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Dairy processing: Non-thermal processing systems

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Introduction

The following provides recommended practices for non-thermal processing of dairy products.

Fermentation and slow vat

Starter activity is a critical step to monitor in the manufacture of cultured dairy products. After inoculation of the milk with starter, acidity development (lowering of pH) should occur at an anticipated rate. A slow vat is when the desired pH is not achieved within the anticipated timeframe and is an indication that the starter has not produced acid at the desired rate. Many things can affect starter culture activity, including bacteriophages and residues of antibiotics and cleaning agents. When a slow vat occurs other competing organisms, which may include pathogens, may grow and have a detrimental effect on product safety and quality.

Membrane processes

Membrane filtration processes separate milk components on the basis of molecular size and/or electrical charge. While traditional or conventional filtration is typically used to separate suspended particles larger than 10 µm, membrane filtration separates substances of molecular sizes less than 10 µm. Those particles that have a molecular weight lower than the filtering threshold of the membrane will permeate the membrane.

The membrane separation techniques used in the dairy industry include the following:

Microfiltration (MF): Mainly used for the reduction of bacteria in skim milk, whey and brine. It is also used for defatting whey intended for whey protein concentrate (WPC) and for protein fractionation. MF membranes have a pore size of 10 - 1 - 101 µm. The resulting permeate of MF separation is water, minerals, lactose and some proteins.

Ultrafiltration (UF): Typically used for the concentration of milk proteins in milk and whey and for protein standardization of milk intended for cheese, yoghurt and some other products. UF membranes have a pore size of 10 - 2 - 10 - 1 µm. The resulting permeate (defined as the filtrate, the liquid passing through the membrane) of UF separation consists of water, minerals and lactose.

Nanofiltration (NF): Is associated with the concentration of organic components by removal of part of the monovalent ions like sodium and chlorine (partial demineralization). It is generally used for the dehydration of whey, UF permeate or retentate. NF membranes have a pore size of 10 - 3 - 10 - 2 µm. The resulting permeate of NF separation is water and minerals.

Reverse osmosis (RO): Consists of the concentration of solutions by the removal of water. RO is generally used for the dehydration of whey, UF permeate and condensate (defined as the retentate or the retained liquid). RO membranes have a pore size of 10 - 4 - 10 – 3 µm. The resulting permeate of RO separation is water.

Electrodialysis polar membranes systems: Electrodialysis is dependent on ionic mobility and preferentially removes monovalent ions. These systems consist of sets of paired membranes, each of which has whey passing on one side and water passing on the other. A direct current is applied, one membrane in each pair functioning as an anode and attracting negatively charged ions (anions), while the other functions as a cathode and attracts positively charged ions (cations). The membranes are porous to both anions and cations, which pass into the water. This type of membrane system is applicable to the production of demineralized whey powder.

Some examples of membrane systems include UF of milk prior to the manufacture of soft cheeses such as Brie and Camembert. The water, lactose and dissolved salts are forced under pressure through the membrane pores or by diffusion through the membrane. These three components of the milk are removed (permeate) resulting in the progressive concentration of the fat and protein (retentate). MF is also used to concentrate the milk prior to cheese making where the casein micelles are concentrated and results in the removal of more whey proteins from the cheese. Use of membrane technology for cheese making has demonstrated improved yields, savings in rennet, and reductions of manufacturing time, labour, and space. A more uniform final product is also produced due to the improved drainage. Another example of membrane technology is the UF of whey where water is removed as permeate together with some of the lactose and minerals and results in the manufacture of whey protein concentrates and whey protein isolates (retentate).

Membranes are available in various forms: pipe; hollow fibre, plate and frame; and spiral wound. The material used to manufacture the membrane determines its resistance to temperature, pH and oxidizing agents and thus governs its operating conditions and cleaning techniques.

Generally, acid conditions are used to remove minerals and chlorinated alkali conditions with or without surfactants are used to break down protein deposits. Sometimes enzyme cleaners are also used to remove protein. For UF membranes, the chlorine (Cl) content should be maintained at the manufacturers recommended level throughout the wash cycle; Cl is added to the wash solution until no further decline in Cl level (ppm) is observed. After cleaning, the system is left full of water in general with an inhibitor for bacterial growth if the storage exceeds a critical time to prevent drying of the membranes. NF and RO membranes cannot tolerate chlorine or strong oxidizers and therefore cleaning should be accomplished with alkali, acid and surfactants.

Membrane fouling is defined as the deposition and accumulation of components of the starting liquid on the membrane surface and/or within the pores of the membrane, causing an irreversible flux decline during processing. The membrane is considered to be clean when the permeate flux rate (l/m2/h) (defined as the rate of extraction of permeate measured in litres per square meter of membrane surface area per hour) and pressure (this is a combination of permeate flux rate and pressure monitoring) is in the same range as they were prior to the start of operations. A Total Plate Count of clear water drained from the system can be used to verify proper cleaning.

Brine control

For brine used in cheese manufacturing, it is important that the brine is of good quality. Although pasteurization or UV light treatment of brine is recommended, controlling the following four factors (salt concentration, microbial concentration, particulate matter, temperature) can also be effective.

If the temperature is controlled by doing the salting process in a cold room, improper maintenance of the cold room may result in the formation of condensate and mould growth which may be a source of contamination.

Starter preparation

Some processors may prepare bulk starter in tanks or cans before it is used in cultured products manufacturing.

Starter preparation involves the mixing of the starter culture and media, followed by an incubation step to propagate the culture. The resulting starter is added post pasteurization to milk and therefore it is critical that the media is pasteurized under controlled conditions (time and temperature), and is maintained in a hygienic condition, to ensure no pathogens are present prior to mixing it with the starter culture.

Equipment

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