capacity is reduced at 90 % demineralization.
Water treatment, electric power, chemicals and steam account for the operating costs of a demineralization plant. Waste water treatment is a particularly heavy item. During production, lactose leaks through the membranes at a rate of 7 – 10 % at 90 % demineralization. The phosphate removed from whey also accumulates in the waste stream. The cost of electric power amounts to 10 – 15 % of the processing cost, while the chemicals used in the process, mainly hydrochloric acid, account for less than 5 %. The cost of steam used for pre-heating the product and cooling costs for control of process temperature are 10 – 15 %, depending on the de-mineralization level.
Electrodialysis is best for demineralization levels below 70 %, where it is very competitive, compared to ion exchange.
In contrast to electrodialysis, the process which removes ionizable solids from solutions on a continuous electro-chemical basis, an ion exchange process employs resin beads to adsorb minerals from solution, in exchange for other ionic species. The resins have a finite capacity for this, so that when they are completely saturated, the adsorbed minerals must be removed and the resins regenerated before reuse. Normally, the resins are used in fixed columns of suitable design.
Ion exchange resins are macromolecular porous plastic materials, formed into beads with diameters in the range of 0.3 to 1.2 mm for technical applications. Chemically, they act as insoluble acids or bases which, when converted into salts, remain insoluble. The main characteristic of ion exchange resins is their capacity to exchange the mobile ions they contain for ions of the same charge sign, contained in the solution to be treated. A simple example of this reaction is shown for sodium chloride removal, where R is the exchange group bound to the insoluble resin.
Cation exchange R – H + Na+ = R – Na + H+ resin in H+ form
The reaction above is deliberately written as an equilibrium, because the direction in which the reaction goes depends on the ion concentration in the liquid and in the solids phase of the resin. The equilibrium is characterized by a constant. On regeneration the reaction is reversed when the sodium-laden ion exchange resin is treated with, say, a 4% hydrochloric acid solution.
The high concentration of hydrogen ions in the acid drives the equilibrium to the left.
The equilibrium constant varies depending on ion species, which gives the selectivity of ion exchange processes. Generally speaking, multivalent ions have higher selectivity than monovalent ones and ions of the same valence are selected by size, large ions having higher selectivity. For cations typically found in dairy process streams, selectivity decreases in the order Ca2+ > Mg2+ > K+ > Na+.
Similarly, anion exchange selectively can be classified in the following way: citrate3– > HPO42– > NO3– > Cl–.
ln practice, this means that the ion exchanger, after being exhausted by a liquid containing different ion species, will exist in different forms along the length of the column as described in Figure 15.12. This figure shows what happens in a column treating ordinary raw water in a cation exchanger loaded in the hydrogen ion form. The situation after regeneration with acid is also shown. It can be seen that the ions that remain longest in the cation-exchange column are Na ions. This can be understood from the selectivity order described above.
Going back to the picture of the exhausted cation-exchange column in the figure, the segregated distribution of ions means that Na ions leak first, followed by Mg2+ and Ca2+ ions. An initial ion leakage in the exhaustion phase may occur when the ion exchanger is not fully regenerated, but after a while the Na+ ions are eluted and replaced by H+ ions (see Figure 15.12). The status of the lower part of the ion exchanger determines the leakage of ions from the process liquid.
lon exchange resin characteristics
Ion-exchange resins in industrial use today are based on polymeric plastic materials to build up the porous matrix structure. Common materials are polystyrene/divinyl benzene and polyacrylate. Functional groups are chemically bound to this matrix structure. Typical groups are:
- Sulphonic groups -SO3-H+ (strong acid cation exchanger)
- Carboxyl groups -COO-H+ (weak acid cation exchanger)
- Quatenary amine N+ OH- (strong base anion exchanger)
- Tertiary amines NH+ OH- (weak base anion exchanger)
Both strong base and strong acid exchangers are fully ionized in the whole pH interval (0 – 14). Weak base and weak acid ion exchangers have a restricted pH area in which they are active. Weak acid cation exchangers cannot normally be used in the low pH range (0 – 7), because the carboxyl groups are mainly present in their free acid form, as determined by their acid/base dissociation constant (often expressed as pKa = –10 logarithm of the dissociation constant). At pH values higher than pKa the carboxy-lic groups are in their salt form, and can consequently participate in ion exchange reactions. As a contrast, weak base anion exchange resins are only active in the low pH range, 0 – 7.
