New types of milk-based products by high pressure

High pressure technology offers new opportunities for nutritional and healthy milk products. Based on skim milk and added whey protein or hydrocolloids, high pressure makes it possible to produce milk products ranging from yoghurtlike to pudding-like, but without the sour taste and with less sugar. Moreover, high pressure is a minimal food processing method which addresses consumer preferences and reflects the human ethics of natural, tasty, clean-labelled and eco-friendly products.

The production of various types of milk products like yoghurts involves, in short, a souring processing step to form the gel network and typically requires bacterial fermentation of a starter culture and an incubation step involving heat that can be relatively time consuming. In such a sour milk gel, the gelation is based on the acid-induced dissociation of the casein micelles and the thermal-induced denaturation of the whey protein β-lactoglobulin (β-Lg) which link together and form a strong gel network. It has been discovered that aqueous solutions of β-Lg and of whey protein subjected to high pressure (HP) treatment also form gels, and the perspectives of designing new types of dairy products blaze up.

The opportunity to produce new types of food products using HP processing is based on the specific actions of pressure on food constituents. HP technology is a minimal, cold processing method, which means that the covalent bonds do not break while noncovalent bonds are affected, thus it can be used to modify macromolecules like proteins resulting in changes of the functionality of food proteins. HP processing relies on the application of hydraulic pressures between 100 to 800 MPa (the same as 1,000 to 8,000 atm), and due to the isostatic principle, the food material will experience exactly the same pressure instantaneously throughout and no gradients, like during heat treatment, are rising. Therefore, irrespective of the size, shape and composition of the food, the results are a uniform treatment. Other advantages of high pressure are that many types of spoilage and pathogenic microorganisms are killed (without chemical additives), maintenance of flavour, colour, nutritional value, inactivation or activation enzymes (dependent on type of enzyme) and energy efficiency.

The base in producing milk-based gels by HP processing is a mixture of milk powder and whey protein powder, thereby taking advantage of the good gelation properties of β-Lg. Next is the use of high pressure as an active processing step thereby utilising the HP technology to its full potential. The un – complicated production of HP milk gels is presented by Figure 1, together with the principle of high pressure treatment. Overall, the aim of the research is to produce neutral milk based gel products similar to the well-known yoghurts ranging in viscosities from drinkable to spoon-able, but obviously without the sour taste.

Upon manufacturing, distribution and storage at the retailer or consumer of milk gel products, the important gel characteristics, gel elasticity and non-incorporated water, must be controlled. These parameters are found to depend on applied pressure, duration of the HP treatment and concentration of whey protein. The production of a skim milk based gel were investigated by reconstituted milk fortified with 5, 10 or 15 per cent w/w natural whey protein isolate (WPI) and subjected to HP treatment at 200, 300, 400, 500 or 600 MPa for 15 or 30 minutes, and subsequently stored for one or five days at 5ºC. From the obtained results, it was immediately apparent that the gels could be classified in three types of gels; soft gels with elasticity << 100 Pa, intermediate gels with 100 Pa << elasticity << 1000 Pa and strong gels with elasticity >> 1000 Pa. Table 1 summarises the investigation on how the milk gel characteristics; gel elasticity (G’), nonincorporated water (NIL), and whiteness (L*) are affected by pressure, holding time, WPI concentration, and storage. Some remarkable effects on the gelation of milk-based whey protein solutions were observed in the elasticity and water binding capacity due to the con – centration and pressure variations. The soft gels based on milk added 10 per cent WPI with low amount of NIL suggest a rather rough structured gel network capable of retaining water during pressurisation and/or on pressure release. Upon increasing the WPI concentration to 15 per cent, the gel elasticity increases due to the additional protein-protein interactions, resulting in a stronger network formation. The increase in NIL (up to 400 MPa) expresses a lower water binding capacity caused by a denser network structure formation which expels water during the HP treatment. Above 400 MPa, the large improvements in the elastic strength suggest an even greater structural enlargement. At this pressure level, β-Lg is fully or partly denatured/unfolded and casein micelles dissociate into smaller units, which together enhance gel formation. Mechanistically, pressure denaturation of β-Lg facilitates intermolecular exchange reactions between the free thiol group and disulphide groups of other β-Lg and casein molecules, resulting in oligomer formation. Also, noncovalent physical interactions such as hydrophobic, electrostatic and hydrogen bonding are important for further aggregation and gel formation of pressure induced protein oligomers. Less convincing from an industrial point of view was that these milk-protein gels were lacking mouth feel and creaminess.

Hydrocolloids are often used in the dairy and dessert industry and their functions are stabilisation, thickening, emulsifying, gelation and water retention. Typically, a small amount of hydrocolloids, like carrageenan, locust bean gum (LGB), alginate and xanthan gum have a large effect on the respective function. It was, therefore, of interest to investigate the role of such hydrocolloids in the production of HP milk gels and various students’ case studies were conducted to show the effect on the texture, appearance and taste.

