Short CommunicationSonocrystallisation of lactose in concentrated whey
Introduction
Lactose is the most abundant carbohydrate found in milk (4.4–5.2%) and a major constituent of many concentrated and dried milk and whey products. Lactose must first be crystallized before many of these products can be spray dried. Commercial manufacture of whey powder involves concentration of whey, often by evaporation, followed by batch crystallisation which is initiated by rapid cooling or by seeding directly with lactose over many hours (up to 20 h) to ultimately yield up to 80% crystallized lactose [1]. These processes offer limited control and improving the efficiency of crystallisation will benefit the dairy industry [2]. Crystallisation of lactose consists of three phases; the first is supersaturation followed by nucleation (appearance of crystals) and crystal growth. During the crystallisation process it is critical to control crystal purity, shape and size but traditional paddle mixers are known to create non-uniform mixing. Irregularities cause random fluctuations in supersaturation, resulting in uneven and irregular crystal size and growth occasionally forming agglomerates [3], [4].
The overall crystallisation process is slow and lactose recovery can be improved. Sonication is known to reduce crystallisation induction times and increase the rate of nucleation in a number of processes including the crystallisation of fats [5] and pharmaceutical lactose [6] in a process known as sonocrystallisation. Sonocrystallisation is most effective when ultrasound is delivered at the nucleation phase [7].
Ultrasonic cavitation can enhance the rate of reaction and facilitate mass transfer in liquid. Studies have shown that sonocrystallisation generally exhibits four characteristics which are not typical of crystallisation without sonication. These are faster primary nucleation, ease of nucleation, initiation of secondary nucleation and production of smaller and purer crystals [8]. Ultrasound in the presence of an anti-solvent such as ethanol was used to increase the yield of lactose crystallisation [7], [9], [10], [11], in acetone [12] and in glycerine solution [4]. More recently, these characteristics were reported in a simple aqueous system without anti-solvent [13].
Much of the laboratory data reported in literature is based on direct contact sonication. In this approach, a titanium ultrasonic probe was immersed directly into the product. Because the energy density is greatest at the surface of the sonotrode it will cause gradual pitting and degradation. Although the risk associated with such practice is minimal, there is concern that erosion of the sonotrodes may result in product contamination [14]. A non-contact alternative to direct contact sonication exists and this design permits modular implementation and in-line operation. These sonication cells are designed with multiple low power transducers attached to the outer surface of the metal cell, eliminating the need for sonotrodes. Sound waves propagate through the metal surface overcoming sonotrode erosion and improving energy distribution. These generate lower power densities than sonotrodes but efficiently initiate lactose nucleation and have been implemented industrially outside the food industry [6], [15].
Sonocrystallisation of lactose is known to occur in the presence of anti-solvents and in aqueous solutions but the effect of ultrasound on lactose crystallisation in concentrated whey remains unknown. Since the use of anti-solvent in the manufacture of food grade lactose is unlikely to be feasible at a commercial scale, the aqueous lactose study [13] is probably the most relevant reference publication for industry. In the current study, commercially manufactured whey concentrate was sonicated at pilot scale using non-contact equipment to study the effects on lactose crystallisation.
Section snippets
Materials and methods
Concentrated whey was sourced directly from a commercial dairy factory (Northern Victoria, Australia). Whey was concentrated to 32 ± 2% lactose by evaporation at 55 °C. Concentrated whey was then flash cooled to 30 ± 1 °C to initiate lactose crystallisation. Two sonication studies were conducted (Fig. 1).
In the first, sonication was performed with a 20 kHz Sonolab SL250 non-contact sonicator (Prosonix Ltd., Oxford, UK). The processing chamber was 15.4 cm in diameter with a capacity of 2.1 L.
Results and discussion
Crystallisation of lactose in commercially concentrated whey was significantly increased by the application of ultrasound at a low energy density of 3 J/mL and a flow rate of 2 L/min (Fig. 2). A similar observation was made for the various flow rates (0.75, 1.2 and 2 L/min) and power inputs explored (3–16 J/mL) (data not shown). Regardless of the sonication intensity and flow rate, the least number of lactose crystals was observed in the control solutions at T0. A greater number of lactose crystals
Conclusion
Sonication initiates rapid lactose nucleation in concentrated whey but the rate of sonocrystallisation slows after the initial period of accelerated growth. A fast rate of reaction can be maintained for longer by applying a second ultrasonic treatment at the metastable limit to stimulate further nuclei formation. Although the yield of crystallized lactose is limited by the solubility of lactose, the resulting crystals are smaller than conventional stirring and the process delivers greater
Acknowledgements
The authors would like to acknowledge Dairy Australia for technology transfer funding and Prosonix Ltd. for supplying the ultrasonic equipment and technical support.
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