From the ease-of-regeneration point of view it is beneficial to use weak resins whenever possible. They can be regenerated with acid/base respectively with an excess of only 10 – 50 % of the theoretically needed amount. Strong resins require an acid/base excess of 300 – 400 % compared with the theoretical value for regeneration. For demineralization according to the classical procedure, a strong acid cation exchanger regenerated in the hydrogen form is combined with a weak base anion exchanger working in the free base (hydroxyl) form. It is not possible to use a weak acid cation exchanger instead of a strong one, because of the very advantageous equilibrium for exchange of cations for the hydrogen bound to the hydroxylic groups.
Other important characteristics of ion exchangers, which are not further discussed, are:
- Ion exchange capacity
- Swelling properties
- Mechanical strength
- Fluidization during backflushing of the bed
- Pressure drop
- Flow-velocity restrictions
- Water rinse requirements after regeneration
Ion exchange processes for demineralization
Demineralization by ion exchange has long been an established process for water treatment but has also been adopted for 'de-ashing' of whey. Whey is not a uniform product as to composition. Whey from an acid casein/cheese curd has a pH of 5.9 – 64.3, while the pH of sweet whey is 6.3 – 6.6. The main difference between these two types of whey, apart from the acidifying medium, is the high level of calcium phosphate in the acid whey. It is good practice to use the cations as the base for calculating the salt load in whey because the anions, e.g. citrates and phosphates, are involved in proteolytic reactions. This complicates the calculation of specific ion contents. The figures for cation content are typical of sweet and acid whey respectively and are shown in Table 15.6.
Whey can consequently be characterized as a liquid with a high salt load which, as a consequence, results in short cycles when ion exchange is applied. This, in turn, results in high costs for regeneration chemicals, if they are not recovered.
Conventional ion exchange for demineralization
A simple demineralization plant using ion exchange is shown in Figure 15.13. The whey first enters the strong cation exchanger, loaded in H+ form, and continues to anion exchange in a weak base anion-exchanger in its free base form. The ion exchange columns are rinsed and regenerated separately with dilute hydrochloric acid and sodium hydroxide (ammonia). Once a day, the columns are disinfected with a small amount of active chlorine solution.
The following net reactions take place during demineralization (NaCI is used to illustrate the salts of whey and R represents the insoluble resin exchange site).
Anion exchange: R – OH + H+ + Cl– —— R – CI + H2O
The various flows in the ion exchange process include the following steps:
- Exhaustion 10 – 15 bed volumes of whey can be treated per regeneration. The bed volume is based on the bed volume of the cation exchanger.
- Displacement of whey
- Contact with regeneration solution
- Water rinse
The ion exchange columns are often made of rubber-lined mild steel, to avoid corrosion problems. The conical shape is used specially for the anion exchanger to allow for swelling of the bed, during transition from the free base form to the salt form. Counter-current flow is often used for regeneration of the cation exchanger. Thus, when whey is treated in downward flow, regeneration takes place in upward flow. This system reduces consumption of regeneration chemicals by as much as 30 – 40 %, but at the expense of a more complicated design. The plant can easily be automated. Two or three parallel ion exchange systems are needed for a continuous flow of whey. A normal cycle time is six hours, four of which are used for regeneration.
Whey is a liquid with a high salt load, which means short runs between regenerations. It also means a high consumption of regeneration chemicals and a high salt load in the waste from both ash removal and the required surplus of regeneration chemicals. Rinse-water consumption is also high, especially for washing out excess sodium hydroxide from the weak anion resin.
Losses of whey proteins occur on the columns due to denaturation/ absorption. This is caused by great pH variation in the whey during the ion exchange process. Consumption of regeneration chemicals accounts for 60 – 70 % of the operating costs of the process.
The process is primarily designed for 90 % demineralization, but any demineralization rate can be chosen if a by-pass system is used.
The destination of the solids in whey for different product mix options is given in Figure 15.14 below
Chromatographic isolation of lactoperoxidase and lactoferrin
Generally speaking, the use of natural bioactive agents is of great interest to products like infant formulas, health foods, skin creams and toothpaste. Examples of such components are the bioactive proteins lactoperoxidase (LP) and lactoferrin (LF), which exist at low contents – typically 20 mg/l of LP and 35 mg/l of LF – in whey. A chromatographic process is usually used to isolate LP and LF from whey.
The basic principle underlying the process is based on the fact that both LP and LF have isoelectric points in the alkaline pH area, which means that these proteins are positively charged at the normal pH of sweet whey (6.2 – 6.6). The rest of the whey proteins (e.g. β-lactoglobulin, α-lactalbumin and bovine serum albumin) are negatively charged in the same pH range. A fundamentally suitable process for isolation of LP and LF is, therefore, to use a specially designed cation exchange resin for selective adsorption. The LP and LF molecules thus bind to the negatively charged functional group of the cation exchanger by charge interaction, leading to the fixation of these molecules on the ion exchange resin while the other whey proteins pass through because of their negative charge. To make the process industrially viable, some basic criteria have to be satisfied. One of them is the need for a 'particle-free' whey to maintain a high flow rate during the loading phase because very large volumes of whey have to pass the ion-exchange resin to achieve saturation. Cross-flow microfiltration (MF) with a pore size of 1.4 µm, operated under a uniform transmembrane pressure (UTP), has proved to be a successful technique for getting particle-free whey. Stable flux of 1 200 – 1 500 l/m2h is easily sustained for 15 – 16 hours. This type of whey pre-treatment avoids build-up of increasing back pressure over the ion exchange column.
