Milk and dairy products are an excellent source of proteins, vitamins and minerals. These products have been traditionally consumed fresh, which led to numerous outbreaks and commercial loses. Thermal pasteurization greatly improved shelf life and food safety, but organoleptic quality of the resulting products was affected. Since late XIX century, High Pressure Processing (HPP) positioned as a valid alternative to heat pasteurization to maintain taste, color and texture of dairy products at the same time that spoilage microorganisms and pathogens are inactivated.
It was back in 1899 when the idea of applying intense pressures to food crossed Bert Holmes Hite’s mind. Of course, milk was the chosen model. Quick spoilage and recurrent outbreaks linked to the presence of pathogens in fresh or heat pasteurized milk probably pushed this researcher from the University of West Virginia, USA to take the decision. Interestingly, he was already seeking for alternatives to preserve color and flavor of this delicate dairy product, aiming to avoid the side effects that incipient thermal sterilization caused on milk. Results from this very first study on the effects of High Pressure Processing (HPP) on food are not surprising in our time, but they were back in the old days: shelf life increased various days due to microbial inactivation.
Research on HPP steadily continued, but technical limitations prevented industrial implementation until the early 90’s, when HPP equipment became reliable and productive. Since then, multiple foodstuffs entered the market, including dairy products. Keep reading to get an overview on the most promising applications of HPP in the dairy industry.
Who in the dairy industry benefits from HPP?
Spoilage microorganisms and pathogen inactivation with minimal changes in sensorial properties opens a wide range of opportunities for dairy producers. The Mexican company Villa de Patos quickly took advantage of this, launching the first HPP milk in the world. They claimed it was fresh and posed an increased shelf life under refrigeration. Few years later, the New South Wales Food Authority approved the commercialization of HPP milk in the Australian state. The company Made by cow developed a process for the safe production of never heated and non-homogenized milk. The original yellowish color and creamy texture of the milk from the Jersey cow breed mimics the slight changes in the visual properties that HPP could cause. Pressure induces colloid disruption due to the dissociation of casein micelles in sub micelles (Huppertz et al. 2006). These sub micelles furtherly aggregate into big clusters, which gives light free way through the milk. Light is reflected in the lipid phase (where carotenoids are) and give to milk this final yellowish appearance (Chawla et al. 2011).
HPP is not only able to inactivate spoilage microorganisms to extend shelf life of milk, but also inactivates pathogens (> 5 log CFU/ml) to guarantee food safety (Figure 3). Nonetheless, neutral pH (between 6.5 and 6.7) and high nutritional value of milk makes recovery of injured bacteria likely to occur, so further research is needed to accurately establish safety of HPP milk and overcome regulatory issues related with the labeling and claims.
Yogurt industry can also benefit from HPP in various ways. For instance, processing milk previous fermentation improves viscosity of final set yogurts and significantly reduces syneresis. This can be attributed to whey protein denaturation and micelle disruption (Harte et al. 2003). Application of HPP to yogurt after fermentation have other advantages in terms of shelf life extension. Nonetheless, the use of pressure resistant starter cultures is required in order to ensure viability of strains after processing. Some regulations demand a final concentration of starter cultures of at least 108 CFU/g to consider the resulting product of fermentation as yogurt. Fonterra Co-operative Group developed a process for yogurt making that includes the use of selected pressure resistant starter cultures (US7854950B2) The use of these strains guarantees their viability after HPP and extends shelf life of yogurt due to spoilage microorganisms’ inactivation (Figure 4).
The destabilization and disruption of proteins mentioned above that HPP causes on milk components does not affect functionality of bioactive molecules. This gives the opportunity to develop functional products that would not be possible with the use of traditional preservation methods, such as thermal pasteurization. Colostrum is the first form of milk produced by mammals. Its high nutritional value resides in the elevated concentration of immunoglobulins, which are extremely heat sensitive. HPP maintains the functionality of these antibodies because structure is maintained. New Image Group launched together with Fonterra the first HPP colostrum, claiming an elevated content in immunoglobulins (Figure 5).
High pressure processing also finds a way into more traditional applications, such as fresh cheese or mozzarella. High water activity make these products highly perishable. Since HPP is applied on already packaged products, spoilage microorganisms resulting from manual handling and cross contamination are inactivated. For instance, Lactococcus sp. and yeasts were inactivated below the detection limit (102 CFU/g) in fresh curd cheeses processed at 600 MPa for 5 min (Daryaei et al. 2008). L. monocytogenes can be effectively controlled as well in fresh cheese, causing reductions below the detection limit (0.91 log CFU/g) when processing at 600 MPa (Tomasula et al. 2014). Semi-cured or soft cheese-snacks are popular and convenient products that already benefit from HPP (Figure 6).
HPP also extends shelf life of regional cheeses with very specific and delicate organoleptic attributes that confer them extra value, such as the traditional “Torta del Casar”: a Spanish cheese made from raw ewe’s milk. Spoilage microorganism can be effectively inactivated and proteolysis is delayed. Therefore, pH remains stable during shelf life and undesirable bitterness is prevented (Delgado-Martínez et al. 2019).
A very popular dairy alternative gaining relevance in recent years are fruit smoothies that include yogurt in their formulation. HPP preserves the taste and vibrant colors of fresh fruit at the same time that spoilage microorganisms and pathogens are inactivated. These products become an excellent option to include in our diet all the vitamins and antioxidants from the fruit and the nutritious protein and minerals from the yogurt (Figure 7).
HPP milk, limitations in the dairy industry
Despite the numerous and potential applications that high pressure processing offers to the dairy industry, it is estimated that only around 2% of the global HPP equipment is working on dairy applications. Multiple factors may account for the limited implementation.
Milk and dairy products follow worldwide very strict regulations, which sometimes confines innovation capacity of producers. According to FAO’s Codex Alimentarius, “raw milk” is defined as “milk which has not been heated beyond 40 ºC or undergone any treatment that has an equivalent effect”. In the European Union, HPP is considered as a processing step with an effect on microorganisms equivalent to heat pasteurization. Guidance for the implementation of Regulation (EC) No 852/2004 clearly indicates that “HPP with the purpose of decontamination should be considered as processing”. For this reason, labeling and claiming HPP milk as “fresh” or “raw” seems complicated in the European market. Nevertheless, HPP milk would keep its fresh attributes like HPP Juice does.
In the United States, commercialization of milk is only possible when an equivalent treatment to heat pasteurization is applied to the product, which would include a 5 log CFU/ml reduction of Coxiella brunetii. There are no scientific reports until today showing the lethal effect of HPP against C. brunetii on milk.
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