Skim milk containing 0.5 or one per cent of two commercial carrageenans, type LP-60 or PMD (CP Kelco) gave different gel strengths at pressure treatments between 300 to 800 MPa (30 minutes at 5°C). Overall, in the low pressure interval from 300 to 500 MPa, no increase in strength for the respective milk and carrageenan combination was measured, while the gel strength increased significantly upon increasing pressure up to 800 MPa. Moreover, as expected, increasing the concentration of carrageenan from 0.5 to one per cent increased the gel strength, especially in the high pressure interval (600 – 800 MPa). Unexpectedly, the two types of carrageenan, where LP-60 contains mainly κ-carrageenan and PMD contains mainly ι-carrageenan, gave same high gel strengths at one per cent after HP treatment at 600, 700 and 800 MPa. The dissociation of the casein micelles plays an essential role in the gelation process. Upon pressure treatment, not only do casein micelles dissociate into smaller units but also casein proteins and calcium ions will be released, and increasing pressure results in enhanced micelle dissociation following increased concentration of caseins and calcium ions. Thus, increased pressure will promote the interaction of calcium and ι-carrageenan and the interaction of free caseins and/or smaller micelle units and κ-carrageenan both stabilising the gel network, explaining the similar gel strength obtained with the two types of carrageenan. A mixture of ι- and κ-carrageenan (one or two per cent 700 MPa, 30 minutes, 5°C) showed synergistic effect on gel strength, indicating that further trapping of each carrageenan in the network of the other as well as some induced electrostatic interactions with calcium as the cross-linking agent gave even greater structural enlargement. A similar synergistic effect was observed with skim milk added one per cent xathan gum and one per cent locust bean gum subjected to pressuretreatment at 700 MPa for 30 minutes, whereas no gels were formed when milk solutions containing either of the individual hydrocolloids (in two per cent) were subjected to the same HP treatment. Apparently, the dissociation of micelles exposes hydrophilic and charged protein domains to the solvent and other constituents and thereby make them accessible for different intermolecular bonding with the two types of biomolecules.

With the aim of producing a tasty milk drink, two new neutral milk protein stabilisers were tested, the high protein content powder PSD O47 (CP Kelco), special for set yoghurt and the multi-functional food ingredient Simplesse® (CP Kelco), which is microparticulated whey protein powder imparting creamy texture. Skim milk with varying concentrations of one to four per cent of the individual or mixed powder was HP-treated at 400 or 600 MPa for 15 minutes and resulted in two potential soft gel products; 2.5 per cent PSD O47 + one per cent Simplesse® at 400 MPa and one per cent PSD O47 + three per cent Simplesse® at 600 MPa, both having viscosities similar to a commercial drinkable yoghurt but with a pH of 6.8. In order to improve flavour and colour of the products, natural strawberry juice was added. Firstly, the addition of 20 or 30 per cent strawberry juice did not change the viscosity of the selected gels immediately after pressurisation or upon storage up to five days in a refrigerator nor did the pH change though it was considerably lower; around 5.8 and 5.4 for the 20 and 30 per cent juice, respectively. Secondly, the colour was faintly pink and stable during storage. Finally, the sensorial evaluation revealed that the milk products all had a pleasant scent of strawberry and a good taste of strawberry.

In recent years, an increasing interest has been given to nutritional and healthy food, often the same as foods low in fat, sugar and additives. High pressure processing offers new opportunities for such milk products due to modification of biomolecule structure and functional properties as described above. In order to make a healthy milk dessert type, more ingredients were added to the skim milk, thus various combinations of PSD O47, alginate, honey, sugar, vanilla sugar, lemon juice, strawberry and banana were mixed with the skim milk prior to HP treatment. Overall, an improvement in taste and creaminess was obtained with the addition of vanilla and the addition of sugar resulted in better water binding capacity. However, the problem regarding the browning of the fruits was not overcome, and the fruits were taken out from the following trials. One combination of skim milk, PSD O47, alginate, sugar and vanilla sugar gave a firm, pudding-like gel that was shiny, yellowish, not sticky, and had a sweet vanilla taste, quite comparable to commercial ready-to-eat milk-based vanilla desserts. The nutritional value was calculated for both milk products (see Table 2) and it can be seen that the HP produced milk product has a con – siderably lower content of calories as a result of lower carbohydrate and fat contents.

These case studies show that there is a good foundation for producing soft or firm milk gels using HP. However, the pressure-induced gelation of milk, protein and hydrocolloids solutions is a complex phenomenon and the resulting gels are influenced by numerous parameters. The research into the mechanism, structure, texture and perception of milk based gels formed during HP treatment is of huge interest from a scientific, industrial and consumer perspective. Thus, further studies are needed to produce new high quality milk products and to elucidate the mechanism of gelation under pressure.

About the author

Vibeke Orlien

Vibeke Orlien has an MSc in mathematics and chemistry and a PhD degree in food chemistry. Since 2008, Vibeke has been employed as an associate professor in food chemistry at the Department of Food Science, Faculty of Life Sciences, University of Copenhagen. In 2010, Vibeke became head of the Food Chemistry research group at Department of Food Science. Her major research area is high pressure processing of food and the effects on food components and their properties.