Concentrating on Quality - Membrane Filtration

Abstract

Cross Flow membrane filtration offers some unique benefits to producers. It provides an economic means of water removal which can easily be integrated into existing concentration and drying systems, but perhaps the most exciting feature of the technology is the ability to simultaneously concentrate and purify, thereby making possible enhanced product quality and new product development.

In this paper two case studies are presented which seek to demonstrate how these properties have been exploited.

Background

Unlike conventional dead end filtration, in a cross flow filtration system the retentate flows parallel to the membrane surface at a velocity sufficient to prevent or at least minimize "cake" formation, in this case the formation of a concentration polarization layer.

The cross flow velocity is typically in the range 2-6 m/s dependant upon membrane type, configuration, and application.

Membranes fall into two basic categories, polymeric and inorganic - ceramic and stainless steel.

Various membrane configurations have been developed, each having their own specific strengths including tubular, hollow fibre, flat sheet and spirally wound.

The filtration spectrum is classified into four areas reflecting the degree of filtration and mechanism. Typical applications and parameters are indicated in Table 1.

Case Study 1 - Whey Processing

In the course of cheese production approximately 90% by volume of the liquid milk entering the plant is converted to whey - a liquid by-product of cheese manufacture. This was formerly considered an inconvenient and difficult to dispose of effluent stream, often dumped into the sea, sprayed on fields or fed to cows. Today, economical processing of the whey is of critical importance to the economics of cheese manufacture, as the cost of effluent disposal is in the order of £200 per tonne.

As a rule of thumb, any cheese factory producing in excess of 300m3 per day of whey can process whey profitably. However, as many cheese dairies do not have this volume of scale it is common to pre-concentrate the whey at the cheese factory and tanker it to a dedicated facility for the downstream processing.

Let us consider by example a cheese plant producing 750 tonnes per day of whey looking to improve their process economics, and using 2 TVR evaporators in parallel, to concentrate the whey from approximately 6% solids to 35% prior to it being taken off site for finishing and drying elsewhere.

Five alternative strategies can be considered:

• To do nothing, apart from minor optimization of the existing evaporation plants.
• To install an RO plant ahead of one of the evaporators.
• To install an RO preconcentrator plus NF plant and mothball both evaporators.
• To install a single energy-efficient MVR evaporator.
• Not to process on-site but instead to tanker the whey at 6% TS concentration and drying elsewhere.

The results of the evaluation are set out in Table 2, which it should be noted excludes the subsequent processing, i.e. concentration and drying.

It should be noted that in the case of option 3, the resulting solids concentration is approximately 22% from the nanofiltration plant rather than 35% from the evaporator and hence an increase in subsequent processing cost - evaporating to approximately 52% TS prior to spray drying - during subsequent processing.

However, due to the unique separation characteristics of the nanofiltration membrane the resulting product will be part demineralised by approximately 40%, which itself is reflected in the value of the product.

In this case the preferred solution is option 2, the use of an RO plant for preconcentration of the whey which gives the most favorable return over a five year period.

If a longer payback is considered, then option 4, a new MVR evaporator also provides an attractive solution.

In this example, preconcentration by membrane filtration has a number of distinct advantages:

• Optimal transition of 8-12% DS.
• Low investment cost.
• Low energy consumption.
• Reduced space requirements.
• Ease of integration

In the case study we have considered only the concentration and its subsequent drying to form whey powder, which is used extensively as a food ingredient in addition to or part replacement of skim milk. By using membrane filtration however, it is possible to further process the whey to produce a range of value added products.

To understand what is possible, we first need to consider the relative composition of whole milk and whey milk.

The whey proteins referred to have the following approximate composition.

In Fig. 1 the various options for whey processing are highlighted:

By concentration of the whey by ultrafiltration, it is possible to produce a product with a protein content in the range 35-50% and a solids content of up to 15% TS which for reasons of economy is further concentrated to 40-45% TS prior to drying in a multistage spray drying process.

By addition of diafiltration water in a second process stage, the lactose concentration can be reduced as the lactose will pass through the ultrafiltratiion membranes whilst the whey proteins are retained to achieve protein content in excess of 80%.

The process is illustrated in Fig. 2:

In order to further refine the WPC to achieve protein content in excess of 90%, the fat must be removed by microfiltration, normally a ceramic system, installed between the two ultrafiltration stages as shown in Figure. 3.

The resulting product is referred to as whey protein isolate WPI, in this case WIP-90, and can be seen as an almost pure source of protein, with highly desirable functional properties, with a value similar to cheese itself.

The relative market size, market price and production costs are given in Table 5.

Until recently that was the end of the story. Today however, by combining chromographic separation with membrane filtration it is possible to separate out the individual whey proteins, with protein purities in the range 80-90% which has opened up some new and exciting possibilities.

We have seen that whereas human milk is predominantly alfa-lactalbumin and lactoferrin, the major whey protein is beta-lactoglobulin.

If this can be selectively isolated, it is then possible to produce infant formula much closer in composition and functionality to human milk, which represents an $8-10 billion global industry.

Other high growth, high margin products using the isolated whey proteins include fermented products such as the new range of flavoured beverages, probiotic yoghurts and sports drinks, which claim significant health benefits including the control and even reduction of cholesterol. So, particularly with falling cheese prices, cheese may be seen one day as the by-product of protein production.

Next (Case Study 2 - Enzyme Production, and Separation Technology)