This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
The disposal of whey is a crucial problem for dairy industries because of strict environment regulations. Management of this waste stream is required implementation with the most economical alternative methods for disposal. At the same time, there is increasing demand of lactose present in the whey. The presence of lactose in whey is responsible for more than 90% BOD of whey. Recovery of this component from whey may solve the problem of whey BOD to some extent. In the present paper, we have reviewed the different processes like; membrane separation, conventional crystallization, anti-solvent crystallization and anti-solvent sonocrystallization for the recovery of lactose from whey.
The treatment of waste water usually coming from small and medium scale manufacturing plants is a very crucial problem because of strict environment regulations. The solution of this problem is a challenge for engineers to purify a process with combination of both economical and environmental acceptability (1). One example of process industry is dairy industry, where numerous effluents are generated; out of which few effluents contain nutritionally valuable constituents such as proteins, lactose, fats, etc. One such effluent is whey. The dairy products are obtained from raw milk using processes such as chilling, pasteurization, and homogenization. During this process, by-products like buttermilk, whey, and their derivatives are generated which are considered as waste. Production of whey in the world appears to be in the order of 85 million metric tons with increasing rate of about 3% per year of cheese production (2). Among all nations of world, India is also one of the largest producers of milk and dairy products, hence, generates dairy based wastewaters. The production of milk crossed 85 million tones in the year 2002 and grows at the rate of 2.8% per annum (3). In India, the production of paneer has been increased substantially, resulting in an increased accessibility of whey. Production of paneer is estimated at 1, 50,000 tones per annum which produced 2 million tones of whey per annum (4). In 2001, around 2.58 ´ 106 tones of paneer whey was produced only in India (5). The study carried out in Serbia says that 11 big and medium scale dairies produces approximately 43,800 tones year-1 of whey or milk ultrafiltration permeate, out of which 3,212 tones year-1 is used for cheese production, 6,096 tones year-1 as animal food and 34,493 tone year-1 is discharge into a water bodies (6). Whey contains 30,000 to 50,000 mg liter-1 biochemical oxygen demand (BOD) and a high chemical oxygen demand (COD) which are responsible for high polluting potential of whey and also waste treatment of whey is uneconomical (7, 8). The large quantity of whey produced throughout the production of cheese and the increasing capacity of cheese plants make compulsory for dairy product manufacturers either to process whey or to dispose of it under environmental acceptability (9). Whey has been considered as a waste for a long time since it has very low concentration of its components and unavailability of technically sound low-cost recovery process. It is estimated that 40-50% of the whey produced is disposed of as sewage or as fertilizer to be used for agricultural lands with the rest being used as animal feed. The main components of whey are 5-6% lactose, 0.8-1% protein, and 0.06% fat and mineral salts with varying concentration (10). Disposal of liquid whey is not economical because of high BOD and water content. It is reported that lactose content of whey is responsible for more than 90% whey BOD (11). The most economic way for industries to dispose of whey is to convert it into products which have commercial importance or to recover valuable components (lactose) from whey as lactose has good nutritional and functional properties and can be used in the food industry and in the cosmetic, pharmaceutical, and medical industries (12-14). Therefore, the dairy industry must have to either decrease the lactose content from whey or recover huge amounts of lactose from whey before the disposal of it into the environment (15).
Lactose is a major carbohydrate in the milk and whey. In the dairy industry, lactose is recovered from whey and whey permeates. The pure lactose recovery process generally involves concentration by evaporation, crystallization, separation, refining, drying and milling. Lactose is a disaccharide consists of one glucose molecule linked to a galactose molecule and in aqueous solutions lactose presents in a and b forms as shown in Figure 1 (16). Lactose can exist as either amorphous lactose or crystalline lactose, as either a-lactose or b- lactose or as a mixture (17). The physical properties of lactose are shown in Table 1.