The ion-exchange resin has a total capacity to adsorb 40 – 45 g of LP and LF per litre of resin before breakthrough occurs. With a resin bed volume of 100 l, almost 100 000 l of whey can be treated per cycle.
With properly chosen conditions for the elution of adsorbed bioactive proteins on the column, it is possible to obtain very pure fractions of LP and LF. Salt solutions of different strengths are used for this step. The proteins in the eluates occur in fairly concentrated form, of the order of 1 % by weight. The ion exchange step thus concentrates LP and LF by a factor of almost 500 compared to the original whey. Further processing of the eluates by UF and diafiltration yields very pure protein products (approximately 95 % purity). Finally, after sterile filtration in a cross-flow microfilter with 0.1 – 0.2 µm pores, the protein concentrates are freeze-dried. The overall process is illustrated in Figure 15.15.
Lactose is a disaccharide consisting of the monosaccharides glucose and galactose, as shown in Figure 15.16. Lactose exists in two isomeric forms, α-lactose and β-lactose.
They differ in the spatial arrangement of the hydroxyl group at the C atom in the glucose molecule, and thereby also, amongst other things in:
- Crystal shape
- Melting point
- Physiological effect
Lactose can be split hydrolytically, i.e. by bonding of water, or by an enzyme. The lactose-splitting enzyme β-galactosidase belongs to the hydrolase group. Figure 15.16 shows enzymatic splitting of lactose into galactose and glucose.
Lactose is not nearly as sweet as other types of sugar. Figure 15.17, which indicates the relative degree of sweetness of different types of sugar. Hydrolysis of lactose consequently results in considerably sweeter products.
Some people lack the enzyme that decomposes lactose and therefore cannot drink or eat any significant quantities of milk products. This is called lactose intolerance. Hydrolysis of the lactose in the milk products allows these people to utilize the high-quality proteins, vitamins, etc. in milk products.
Some defects, such as sandy texture in ice-cream (crystallization of lactose) are practically eliminated by lactose hydrolysis.
Figure15.18 shows a process for enzymatic hydrolysis of lactose in whey.
Pre-treatment in the form of demineralization is not essential, but it improves the taste of the final product. After hydrolysis the whey is evaporated. A syrup with a dry solids content of 70 – 75 % is then obtained. 85 % of the lactose in this syrup is hydrolysed and can be used as a sweetener in the baking industries and in the manufacture of ice-cream.
During production, the enzyme is inactivated by heat treatment or by pH adjustment. It cannot be used again. Instead of using free enzymes, it is now possible to bind the enzyme to different types of water-soluble and non-water-soluble carriers. Such systems with immobilized enzymes can be used for continuous lactose hydrolysis. The enzyme, which is expensive, is not consumed and can be used to hydrolyse large amounts of product. This increases the profitability. The technique has not yet been developed to any great extent.
Lactose can also be split by means of acids in conjunction with heat treatment or by passing a cation exchanger in hydrogen form at high temperature, around 100 °C. The required degree of hydrolysis is determined by selection of pH, temperature and holding time. As brown discolouration occurs during hydrolysis of the whey, active carbon treatment is recommended.
It has been established that non-protein nitrogen products can be used as partial replacement for natural protein in ruminants because certain mic-robes in the cattle rumen can synthesize protein from urea and ammonia. However, in order to get a balanced feed of nitrogen and energy, urea and ammonia have to be transformed into more suitable forms, which slowly release nitrogen to the rumen for improved protein synthesis.
Lactosyl urea and ammonium lactate are two such products based on whey.
Briefly, the procedure for production is as follows: after separation the whey is concentrated up to 75 % DM, typically in two steps. After addition of urea and edible sulphuric acid, the whey concentrate is held at 70 °C for 20 hours in a jacketed tank provided with agitator. Under these conditions the urea reacts with the lactose to form lactosyl urea.
Following the reaction period, the product is cooled and transported to a factory producing concentrated feed (pellets for instance) or direct to farmers.
The process technique involves fermentation of the lactose in whey into lactic acid and maintaining the pH with ammonia, resulting in formation of ammonium lactate. After concentration to 61.5 % DM, the product is ready for use.