Lactose Recovery Processes

Crystallization is the most widely used process for lactose recovery. Commercial lactose recovery via crystallization from ultrafiltrate or concentrated whey yields 400,000 tons of crystalline lactose annually [49].

The lactose manufacturing process aims at maximizing the yield of crystal mass in a minimum time, and to produce crystals that can undergo the washing step with a minimum loss. In order to increase the yield and reduce the reaction time, novel methods such as antisolvent crystallization, ultrasound crystallization and combined anti-solvent ultrasound crystallization have been examined in the last several years [50-52]. Food-grade lactose recovery in industrial scale is stated to be approximately 65% due to scaling of minerals in the process as well as incomplete washing steps [46]. Due to its higher lactose/solids ratio after protein removal, whey permeate has become an attractive source of lactose. The crystallization process involves several steps including evaporation, crystal formation, centrifugation, washing, and spray drying [46, 50, 53].

In the first step, the initial amount of solids is increased to 50-70% by evaporation. It is advantageous to remove the minerals by electrodialysis or ion exchange resins to avoid fouling on the heat exchanger surface during evaporation [53]. The evaporation step is followed by the initiation of the crystals growth, where the concentrated permeate is fed into a tank and the crystals form either spontaneously or by seeding during a gradual and controlled cooling process. After reaching the desired level of growth, crystals are removed via centrifugation to produce crude lactose slurry. Crude lactose is further washed and centrifuged to increase the purity of the crystals, a process in which it is difficult to control the crystal shape and size as well as the final purity of the crystals.

The presence of proteins and minerals has been associated with the formation of small lactose crystals and lower recovery yields. Partial removal of whey permeate salts by nanofiltration has led to an increase in lactose recovery yield of 6-10% [53, 54].

High evaporation costs and long crystallization times have prompted the search for alternative methods for lactose recovery. Several investigations have described processes to increase the lactose yield, speed up the crystallization and control desired parameters. Some of the proposed techniques include membrane filtration, anti-solvent crystallization, sonocrystallization and anti-solvent sonocrystallization [51, 52, 55, 56].

The use of membranes with different molecular weight cut-offs enables the partial removal of whey permeate salts which affects both lactose solubility and lactose recovery yield. Sequential filtration steps can be used to achieve desired yield and degree of purity; most commonly ultrafiltration (UF) and nanofiltration (NF). Atra et al. [55] examined the performance of nanofiltration of whey permeate. A lactose yield higher than 90% was achieved at 30°C and concentration factor 5. With respect to lactose recovery, membrane processes are most effective compared to crystallization, but less profitable for small and medium scale recoveries due to high capital costs, recurring costs and limited membrane life associated with any membrane filtration system [56].

The addition of anti-solvents leads to a decrease in lactose solubility without creating an additional liquid phase. Due to the reduced lactose solubility, supersaturation and hence crystallization occur faster. Patel and Murthy [51] demonstrated that the recovery of lactose could reach almost 90% when using 85% v/v acetone in conjunction with appropriate process conditions such as stirring time and speed. Furthermore, seeding also decreased the total required crystallization time, and a lactose recovery greater than 90% of lactose was reported.

Ultrasound-assisted crystallization (sonocrystallization) is a new technology that relies upon the use of ultrasound to alter induction periods and saturation. It is typically combined with anti-solvent crystallization (usually acetone or ethanol) to develop anti-solvent sonocrystallization. This process can further decrease the crystallization time to several minutes by reducing the solubility of lactose in water and by inducing a more rapid nucleation event. With this technology, a five-fold decrease in the crystallization time of lactose has been achieved [56].

The removal of lactose via crystallization yields a new stream known as delactosed permeate (DLP). Until recent years, little effort has been made to investigate uses of this novel waste stream. While the lactose content is still quite high on a solids basis (nearly 60%) the mineral content, often measured as ash, increases from 8% to nearly 30% after lactose crystallization. This vast increase in minerals, including an increase to over 3.5% calcium, has led to its use as a dried supplement in animal feed. As with whey permeate, DLP is also spread on land for use as a fertilizer, with fewer deleterious effects due to a lower BOD [57]. Additionally, DLP has been used as a salt replacement in food products [14]. Before drying, a typical moisture content of DLP is 60-70%. The high moisture content of DLP [58] represents a challenge for the development of an economically feasible process for this new stream. Because lactose is commonly crystallized from whey permeate in large cheese manufacturing plants, DLP could pose unique issues to the dairy industry as a new waste stream with which to contend.

 
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