HPP combined with other food processing techniques

High Pressure Processing (HPP) is a non-thermal preservation technology that extends shelf life and ensures food safety, at the same time that sensory and nutritional attributes of fresh foods are retained. HPP is efficient inactivating microorganisms such as Listeria monocytogenes, Escherichia coli or Salmonella spp. Industrially applied and potential food processing techniques that can be used in combination with HPP to launch new products into the market are here discussed.

Despite tremendous efforts made by the food industry to ensure the innocuousness of their products at the end of the production chain, infections linked to the consumption of contaminated food are somewhat frequent. The listeriosis outbreak that took place in South Africa between 2017 and 2018 is probably the most dramatic recent event related to the consumption of a popular ready-to-eat (RTE) meat product; but more cases are continuously reported worldwide, such as the Salmonella and E. coli infections linked to ground beef and bison meats in the US (2019) or the L. monocytogenes infection associated to roasted pork products in Spain (2019).

Many food processors rely on good manufacturing practices or a single processing technique as a control point to prevent cross-contamination with pathogens. Nonetheless, poor cleaning and disinfection procedures or inadequately optimized processing operations might increase the potential risk of microbiological hazards to human health.

High Pressure Processing (HPP) is a non-thermal preservation technology able to inactivate spoilage microorganisms and pathogens in already packed solid and liquid products or in bulk when applied solely to liquids. Nutritional and sensory attributes of processed food remain intact and are similar to the fresh products, which represents a big advantage compared to traditional heat pasteurization methods that drastically change the quality and nutritional attributes of the food. HPP can be used alone or in combination with other food processing techniques to maximize the benefits of the technology. Here we will discuss some of these currently used and potential combination strategies.

Industrially applied strategies combined with HPP

It certainly sounds astonishing that maximum pressure at which food is subjected in industrial HPP units equals six times the pressure found at the bottom of the Marianas Trench, but even more shocking is the fact that some microbial species manage to survive at these conditions. Bacterial spores are among these resistant organisms, and Clostridium botulinum is probably the most pressure resistant spore former of public health significance.

C.botulinum spores are not dangerous as long as they fail to germinate and grow in food products. Physicochemical parameters of foods determine whether spores of the pathogen can grow and release the potent botulinum neurotoxin. It is generally recognized that products with pH below 4.60 or water activity (aw) below 0.94 do not support spore outgrowth so remain safe for consumption.

Lowering the pH of low-acid products is the most straightforward way to eradicate the risk associated with pathogenic spore formers. Using natural ingredients such as citric juices or organic acids gives the chance to develop clean label formulas with a balanced or even improved sensory profile. Vegetable smoothies, root juices or coconut water are typically blended with lime or lemon juices to adjust their final pH and create flavors that meet customers’ expectations.

Figure 1. HPP formulas including citric fruit juices to develop interesting flavors and to adjust pH

Figure 1. HPP formulas including citric fruit juices to develop interesting flavors and to adjust pH

In the particular case of coconut water, ongoing research conducted by Hiperbaric is trying to validate that the product does not support growth and toxin formation of C. botulinum, which would discard the pathogen as the pertinent microorganism in the low-acid beverage.

Another possible technique in combination with HPP consists in the use of natural additives that enjoy a clean label image. USDA recently approved celery powder, sea salt or beet juice as a natural source of nitrites if used in combination with a source of ascorbate, such as cherry powder. This strategy is currently applied by meat producers to develop safe and more natural products: HPP eliminates vegetative microorganisms (e.g. L. monocytogenes, lactic acid bacteria, molds, yeasts…) and producers can get rid of sorbates, ascorbates or other chemical additives. Nitrites from the natural ingredients serve as control of spore formers surviving the HPP process (Figure 2).

Figure 2. Uncured Hormel Foods’ and Applegate’s meat products including natural nitrite sources as spore control

Figure 2. Uncured Hormel Foods’ and Applegate’s meat products including natural nitrite sources as spore control

Conversion of nitrates to functional nitrites is catalyzed by microorganisms, so lactic acid bacteria starter cultures may be added for this purpose. Organic sugars are typically used as ingredients to boost microbial growth following HPP and achieve efficient conversion rates (Figure 2).

Potential combination of food processing techniques

It is obvious that spore-former bacteria represent a major limitation for companies willing to launch new HPP formulas because of their pressure resistance and potential development in the final product following the process. Alternatives previously discussed to control their growth might not apply to all products due to flavor incompatibilities or even regulatory constraints. Some other potential alternatives are under study and could soon become a suitable solution.

A combination of high pressures with high temperatures in a process called high pressure thermal sterilization (HPTS) has the potential to produce ambient-stable packed foods subjected to a lower thermal impact compared to traditional sterilization processes. In this case, high pressure allows for rapid and homogeneous heating and cooling due to adiabatic effects during pressure build-up and pressure release (Figure 3).

Figure 3. Temperature profiles in traditional retort (red) and HPTS (blue) processes. Steps during pressurization are indicated (adapted from Barbosa-Cánovas et al., 2014)

Figure 3. Temperature profiles in traditional retort (red) and HPTS (blue) processes. Steps during pressurization are indicated (adapted from Barbosa-Cánovas et al., 2014)

Lower thermal impact results in improved color, texture, taste and nutrient content compared to heat sterilization. From a regulatory point of view, this innovative technology received the approval from the FDA back in 2009 when a petition for the commercialization of a mashed potato product was accepted.

Nonetheless, technical limitations regarding the reliability of industrial machines, difficulties to monitor temperature and non-homogeneous temperature distribution during the process make industrial implementation complex. In addition, elevated food processing costs would be reflected in the final product price and consumers’ may not find quality improvement sufficient to justify higher prices.

A strategy to address some of these limitations consists in the use of isolated processing canisters. Preheated pressurizing fluid (water) and products are loaded into the canister and conventional HPP units can be used to pressurize preheated batches. High reliability and low processing cost of existing HPP machines would significantly reduce prices and make monitoring easier and more accurate (Figure 4). Ongoing research is trying to find adequate isolating materials to develop reliable canisters as documented in a patent application from CSIRO in Australia.

Figure 4. HPTS process using isolated canisters and conventional HPP unit

Figure 4. HPTS process using isolated canisters and conventional HPP unit

Another strategy with the potential to be used in combination with HPP is the so-called biopreservation. Bacteriocin-producing lactic acid bacteria (LAB) to control undesirable microorganisms in foods has been extensively evaluated. Nonetheless, the main boundaries of this approach rely on LAB growth variability, non-homogeneous composition of food and difficulty to control storage temperature.

The BLAC HP project, in which Hiperbaric collaborated together with other industrial partners, aimed to develop a new strategy for stabilization of refrigerated meat products combining HPP and biopreservation using LAB. Thanks to the pressure tolerance of some lactic acid bacteria species, the combination is expected to control pathogenic spore formers without the addition of chemical preservatives such as nitrites.

Contact us if you want to know more about food processing techniques and how HPP can improve food safety and shelf life.

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