Six Elephants on a Dime!

Don Mercer

Many of the methods used to process the foods we eat involve heating them to a specified temperature and holding them there for a given period of time before cooling.  This method of thermal processing is highly effective and dates back to the 1800s when Louis Pasteur provided a scientific understanding of the relationship between time and temperature in the destruction of microorganisms.

Some foods are too delicate to undergo heating without suffering a severe reduction in quality attributes such as taste and aroma.  One alternative to thermal processing was discovered in the late 1890s by Bert Hite at the West Virginia Agricultural Experiment Station in Morgantown.  His findings on “The Effect of Pressure in the Preservation of Milk” were published in a Research Bulletin in June 1899.  He showed how extremely high pressures could destroy many of the microorganisms in raw milk without the need for heating it.  The net effect was an improvement in the keeping properties of the milk. 

Sadly, Dr Hite’s technique seemed to have been all but forgotten for nearly ninety years before re-emerging within the scientific community.  Recently, there has been renewed interest in high pressure processing as different options are sought to replace thermal processing of heat-sensitive foods.

The high pressure process involves sealing food materials in a flexible package which is placed in a treatment chamber.  The chamber can then be filled with water and subjected to phenomenally high pressures delivered by sophisticated pumps.  Scientific literature on the subject reports pressures as ranging from 300 to 600 megaPascals (MPa).  A quick conversion to more user-friendly pressure units gives us a range of 43,500 to 87,000 pounds per square inch (psi).  Considering that car tires are inflated to about 32 to 36 psi, the significance of the pressures used in high pressure processing becomes immediately apparent.

For those of us who are visual types, we can look at this in a slightly different way.  Let’s take a dime (10 cents) which has a surface area of about 0.4 square inches (~260 mm² ).  Then, we can consider the weight of an average female African elephant as being about 5,000 pounds (or ~2270 kg, I have not weighed one personally).  Six elephants perched on a dime will give 30,000 pounds (~11338 kg or 11.3 tonnes) on that small surface area − or the equivalent of about 75,000 psi (or ~520 MPa)! 

Under such high pressures, the contents of each flexible package are squeezed to the point that the spacing between large molecules is compressed.  While this compression has little or no effect on the food itself, if can rearrange the structure of DNA molecules in any microorganisms that are present.  As a result, the microorganisms can no longer grow or reproduce or become irreversibly damaged, and the product becomes safe for human consumption.  In addition, the structure of certain enzymes that degrade food quality can be altered so that they are no longer able to participate in spoilage reactions.  This gives the same benefits as thermal processing without the need for the addition of heat.

High pressure processing has been successfully applied on a commercial scale to orange juice, tomato puree, guacamole, oysters, and pork; as well as jams and sauces, fruits and vegetables, dairy products, and seafood such as shrimp.  Production facilities are operational in many countries including the USA, Europe and Japan.  Future developments should extend its usage and make it even more commercially viable.

The pressure of six elephants standing on a dime is equivalent to 75,000 psi (~520 MPa).

 Dr Don Mercer is Associate Professor in Food Science, Department of Food Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada; e-mail: dmercer@uoguelph.ca

Permission to reproduce this article is greatly appreciated and acknowledged.