Response to article on Food for Astronauts

When I read the article entitled “Vitamin Stabilization for a Long-Term Spaceflight,” I was disappointed with the clear lack of understanding of some of the processing technologies introduced in the article, specifically related to high pressure processing (HPP) and pressure assisted thermal sterilization (PATS).

The article states, “HPP has the advantage, compared to retorting, of allowing a 3°–4° temperature reduction per 100 MPa pressure increase and also provides for relatively uniform heating because pressure remains constant throughout the sample.” Not only do the temperature values not contain a unit (rendering them nearly useless), the statement is actually the opposite of the well-documented phenomena referred to as “compression heating.” In the early 1900s, Bridgman measured the properties of water under pressure, and he observed that there was about a 3°–4°C temperature increase, not a decrease. The compression heating values measured by Bridgman are standards at the National Institute of Standards and Technology. Numerous published works have also documented compression heating.

The previous statement cites a compression heating value that is only specific to water, and is not representative of other food or packaging materials. Published work out of the HPP lab at Ohio State (including one study I led) shows compression heating values that are actually quite a bit higher, up to 9°C per 100 MPa for fatty material. In addition, compression heating values can differ, depending on the initial temperature of the material being compressed, which is different whether you are considering HPP (chilled or ambient temperature) or PATS (80°–90°C). While I do understand that the main point of the article was not to give a dissertation on HPP and PATS, the lack of specificity is misleading. These compression heating differences are quite important in determining process lethality and uniformity during HPP, and particularly PATS.

Another concern I had was in the first part of the article, in “Vitamin Loss Concerns,” where PATS is referred to. This part of the article makes no reference to HPP at all, which is much more widely used than PATS. To my knowledge, based on attending seminars at the International Nonthermal Processing Conference in October 2014, PATS has not been commercially used yet due to system constraints and packaging issues. The delineation between HPP and PATS was blurred throughout the article. Even more concerning is that the article lists the sensitivity of microorganisms to pressure as follows: “yeasts/molds > gram negative bacteria > gram positive bacteria.” This statement is generally true; however, it neglects to mention the resistance of bacterial spores to pressure, which are the target for sterilization!

Overall, the advantages of using HPP and PATS for vitamin retention were poorly represented and not grounded in fact. I hope the review process is more thorough from this point forward to avoid mistakes like these.

Kaitlyn Casulli, MS Candidate, Michigan State University

The authors of the article offered the following reply to the preceding Letter to the Editor.

Our article was by no means a treatise on high pressure processing/sterilization but a description of our NASA research on vitamin stabilization—of which PATS is only one of several formulation, encapsulation, and processing strategies we are investigating—and the associated workshop held at NSBRI headquarters. Ms. Casulli’s letter refers exclusively to a two-sentence summary of a presentation on both HPP and PATS technologies provided by one of the workshop speakers, and it does not acknowledge the overall thrust of the article. Our reply to her specific comments follows.

1) While Ms. Casulli is, of course, correct about the occurrence of compression heating during HPP/PATS, the temperature comparison in the manuscript is clearly stated as relative to retorting. PATS is being investigated (in addition to MATS and irradiation) because of its ability to sterilize products using comparatively reduced integrated time-temperature exposure—which may reduce damage to vitamins.

2) Of course, the exact degree of compression heating due to HPP/PATS varies according to the food matrix, which is why there are specific petitions for different foods. The processing of our product involved parameters particularly specified for that formulation, followed by established procedures for incubation testing and microbiological validation. A detailed discussion of PATS/HPP regulatory issues would have been far beyond the scope of our manuscript.

3) And, of course, while PATS processing is not widely commercially used, HPP is a pasteurization process and thus entirely inappropriate to ensure five-year food stability.

4) The lethality sequence in question refers to HPP (stated in the article), which is obviously why spores were not included. Inactivation of spores requires that HPP be used in combination with other treatment.

5) Regarding the comment about the advantages to using HPP/PATS for vitamin retention not being “grounded in fact”: Well, we have a multiyear project funded by NASA to find out.

—Ann Barrett, PhD, Senior Food Engineer, Food Engineering & Analysis Team, Combat Feeding Directorate (CFD), U.S. Army Natick Soldier Research Development and Engineering Center (NSRDEC); Danielle Froio, Senior Materials Engineer, Advanced Materials Engineering Team, CFD, NSRDEC; and Michelle Richardson, Senior Food Technologist, Food Processing, Engineering and Technology Team, CFD, NSRDEC.