BY-PRODUCTS TECHNOLOGY

Module 3. Processing and utilization of whey

Lesson 21
MANUFACTURE OF CONDENSED WHEY PRODUCTS

21.1 Introduction

Whey is a highly perishable and its use in liquid form is limited to only small quantities. It can be preserved by removing its water and so the concentration and drying of whey assumes a special significance. With the development of membrane technology, it became economically feasible to concentrate whey using reverse osmosis (RO) to 28% TS retaining all functional properties of whey proteins. The potential use of RO can be as a pre-concentration step prior to transportation of whey concentrate to a central processing facility for further processing for the manufacture of whey powder, whey protein powder, demineralised whey powder and lactose. Whey concentration is carried out with the following objectives:

a) Volume reduction for transport to other processing facilities.

b) Pre-concentration for drying into whey powder.

c) Manufacture of condensed whey: plain or sweetened.

21.2 Production of Concentrated Whey

Usually the whey is prepared as plain or sweetened condensed whey depending on its end use and its targeted shelf life. Various steps involved in the production of concentrated whey are as follows:

21.2.1 Clarification

The whey always has curd fines, which confer serious risk of blocking heat exchangers channels or fouling RO membranes. These curd fines also adversely affect the solubility properties and flavour of the end product. These curd fines are usually removed by a combination of treatments such as settling, screening and clarification. For large-scale operation, it is usual to employ a self-discharging clarifier with an enlarged solid-holding capacity. Also used commonly are the hydrocyclones, for recovering curd fines from whey.

21.2.2 Separation and pasteurization

In order to obtain flavour stability in concentrated whey, the whey is separated to remove fat. It is followed by whey pasteurization for optimum microbiological quality, to inactivate rennet and to ensure storage stability. It is then stored at 5°C till concentration and drying.

21.2.3 Concentration

The clarified and defatted whey may be concentrated economically with the integration of RO for pre-concentration and multi-effect evaporator. The main difference between evaporation and reverse osmosis is that evaporation removes water by the application of heat, which turns the water into steam followed by condensation of the vapour, whereas RO is a simple filtration process requiring mechanical energy. As compared to evaporation, RO takes place at low temperatures and consumes less energy (Table 21.1). However, RO concentration is limited by the osmotic pressure of the concentrated solution and for whey, the maximum level is approx. 25-28% TS. RO concentration of whey will at some point become uneconomical to continue the concentration because of a low flux at higher concentration. Typical economically viable TS levels in RO concentration are 21% for sweet whey and 17% for acid whey.

Evaporation, on the other hand, is only limited by the solubility of the solutes at the evaporation temperature and thus a TS level of 60-65% is obtainable. The most modern concentration installation consists of a 5-7 stages evaporation system. By adding more stages to the evaporator, the specific steam consumption is reduced. A seven-stage plant consumes 50 % less steam than a three-stage plant.

Acid whey foams less during evaporation in evaporators than does sweet whey. When sweet whey is difficult to manage in the evaporator a small quantity of acid or a little fat may be added to break the foam.

Table 21.1 Energy cost of efficient evaporation v/s reverse osmosis for whey

There is spontaneous lactose crystallization in vapour separator while concentrating whey to levels above 55% TS. This may be avoided by immediate removal of concentrated whey from the evaporator apart from controlling final total solids contents of whey and increasing the evaporation temperature in the last stage, where lactose is in most concentrated form. For this, the whey evaporators are sequenced in such a way that the higher solids are reached at higher temperature than that prevails in the last stage. For instance, a 7-Stage falling film evaporator can be sequenced 1-2-3-4-7-6-5, where the temperature (°C) in the respective stages is 68, 65, 61, 57, 50, 45, and 39°C.

21.3 Plain Condensed Whey

Plain condensed whey is prepared by pasteurizing the whey and drawing the hot liquid into the vacuum pan. The desirable degree of concentration of plain condensed whey is 35-50% total solids. Whey condensed to higher solids content (more than 55% TS) forms a gel after cooling and is not recommended to be used in any food or feed products due to its coarser texture and low solubility. ).

Plain condensed whey is also prepared as an intermediate product for whey powder making. For this purpose, the whey is concentrated to about 40-60% solids and lactose pre-crystallized before drying. For lactose crystallization, concentrated whey is cooled rapidly to 30°C using flash coolers and seeded with fine-grained α-lactose monohydrate crystals (crystal size of approximately 10 μ) at a level of about one kg per tonne of concentrate. The seeded mixture is stirred for 1 to 3 h in a crystallization tank at 30°C for mass and forced crystallization of lactose to a size lesser than 25 µ. Lactose crystallization may be continued by cooling up to 10^oC for higher level of lactose crystallization.

21.4 Sweetened Condensed Whey

For the preparation of sweetened condensed whey, sweet cheese whey is mixed with sugar equal to the weight of solids in whey (about 6.7 kg sugar for 100 kg whey). The mixture is concentrated using multistage evaporators to at least 76% T.S. . The specific gravity of sweetened condensed whey at 50°C is 1.360 (38.4° Bé). For controlled lactose crystallization that is necessary for producing smooth texture product, the concentrate is then subjected to lactose crystallization by cooling up to 10°C, the similar way as explained under point 21.3. The lactose crystal size must be kept below 25 μ to produce a smooth rather than sandy texture. It may then be packed in barrels or cans. The product does not require any refrigeration for storage.

Sweetened condensed whey is somewhat salty and also has a characteristic whey flavour. Some desalting of the original whey by RO or other procedure would greatly improve flavour and utilization in certain foods. Sweetened condensed whey darkens and thickens in storage, but these changes do not develop to an objectionable level during the first year. The product must be utilized within one year of production to get full advantage of its functional and nutritional properties.

Selected reference

http://www.apv.com/us/applications/dairy/whey/wheypowder/capabilities/Capabilities.asp