Removal of lactic acid from acid whey using electrodialysis
Introduction
Acid whey is a by-product of cream cheese and strained yoghurt manufacture and is also produced from chemical acidification of milk in casein production. This whey stream contains about 20% of the protein (∼1%m/v) and most of the lactose (LT) (∼4.4%m/v) from the original input milk [1]. Sweet whey is generated from rennet cheese manufacture, and while similar in composition, acid whey can contain up to 16 times the concentration of lactic acid (LA) to that in sweet whey [1]. Traditionally, sweet whey is further processed to make whey powder, lactose, whey protein concentrate and demineralised whey powder. One of the essential steps for manufacturing these products is the use of spray dryers. The presence of lactic acid in acid whey prevents a similar process as the acid whey makes the resulting whey powder more susceptible to moisture absorption [2], [3], [4], due to the hygroscopic nature of the lactate ions [5], [6] and this leads to the formation of powder agglomerates and sticky deposits within the dryer that cannot be tolerated in normal operation. Consequently, acid whey is often used as a stockfeed to neighbouring farms or discharged as effluent, rather than providing a valuable source of protein for human diet consumption.
The formation of these sticky deposits is a function of the proportion of lactose that is present in a crystalline rather than an amorphous phase, which in turn depends upon the glass transition temperature of the powder, relative to the dryer operating temperature. In turn, this glass transition temperature is related to the level of lactic acid present in the feed [5]. For example, the temperature of sticking increases from ∼70 °C to ∼95 °C, when the mass ratio of lactic acid to lactose decreases from ∼0.2 g LA/g LT in acid whey to ∼0.04 g LA/g LT in sweet whey powder [5].
The effect of lactic acid in the crystallisation process of lactose/protein powders has previously been studied by Saffari and Langrish [7] and Shrestha et al. [2] who used laboratory scale spray drying units. The ratio of LA to LT was found to correlate well with a decline in spray dryer powder recovery and the glass transition temperature of the recovered powder. Typically, by increasing the concentration of lactic acid from ∼2 g/l (in sweet whey) to ∼6 g/l (in acid whey), the yield of the sprayed dried powder decreased by ∼20% [7]. At the same time, the glass transition temperature decreased significantly. Similar trends were reported by Shrestha et al. [2], who observed that a free flowing powder could be produced only if the lactose solutions contained less than 4.2 g of lactic acid per 100 g lactose. This is equivalent to the ratio of LA/LT in sweet whey.
Therefore, a reduction in lactic acid concentration is necessary before acid whey can be efficiently processed by downstream spray drying unit operations. The neutralization of the lactic acid has been studied as a potential approach to resolve this issue [2], [3] but this approach results in bitter and astringent flavours. Alternatively, electrodialysis has been successfully demonstrated to recover lactic acid from fermentation broths [8], [9], as well as to demineralise sweet whey prior to whey powder production [10], [11]. This unit operation relies on the transfer of charged species from the diluate (feed) stream to a concentrate stream, through a series of cation and anion selective membranes under a constantly applied electrical potential.
While lactose molecules are uncharged, lactic acid is a weak acid that can dissociate into its conjugate base (a lactate ion, CH3CH(OH)COO−) and protons (Eq. (1)). It is expected that lactose will be retained in the diluate stream while lactate and other charged ions will be transferred to the concentrate, effectively demineralising the acid whey.
The dissociation of lactic acid is given by the Henderson–Hasselbalch equation:where the pKa of lactic acid at 25 °C is 3.86 [12]. The association constant at a different temperature can then be estimated by the following correlation [13]:
Williams and Kline [14] patented an electrodialysis approach that used a three compartment stack to reduce the acidity of acid whey for use in ice cream, by neutralisation with caustic solution. Bipolar membranes allow a salt to be split into the corresponding alkali and acid during electrodialysis and this approach has also been used to split salts such as ammonium lactate, formed during fermentation of whey permeate, into lactic acid and ammonium hydroxide [15]. The focus of this prior work with electrodialysis has either been to neutralise the acidity of a whey solution by removing protons, or to provide high purity lactic acid as a product from a high concentration fermentation broth. Conversely, the focus of the present work is the removal of lactate anions at relatively low concentration from a whey solution. The reduction in the concentration of lactate potentially allows for the processing of acid whey in a similar manner as sweet whey, allowing the proteins and lactose to be recovered for sale.
Section snippets
Materials
Raw acid whey samples were obtained from a dairy processing company in Victoria, Australia. The acid whey was treated in a pilot plant at Dairy Innovation Australia Ltd (Werribee, Victoria Australia) using a centrifugal separator (GEA Westfalia Separator, Model SC 6-01-576) to remove the whey cream and most of the casein fines. This stream is referred to as the skimmed acid whey. Part of the skimmed acid whey was further filtered in the laboratory through a 10 kDa ultrafiltration membrane (Koch)
Transfer of lactate ions
The removal of lactate ions from solution as a function of time is shown in Fig. 1. In general, the lactate ions are removed faster at higher temperatures, as the resistance of the membrane [32] and the solution is lower [33]. The solution viscosity is also lower at a higher temperature (1.52 cP at 10 °C vs 0.68 cP at 45 °C for a 4.93% lactose solution [34]), aiding lactate removal. For the artificial solutions at 5 °C, the initial rate of lactate ion removal in the first 60 min is slower than that
Conclusions
Electrodialysis was operated in batch mode to remove the lactate ions from acid whey at three temperatures and two solution pH, typical of the operating conditions in dairy streams. For both the artificial solutions and the acid whey samples, the lactate ions were removed at a slower rate compared to other anions present in acid whey, but the removal of these ions was more efficient at higher operating temperatures. The rate of removal was also improved slightly by increasing the pH from 4.6 to
Acknowledgement
This research was supported under Australian Research Council’s Industrial Transformation Research Program (ITRP) funding scheme (Project Number IH120100005). The ARC Dairy Innovation Hub is a collaboration between The University of Melbourne, The University of Queensland and Dairy Innovation Australia Ltd.
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