Concentration and demineralization of whey

of wheys exist: “ sweet ” whey and “ acid ” whey ( Table 6.1 ), but industrially the diffe-rent combinations of coagulation operating conditions (temperature, heating time, nature of starters, etc.) give rise to various types of wheys. Sweet whey is the most often encountered type and originates from the manufacture of cheese, where the processing is based on coagulating the casein by rennet, an industrial casein-clot-ting preparation containing chymosin or other casein-coagulacasein-clot-ting enzymes. Sweet whey is produced from cheeses such as mozzarella or cheddar and is obtained after the destabilization of casein micelles by enzymatic hydrolysis of the κ -casein using rennet. The final pH of sweet whey is close to the initial milk pH, ~ 6.0 – 6.6. Its mineral content is similar to that of milk, and it contains the soluble glycomacro-peptide portion of the κ -casein that is released in the serum phase under the action of chymosin. “ Acid ” whey is produced from lactic acid fermentation for fresh cheese or from chemical acidification of milk down to the isoelectric point of the caseins (about 4.6). Acid whey has a higher mineral content than sweet whey because of the release of minerals from the casein micelles (mainly calcium and phosphate) into the serum phase under acid conditions. Its final pH is also very acid, close to 4.6 ( Table 6.1 ).

More recently the use of skimmed milk microfiltration using a membrane with a mean pore diameter of 0.1 μ m has originated a new category of “ whey ” [ 2 ]. MF using a 0.1 μ m mean pore diameter membrane makes it possible in one single operation to remove casein micelles (size ~100 – 150 nm) from skimmed milk [ 3 ]. This operation leads to two fractions: a retentate enriched specifically in native casein micelles that can be used for cheese making in order to improve the rennet coagulability of casein and the cheese-making process; and a permeate corresponding to the aqueous phase of milk and containing the native serum proteins (size 2 – 10 nm). The permeate is sometimes called “ ideal whey ” because its composition is close to that of a sweet whey but free of the rennet by-product glycomacropeptide, residual fat and micro-organisms (phage, cellular debris, etc). The pH of the permeate is similar to the pH of milk, and then higher than all pH of wheys that are more acid ( Table 6.1 ). It contains lactose, minerals and serum proteins in their native state, providing the previous milk was not heat-treated.

6.3   Concentration and demineralization of whey

The first application of membranes in whey processing was devoted to the concent-ration of wheys.

RO was mainly applied to the concentration of wheys and various ultrafiltrates on their production site. The concentration was carried out in order to reduce the volume of the product before further transportation or prior to vacuum evaporation and spray-drying so as to save energy. The concentration of ultrafiltrates, containing about 5% total solids (mainly lactose, salt and other minor soluble components

of the milk) leads to valuable uses, such as animal feed, recovery of lactose after crystallization, fermentation of lactose into glucose and galactose as sweetener for confectionery industry, alcohol, lactic acid. RO, known to mainly remove water, is largely used because it is flexible and requires a lower energy consumption (9 – 20 kWh/m ³ water removed) compared to vacuum evaporation ( ≈ 100 kWh/m ³ ).

Concentration of whey by RO is classically performed up to a volume reduction ratio (VRR) of 4, resulting in about 25 – 28% dry matter. The level of VRR is limited by high osmotic pressure, high retentate viscosity, calcium phosphate precipitation and lactose crystallization. The RO permeate can be reused for preparing cleaning solutions, but its composition is not similar to pure water. Urea can pass to some extent, and some salts and low-molar-mass peptides may do so, leading to specific treatment of the reused water.

Today a part of the RO membrane area carrying out the concentration of whey is replaced by NF in order to simultaneously perform concentration and partially demineralization of the product [ 4 , 5 ]. Whey needs to be demineralized before eva-poration in order to make whey powder suitable for certain food applications. The high salt content leads to nutritional imbalance, especially for the preparation of infant formula and becomes a problem when using whey and whey powders as food. Moreover the high salt content of whey, generally ranging from 8 to 10% mine-rals on a dry weight basis, generates numerous processing difficulties especially when concentrating, crystallizing lactose and spray-drying whey (decrease in yield of lactose crystallization, and high hygroscopicity of obtained powders) [ 5 ]. Spray-drying of acid whey treated by NF showed a significant improvement in running parameters and a three-fold reduction in the hygroscopicity of the powder [ 4 ].

Partial demineralization is thus often needed in various situations when manufac-turing dairy ingredients, such as whey protein concentrates, in order to adjust the mineral composition. For ice cream applications, in order to reduce the salty taste of ordinary whey powder, a 50 – 70% overall reduction in minerals is often enough. But in order to mimic the mineral composition of human milk, a reduction of 90 – 95%

in minerals of wheys is generally required. Critical ions for the preparation of infant formulas are Na ⁺ , K ⁺ , Cl ^– and PO ₄ ^3– .

The demineralization of whey can be achieved in various ways [electrodialysis, ion-exchange (resins), nanofiltration] according to the type of treated whey and the required demineralization rate ( Table 6.2 ) [ 6 ]. But the rate of removal of ion varies with the technology used and with the type of ions. Nanofiltration (NF) for example removes monovalent ions (such as Cl ^– , Na ⁺ , K ⁺ , H ⁺ ) and concentrates divalent nutri-tion value ions like calcium with the proteins.

