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Pulse electric field processing can increase shelf life while preserving freshness
 7 min read

Microbes and enzymes both cause spoilage in raw milk. The electrical pulses of pulse electric field processing (PEF) destruct these microbes and enzymes in foods and extend the shelf life. PEF better preserves the flavour, colour and vitamins and thus the freshness of foods compared to common heat treatments. At present, the cost for using this novel technique is still relatively high and PEF is therefore most suitable for premium products, such as dairy bio-actives, liquid infant formula, raw milk cheeses, whey and blended fruit drinks. Looking to emerging consumer trends and market opportunities it is already evident that such novel techniques will become more lucrative as consumers place increasing value on food quality.

PEF tested on dairy products

Researchers from CSIRO Animal Food and Health Sciences in Australia explored the effects of PEF treatments on spoilage, pathogenic microorganisms, unwanted enzymes and other food quality characteristics. They used a wide range of dairy products and tested them under diverse processing conditions. In many instances food quality was enhanced through improved flavour retention, protein functionality, colour, and nutritional profile.

The technique

Pulse electric field treatment applies short, high voltage electrical pulses to food as it passes through electrodes. These electrical pulses inactivate microbes via cell membrane destruction. Furthermore, the electrical and thermal effects of PEF inactivate enzymes via denaturation and chemical changes, such as oxidation and free radial formation. The amount of microbe inactivation depends on the process intensity and environmental factors such as the shape, size and membrane features of the microbe as well as the water content, pH, electrical conductivity and the soluble solids of the food.

Extended shelf life

PEF holds huge potential for extending the shelf life of raw milk prior to downstream processing. The researchers have found for example that PEF treatment effectively targets Pseudomonas, which is the predominant microbe responsible for raw milk spoilage. After using PEF, the shelf life of raw whole milk was extended by 8 days, that of raw skimmed milk by 13 days and pasteurized whole milk by 80 days at 4O°C.

Getting the bad bugs

The dairy industry faces a significant challenge in the control of food born illnesses with a variety of disease caused by bacteria thriving in inappropriately stored dairy products. PEF effectively targets harmful bacterial species, such as Listeria, Salmonella and pathogenic E.coli. PEF process conditions were varied in the different studies. A variety of products were successfully treated, including infant formula, whole milk, a variety of reduced fat milks, fruit juice, liquid whole egg and whey. The number of microbes reduced by a factor 10 to 100,000,000. 

Shelf life of raw milk extended by 8 days

Inactivation of dairy enzymes

Milk contains a large number of enzymes that originate from the cow or from microbes that are present in the milk. These enzymes differ in functionality, process stability and impact on dairy products. Food spoiling enzymes such as lipase, protease and xanthine oxidase have all been found to be susceptible to PEF treatments in raw whole and skimmed milk. Enzyme activity was reduced by 14 – 80%. PEF reduced even the most heat stable enzyme, alkaline phosphatase, in raw milk by up to 67%. Plasmin affects milk and dairy product quality and survives common pasteurisation. It can be inactivated by PEF by up to 90%. Enzyme inactivation depends on treatment parameters and food characteristics. Interestingly, lactoperoxidase was found to be quite resistant to PEF treatments. The enzyme is anti-bacterial and therefore positively associated with milk quality.

Damage of functional proteins

However as PEF is not selective the electrical fields and thermal effects can damage functional peptides and enzymes. For example, PEF can reduce the stability of a range of functional peptides and enzymes in whey, such as beta-lactoglobulin, lactoferrin, apo-lactioferrin and halo-lactoferrin. The degree to which damage occurs is dependent upon treatment parameters. Therefore, consensus is found such that treatment minimises structure disturbances of whey proteins.

Product quality

One of the key attractions of PEF is that it can be applied with modest increases in temperature and minimal heat induced damage.  The effect of PEF processing on the chemical, physical, functional, sensory and nutritional profile of a variety of dairy products has been found to be minimal. The researchers reported almost no effect on milk colour, flavour, milk pH, density or non-fat solid content, such as casein, whey, lactose, minerals and vitamins A, C, E in whole milk. PEF even appeared to increase Vitamin C retention.

Viscosity

Interestingly, whole milk viscosity was unaffected by PEF, whilst that of skimmed milk decreased. PEF has the potential to disrupt fat globule membranes and treatment conditions are critical. PEF treatment can also affect the structure of casein and whey and alter functional properties. Treating milk at a moderate temperature (<50°C) improved rennetability, reduce clotting times and increase curd firmness in comparison to thermally pasteurized milk. PEF treatment of whey protein isolate was found to increase gelation times and decrease firmness, this is of particular value for industrial applications such as spray concentration of whey where whey protein precipitation and gelation are undesirable.

Flavour in cheese

Cheddar cheese produced from PEF treated milk was found to be harder and springier than that produced from conventionally pasteurized milk, with no differences in adhesiveness or cohesiveness. Despite a decrease in the number of adjunct lactic acid bacteria, which accelerate ripening and improve the flavour of Cheddar cheese, there were only minor differences in flavour. Texture was not affected by using PEF treated milk.

PEF retains the bioactive properties of heat sensitive molecules

Synergistic combination with heat treatment

The combination of using PEF and thermal processing results in a product with extended shelf life and superior bacteriological profiles in comparison to conventional pasteurization of milk. The possibility of an 80-day shelf life for PEF treated pasteurised milk holds significant value and potential for the sector. Furthermore, the higher processing temperatures that result from PEF treatment increase enzyme and microbe inactivation. Therefore, the overall amount of energy and PEF equipment power required to achieve shelf life extension can be lower.  This requires significant assessment and optimisation.

Longer clean

PEF treated dairy fluids have the advantage of being low in fouling, as the dairy proteins typically do not coagulate in the low processing temperatures. This implies that PEF systems can potentially operate with longer “clean in place” time intervals than conventional thermal pasteurizers.

Current use

Industrial Scale PEF processing systems for preservation and improved quality is currently being applied for the shelf life extension of fruit juices at a capacity of 8000 litres per hour. The processing costs are approximately €0.02 per litre for the fruit juices. As far as we know, PEF is not yet used in dairy systems. The processing prices will be slightly higher for dairy systems due to the higher electrical conductivity of dairy systems.

Future applications for dairy products

The efficiency of PEF for extending the shelf life and chill stability of fluid dairy systems is evident in the extensive research carried out to date. Such increases in product performance from the application of environmentally friendly processing are a significant development for the dairy industry. PEF will have significant impact on the potential market reach and competitiveness of milk producers, in particular with the changes in regulations and lifting of the Common Agricultural Policy.

References

Buckow, R., Chandry, P.S., Ng, S.Y., McAuley, C.M., Swanson, B.G. (2014). Opportunities and challenges in pulsed electric field processing of dairy products. International Dairy Journal, 34 (2), 199-212.

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Marielle Ramaekers
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