By Don Mercer

Associate Professor, Department of Food Science University of Guelph

This may not seem directly related to food, but in so many ways using the metric system in food processing work just seems to make good sense.

A few weeks ago, we were working on a laboratory exercise dealing with regulatory aspects of food packaging. One of the things the students had to do was measure the dimensions of various cans in order to look up the required net weights and drained weights of each can’s contents. Normally, this would be “duck soup” for the students, to use a food analogy. However, can sizes are expressed in a rather archaic manner using inches and sixteenths of an inch. A can containing 398 mL of product that is three inches in diameter by four and seven-sixteenths inches high, would be abbreviated as being a 300 x 407 can. The first digit represents the number of inches, and the last two digits are the number of sixteenths of an inch. It was rather interesting watching the students scratch their heads over these weird dimensions. It also drove home just how convenient it is to use metric units.

The metric system itself is over two hundred years old and was born out of the French Revolution which began in 1789. In order to have a uniform standard for length, it was decided that a “committee” would investigate a suitable basis. They came up with the idea that one ten-millionth of the distance between the Equator and the North Pole on a line running through Paris would be just about perfect for this purpose. Once the distance had been estimated as accurately as possible with the methods available at the time (this was in the 1790’s don’t forget), they made a metal bar of this exact length and kept it in the Archives as their reference standard. Copies of the bar were then distributed throughout France so that each area would have access to the standardized unit of length. The “metre”, as it was called was then divided into ten to get decimetres; then into ten again to get centimetres; and then into ten again to get millimetres. Prior to this, there were no standards and most countries worked on their own systems of length. Just as an example, a British “foot” was different than a French “foot”.

One of the beauties of the metric system is how nicely things are inter-related. To obtain a basic unit of volume, a cubic metre was considered as being somewhat unwieldy. However, a cubic decimetre filled the bill admirably well. The “litre”, as it was dubbed, was defined as a cube which measured 10 centimetres on each of its sides, giving a volume of one-thousand cubic centimetres.

To obtain a basic unit of weight, it seemed appropriate to base things on the weight of a given volume of water. It was ultimately decided that a “gram” would be defined as the weight of one cubic centimetre of pure water. This meant that a litre of water (i.e., one thousand cubic centimetres) would weigh one kilogram. It was then possible to divide the gram into a series of smaller weights by using steps of ten, just as was done with the metre, as well as the litre. Conveniently, a cubic centimetre is the same as a millilitre, and the weights of these volumes of water are both one gram.