Whey can be demineralized using electrodialysis, which is defined as the trans-port of ions through semipermeable membranes under the driving force of an elec-tric field caused by a direct current source. At the industrial scale, electrodialysis 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

6.3 Concentration and demineralization of whey   137

the whey is circulated until a given ash level (indicated by the conductivity of whey) is reached. The holding time in a batch system can be as long as 5 – 6 h for 90% demi-neralization at 30 – 40 ° C. Owing to the high process temperature the risk of bacteria growth need to be controlled by specific cleaning and disinfections steps. Preconcen-tration of the whey to 18 – 26% dry matter (or up to 30% if the crystallization of the lactose is avoided) is desirable for maximum utilization of installed membrane area and low electric power consumption ( Table 6.2 ).

By using nanofiltration membranes, it is possible to simultaneously retain/con-centrate proteins and multivalent nutrition value ions such as calcium and remove monovalent co-ions (ions with the same charge as the membrane). The demineraliza-tion efficiency in NF is then almost restricted to the removal of monovalent ions. The role of the retention of multivalent co-ions (salts and/or proteins) in the facilitated transmission of monovalent ions has been demonstrated.

In the case of sweet whey [ 5 ], at a pH around neutral (pH 6.0 – 6.6) (membrane negatively charged), the retention of proteins (negatively charged at neutral pH) and polyvalent anions leads to the presence of higher amounts of negative charges in the retentate, which results in an increased transmission of Cl ^– and OH ^– and in a partial transmission of Na ⁺ and K ⁺ in order to maintain the electroneutrality in the permeate fraction. At the opposite, in the case of acid whey (membrane positively charged), the retention of proteins (positively charged at acidic pH) and polyvalent cations result in an increase transmission of Na ⁺ , K ⁺ and H ⁺ and a partial removal of Cl ^– . During nano-filtration of whey for a VRF of 4, the removal of mineral from whey reaches 40 – 60%

and corresponds to 70 – 80% for monovalent co-ions: Cl ^– and OH ^– for sweet whey and Na ⁺ , K ⁺ and H ⁺ for acid whey [ 4 ]. Divalent ions are reduced in the range of 3 – 20%.

By combining with diafiltration, the ash reduction can be driven from 35 – 50% up to 60 – 70% but at the expenses of increased cost, water utilization, and by-products (nanofiltrate) production.

The benefits of nanofiltration are numerous compared to electrodialysis and ion-exchange. That is why in nearly 20 years this membrane operation has become the industrial method of choice for partial desalting whey and become a fast-growing technology in the dairy world for different application purposes. Nanofiltration is

Table 6.2: Demineralization of whey [ 6 ]

Demineralization rate (on dry matter)

Product 30% 50–70% > 90%

Sweet whey (6% dry matter)

Nanofiltration Resins + Nanofiltration Electrodialysis + Resins + Nanofiltration Concentrated whey

(18–26% dry matter)

Electrodialysis Electrodialysis Resins + Electrodialysis

a simple process that has the advantage of simultaneously concentrating the liquid (20 – 22% of dry matter at a VRR of ~ 4), which is often desired, and demineralizing (35 – 50% and even 70% with diafiltration). As whey in most instances has to pass through a concentration stage prior to further processing, the NF option is very attrac-tive because the demineralization is obtained without further cost. In the past, the first concentration was often carried out using RO. However, nanofiltration is more appropriate for the concentration of whey because it simultaneously demineralizes the whey proteins: because of the low osmotic pressure difference between retentate and permeate compared to RO (attributed to the transfer of monovalent ions), the transmembrane pressure is lower and the operation is generally more cost-effective.

Nanofiltration offers low investment costs and simple installations, which are easy to run. Moreover the amount of effluent is greatly reduced in comparison with the other demineralization processes. Electrodialysis and ion-exchange actually lead to high investment and running costs, mainly due to membrane, spacers and elect-rodes replacement as well as wastewater treatment for electrodialysis and high con-sumption of regeneration chemicals for ion-exchange. It has been demonstrated that the running cost of these demineralization techniques are 25 – 55% higher than NF.

In addition, the effluents generated by nanofiltration have a lower BOD com-pared to other demineralization processes, but still require further treatment (clas-sically RO) before sending to the purification treatment plant. The loss of lactose, non-protein nitrogen (N) and protein in nanofiltrate are today lower than those found in electrodialysis or ion-exchange, making retentate more valuable and leading to a permeate with lower BOD. Urea does leak quite extensively. Also, organic acids like lactic and acetic acid can pass through the membrane to a large extent, presenting the possibilities of desacidification of acid whey.

For large demineralization installations (those treating more than 400 m ³ /day), and depending on the proportion of salts to be removed, investment in combining technologies may be of interest ( Table 6.2 ). Today many modern demineralization plants are combinations of classical ion-exchange and/or electrodialysis with NF. By doing this, the ionic load of the ion exchangers is reduced in combination with lower volumes to treat, resulting in principle in a reduction in the size of the columns.

Other applications of NF in whey processing include the concentration and partial demineralization of whey UF permeates prior to the manufacture of lactose and lactose derivatives.