Table 1. The physical properties of lactose.
solubility at 15 oC
Heat of combustion (Cal g-1)
3. Lactose recovery by membrane separation
Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are the four major membrane processes which have become standard unit operations in the dairy industry. Nanofiltration and reverse osmosis are suitable operations for the treatment of dairy streams and to achieve the set targets like; to concentrate the milk constituents in non-food applications, to produce treated water which can be reused in the dairy industry (21). Nanofiltration and reverse osmosis are more efficient in terms of lactose recovery but higher operating pressures are required as compared to ultrafiltration which is one of most potential technology in dairy industry. This process has made it possible for dairy based product manufacturers to improve the quality of its traditional products, to create new food stuffs and to utilize the dairy by-product to be used in entire food industry. The application of UF in the dairy industry was started in the 1970s with the separation and concentration of whey proteins from cheese whey to get protein-rich retentate and lactose-containing permeate (22). In UF the constituents of whey are separated according to molecular sizes and depending on the retention characteristics of the membranes. The protein and fat fractions are retained well while the lactose, mineral and vitamins are separated in the permeate. When ultrafiltration process was studied for lactose recovery from pure lactose solution with three different size of membrane 3, 5 and 10 KDa made up of cellulose material with surface area of 0.1 m2, the recovery of lactose was found 70-80%, 90-95% and approximately 100%, respectively (15). The yield of lactose may depend on the initially content of minerals which can influence lactose solubility in supernatant liquor during crystallization. These minerals, which decreased efficiency of recovery, can be removed by nanofiltration. Nanofiltration process was found effective for removal of mineral salts from whey (23). It was suggested that the combined application of ultrafiltration and nanofiltration may be useful in a recovery of lactose from whey (24). When the process combined of microfiltration (nominal pore size 0.2 μm), ultrafiltration UF3 (molecular weight cut off 5 kDa), ion exchange and reverse osmosis was studied for the recovery and purity of lactose from whey, overall lactose recoverey was obtained 74% with a lactose purity of 99.8%( 25). Also, the high product purity and desired yield can be achieved by combination of ultrafiltration with diafiltration (DF). This process includes three steps: (1) a pre-concentration stage, (2) a diafiltration stage to purify the retentate and permeate, and (3) a final concentration stage to maximize the concentration of high molecular weight solute in the retentate (22). Though membrane processes yields high lactose recovery, they are uneconomical process for the treatment of the whey in concern with small and medium scale dairy processors as membrane processes involve high capital and recurring costs due to limited membrane life and higher operating pressures (26). Despite the high removal of lactose, concentration of milk ions and COD level in permeate remained too high even with reverse osmosis membranes. Therefore, in a single membrane operation, it is difficult to produce water which can be reused in diary industry (21). Another disadvantage of the membrane separation process is that the retentate obtained is a mixture of whey proteins and lactose and other components which cannot be economically separated and exploited commercially to obtain pure components.
4. Lactose recovery by crystallization
Crystallization is a two-step process includes (1) nucleation and (2) growth of nucleus to a macro size. First step involves the activation of small and unstable particles with sufficient excess surface energy to form a new stable phase, which may occur in supersaturated solutions as a result of mechanical shock, the introduction of desired type of small crystals (seeding), or in the presence of certain impurities (20). Factors which influences the crystallization processes are; solubility, supersaturation, seeding, crystal shape factor, agitation, growth rate kinetics, nucleation rate kinetics, agglomeration kinetics etc. (27). Lactose is mostly obtained from whey for many years by crystallization. The commercial development of a continuous technique for crystallization of lactose from whey is require to offer more economic benefits to comparatively small scale processors (19). The conventional lactose crystallization has three basic steps (i) Concentration of whey to 50 to 70% solids by evaporation, (ii) Initiation of crystallization, either spontaneously or by seeding with a small quantity of lactose crystals, and (iii) Separation of lactose crystals by centrifugation. The yield and the purity of crystals depend on the protein and mineral contained of whey, the highest purity and best yields can be obtained from deproteinized and demineralized whey (20). There were two processes used in the recovery of lactose from cheese whey. One process was based on the assumption