What types of fruit & vegetable products are the best candidates for preservation by HPP?
Although high pressure processing achieved much of its early success with applications to halved avocadoes, whole fruits and vegetables are in general not good candidates for HPP. A number of studies have found that detrimental changes occur in cell integrity, and subsequently in the texture of the whole tissue, after high pressure application.
In our laboratory at the University of California – Davis, we conducted HPP studies on onions, persimmons, mangoes and other fruit and vegetable commodities, and found that in general cell integrity is lost at pressures greater than 200 MPa. Inactivation of vegetative microbial cells typically requires pressures of at least 400 MPa, and spore inactivation requires thermally-assisted HPP at even higher pressure levels. This means that, using current systems, the rupture of plant cells will almost always occur if one produces a microbiologically safe product. For this reason, HPP treatment of whole fruits and vegetables or even sliced products will impart an undesirable loss in crispness and crunchiness. Most of the current applications of HPP to fruit and vegetable products are therefore for juices or purees such as guacamole, where tissue integrity is not required.
It’s like comparing apples to oranges
Not all fruit and vegetable tissues may be preserved with equal success, however, and you might say it’s like comparing apples to oranges. Apples, in particular, are notoriously bad for HPP treatment, due to the relatively high amount of dissolved air they naturally contain in their tissues. It turns out that air greatly increases pressure effects and hastens the degradation of plant cell membranes and the loss of textural integrity, as well as causing stress on the package seals during the HPP process. In addition, higher levels of oxygen in plant tissues result in greater enzymatic browning, production of oxidized flavors and oxidation of nutrients such as Vitamin C. Fruits known to have a high content of trapped air include apples, pears and strawberries. In many cases, companies will need to learn about the air content of various fruits and vegetables of interest through experimenting with pilot HPP runs.
Effects of fruit/vegetable variety and maturity
Another factor affecting the quality of an HPP product is the maturity and variety (or cultivar) of the particular fruit or vegetable. In terms of maturity, as plant tissues age, the cell walls degrade and the effects of HPP on degradation of tissue integrity become more pronounced. This results in not only a loss of texture, but a loss in compartmentalization that leads to greater enzymatic browning, oxidation of flavors and degradation of nutrients. In a study on peaches carried out in our group at UC-Davis, we evaluated 9 different varieties of peaches and found dramatic differences in the tissue integrity and degree of browning following HPP treatment. In addition, we compared 3 different maturity stages processed at 0.1, 200 and 400 MPa in a few specific varieties, and determined that although the more mature fruit were slightly more susceptible to damage, the greatest effect was imparted by increased levels of pressure. This is good news for processors, because it may imply that, at the pressure levels required for pasteurization, the raw material maturity is not influential.
Domestic institutions currently carrying out HPP research
National Science Foundation – Center for Advanced Processing & Packaging Consortium between North Carolina State University, the Univ. of California-Davis and the Ohio State University
US Food and Drug Administration/Illinois Institute of Technology/Industry – Center for Process Innovation
US Dept. Of Agriculture – Food Safety and Intervention Technologies Research, Wyndmoor, PA
Also, numerous individual researchers at domestic and international universities.