A cold-pressed juice is made by crushing and pressing vegetables without the heat that a centrifugal extractor or a pasteuriser would add. The result tastes fresh and spoils quickly. For most of the category's short history the answer to that problem has been high-pressure processing, in which the bottled juice is held under several hundred megapascals of pressure in a water-filled vessel for a few minutes. The treatment inactivates vegetative microorganisms while the juice stays cold, and it has become the most widely used non-thermal preservation technology in the food industry (Huang et al., 2017).

Two claims travel with these products. The first concerns texture: that the juice keeps the body, the cloud and the pulpy mouthfeel of something just pressed. The second concerns nutrition: that vitamins which heat would have damaged are still present when the bottle is opened. Both claims rest on a sound physical argument, and both are more conditional than a label can convey. What happens to a juice in the pressure vessel is only the beginning of what happens to it before it is drunk.

This review brings together the published evidence on those two claims for vegetable juices. It considers how pressure acts on a juice, what is known about the particles and pectins that give a cloudy juice its texture, how individual vitamins respond, and how both change in chilled storage. It also marks the places where conclusions for vegetable juices have been borrowed from work on fruit.

The article is a narrative review of the published literature. It draws on the standard reviews of high-pressure processing and of pressure effects on plant foods, and on primary studies of tomato, carrot and mixed vegetable products that are frequently cited in those reviews. Work on orange juice is included where it supplies the only careful storage data, and is identified as such. No formal search protocol was applied and no pooled estimates are offered. The term high-pressure processing is used here for the static, in-package treatment. High-pressure homogenisation, in which juice is forced through a narrow valve, is a different operation with different effects on particles and is outside the scope.

Two principles govern the process. Pressure is transmitted uniformly and almost instantly through the product, whatever the size or shape of the package, so there is no slow-heating centre of the kind that limits thermal processing. And, following Le Chatelier's principle, pressure favours changes that reduce volume. The formation of hydrogen bonds and the disruption of ionic and hydrophobic interactions fall into that class, whereas the breaking of covalent bonds generally does not (Cheftel, 1995; Rastogi et al., 2007). Large molecules whose function depends on weak interactions, such as enzymes and membrane proteins, are therefore vulnerable. Small molecules such as vitamins, pigments and aroma compounds are largely spared (Balasubramaniam et al., 2015).

The treatment is not entirely free of heat. Compression warms water by roughly 3 °C for every 100 MPa, and oils warm more, so a juice loaded cold reaches a noticeably higher temperature at full pressure before cooling again on release (Balasubramaniam et al., 2015). The effect is modest at chilled loading temperatures, but it means that pressure and temperature can never be treated as fully separate variables when studies are compared.

Microbial targets set the commercial conditions. Vegetative bacteria, yeasts and moulds are inactivated at the pressures used commercially, while bacterial spores survive them unless heat is applied at the same time (Considine et al., 2008; Black et al., 2007). In the United States the juice regulation requires a process that achieves a 5-log reduction of the pertinent pathogen (FDA, 2001). For acidic juices that requirement can be met by pressure alone. Many vegetable juices are not acidic: carrot, beet and leafy green juices lie above pH 4.6, the conventional boundary below which spores of Clostridium botulinum do not grow out. Such products depend on strict refrigeration, acidification or blending with acidic fruit, and those formulation choices in turn affect both texture and vitamin stability. Figure 1 summarises the factors that the literature identifies as determining the quality of the finished juice.

Texture in a juice is a matter of dispersed particles and dissolved polymers. A cold-pressed vegetable juice is a suspension of cell fragments, cell clusters and finer colloidal material in a serum that contains soluble pectin, sugars, organic acids and salts. Its apparent viscosity, its resistance to sedimentation and its perceived body depend on the volume fraction, size and shape of the particles and on the viscosity of the serum.

The expectation that vitamins survive pressure treatment is broadly supported, with qualifications that differ from vitamin to vitamin. Table 1 summarises the pattern described in the sections below.

A pressure-treated juice is sold with a refrigerated shelf life of several weeks, far longer than the few days of an untreated cold-pressed juice. Quality at the moment of consumption is therefore decided largely in the bottle. Polydera et al. (2003) followed ascorbic acid in pressure-treated and thermally pasteurised orange juice across a range of storage temperatures and modelled the loss kinetically; the pressure-treated juice lost the vitamin more slowly, and temperature abuse shortened the nutritional shelf life of both. The broader review literature draws the same lesson: comparisons made on the day of processing flatter the technology, and those made at the end of shelf life are the ones that matter (Barba et al., 2012; Koutchma et al., 2016).

Texture drifts over the same period. Where pectin methylesterase remains active, the degree of methylation of serum pectin falls steadily in the cold, and the visible result may be a clear layer, a weak gel or a compact sediment. Residual polyphenol oxidase and peroxidase darken the juice and consume the reducing compounds that protect colour. Packaging plays its part, since oxygen that permeates a plastic bottle wall sustains these reactions long after the dissolved oxygen present at filling has been used up. Processors respond with blends: adding lemon, apple or ginger lowers pH, which improves safety margins and slows some enzymes, and it also changes the product being studied. Much of the commercial evidence concerns such blends and not single-vegetable juices.

Four gaps stand out. The first is coverage. The kinetic and storage literature is dominated by orange, tomato and carrot, with a scattering of berry and apple work, while the juices that define the cold-pressed category, built on kale, spinach, celery, cucumber and beetroot, have been studied far less (Koutchma et al., 2016). The second is the measurement of texture. Studies commonly report a single viscosity value or a turbidity reading. Particle size distributions, flow curves and trained-panel descriptions of mouthfeel are seldom reported together, so the link between measured and perceived texture is weak.

The third gap is the comparison chosen. Pressure-treated juice is usually set against a heavily pasteurised control, which shows the technology at its best. Comparisons with untreated juice at the same age, and with mild modern thermal processes, would be more informative (Knorr et al., 2011). The fourth is reporting. Come-up time, initial temperature, the temperature reached under pressure, dissolved oxygen at filling and the oxygen permeability of the package are often missing, and without them studies cannot be compared or repeated (San Martín et al., 2002). Future work that reports these variables, follows enzymes and vitamins together through the full shelf life and includes low-acid green juices would fill the largest holes.

The published evidence supports a qualified version of both claims. High-pressure processing at low temperature does preserve vitamin C, carotenoids and several B vitamins through the treatment itself, and it does leave the particle structure of a cloudy juice largely as the press made it. It does not stop the enzymatic and oxidative reactions that follow, because the enzymes responsible are among the most pressure-resistant components of the juice. Texture and vitamin content at the end of shelf life depend on residual enzyme activity, oxygen, pH, storage temperature and time at least as much as on the pressure applied. For vegetable juices in particular, the evidence base is narrower than the confidence of product claims would suggest.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.