that the main constituent to be recovered was lactose, while in the other process the recovery of a whey protein was also assumed to be desirable (28). During the seventeenth century in 1633, the first record of isolation of lactose was discovered by Bartolettus, using evaporation of whey. During the eighteenth century, lactose became a commercial commodity with the use of lactose principally in medicine (20). Weisberg (29) reviewed the conventional processes for the manufacture of lactose from whey. The process includes concentration of whey up to 40-65% of total solids by evaporation followed by cooling to give crude lactose. This is then re-dissolved, treated with activated charcoal and recrystalized. Yield of lactose by this method was found in the range of 50-60%. When the cheese whey was subjected to electro-dialysis using ion exchange membrane to reduce the salt content up to 60% and concentrated in evaporator to 60% solids, lactose was crystallized followed by the removal of lactose by centrifugation and recovery of lactose was obtained as high as 87.5% (30). McGlasson and Boyd(28) studied the lactose recovery from cheddar cheese whey using ion exchange resins and found that the purity of lactose obtained was depend on the original content of whey. Lactose of a higher degree of purity about 97.0% was recovered when the original whey was not treated with ion exchange resins to remove protein. More improved conventional process for the higher recovery of lactose is described by Harju and Heikkila (31). However, concentration of whey obtained by evaporation causes precipitation of calcium (complex) salts, which results in fouling or scaling on heat exchange surfaces. Furthermore, these insoluble calcium salts contaminates the lactose crystals during successive lactose crystallization operations. Also, due to low solubility of these salts, they are difficult to remove by washing with water. Hence, pretreatment of whey permeate must be done either before or during evaporation (9). Also, this process is not only uneconomical because of the high evaporation costs but also takes longer crystallization time from 12 to 72 hours. Additionally, the purity of the lactose is significantly influenced by the properties of the initial whey and its protein and mineral content. A few attempts have been done to develop processes for the lactose recovery, different from conventional cooling method (32).
5. Recovery of lactose by anti-solvent crystallization process
The crystallization process in which the organic products are separated from aqueous solution by adding non-solvent compounds which reduce the solute solubility without creating the new liquid phase is known as anti-solvent crystallization. The nucleation and the nature of the crystalline product depend on the conditions of crystallization process under which the material is crystallized. Hence, variations in yield of product can be observed by varying the operational parameters such as supersaturation, temperature, pH, and impurity content. The main problem associated with the lactose crystallization is a long induction time and large meta stable zone width (MZW). Raghvan et. al. (33) has studied the bulk crystallization of lactose from aqueous solution with varied crystallization temperature (293-313 K) for total crystallization time (22-72 hours) and found that the maximum possible yield of lactose was obtained 20-57 %. The long induction time (2-17 hours) and extremely slow growth of lactose crystals was also observed. When sodium hydroxide was added to solutions containing lactose and manganese chloride, insoluble manganese hydroxide and lactose as a complex precipitated. Maximum lactose recovery was found to be 55% at molar ratios of sodium hydroxide to manganese chloride at 2.0 and manganese chloride to lactose at 4.0 (34). Cerbulis (35) was applied Steffen process for the recovery of lactose from cheese whey and found that 81% of lactose was precipitated in a cold precipitation step at temperature 3-5 oC. When precipitant CaO was used in combination of FeCl3, lactose yield was improved to 87-95%. Addition of equal volumes of acetone or methanol gave almost complete precipitation of lactose from whey. Recovery of lactose explored with different anti-solvents is shown in Table 2. The solubility behavior of lactose explored in ethanol-water mixture (36) and acetone-water mixture are shown in Figure 2 and Figure 3. In these studies, it was found that the lactose recovery was greatly influenced by anti-solvent (ethanol or acetone) concentrations. Higher recovery of lactose was attributed to the lactose solubility in solvent, which was substantially decreased with increase in anti-solvent concentration. Variation in the crystallization time and seeding was found to be affected the size distribution of lactose crystals. However, Conventional anti-solvent crystallization includes mechanical agitation, which introduces random fluctuations in the solution and cause heterogeneous distribution of local concentrations, leads to uneven growth of crystals and causes variation in the particle size and morphological features due to poor-mixing (37).
Table 2. Recovery of lactose explored with different anti-solvents.