during the venting period.ViscosityThe internal friction or resistance to flow of a liquid. The constant ratio of shear stress to shear rate. In liquids for which this ratio is a function of stress, the term “apparent viscosity” is defined as this ratio.Water Activity (aw)A measure of water availability in food for microbial growth. The ratio of water vapour pressure of a food to the vapour pressure of pure water under identical conditions of temperature and pressure.YeastsMicro‐organisms classified in the kingdom Fungi. They are spherical or more or less elongated cells. Most yeasts break down sugars to carbon dioxide and alcohol. That process is called fermentation.Z‐valueZ‐value is a term used in thermal death time calculations. It is the temperature required for one log reduction in the D‐value for a specific bacteria.
1 History of Thermal Processing
The application of high temperatures is one of the most commonly used methods to kill or control the numbers of micro‐organisms present within foods and on packaging surfaces. Evidence for its importance in today's food industry can be seen in the percentage of shelf space taken up in any retailer by foods preserved by heat. Along with low temperature storage (e.g. refrigeration and freezing) and dried foods, thermal processing remains of key importance to the worldwide food industry. There have been many attempts to introduce foods preserved by new technologies (e.g. ultra high pressure, pulsed electric fields) into retail food markets, but their significance is still relatively minor in comparison with thermal processing.
Thermal processing is a general term that describes all forms of heat treatments in which micro‐organism numbers are controlled by heat. This includes heat processed container types such as metal cans, plastic trays, pouches, glass jars, and even cartons. It also includes continuous thermal processes that take place outside of the package and are usually linked with aseptic or hot filling. Nowadays, the terms ‘canning’ and ‘thermal processing’ are often used interchangeably; however, as most of the original work was done in cans, thermally processed foods that are in other packaging formats are still often referred to as ‘canned’.
1.1 A BRIEF HISTORY OF THE SCIENCE AND TECHNOLOGY OF THERMAL PROCESSING
The early Greeks believed in ‘spontaneous generation’ (that living things could originate from non‐living matter). Although Aristotle discarded this notion, he still believed that animals could arise spontaneously from other unlike organisms or from soil. His influence regarding this concept of spontaneous generation was still felt as late as the seventeenth century.
The sixteenth and seventeenth centuries were a time of great scientific advancement in areas of chemistry, mathematics, and physics. This was known as the Scientific Revolution which laid the foundations for the Age of Enlightenment in the eighteenth century, a period where science became popular with the ordinary person, and an increasingly literate population was hungry for knowledge, information, and to learn. Evening Science lectures, with demonstrations, were very popular as a form of entertainment for the working class.
In addition at that time, the Industrial Revolution was taking place in Europe in the eighteenth and nineteenth centuries. Great strides were made in the areas of textiles, steam generation, and metallurgy. Steam generation was much more efficient; fossil fuels were used for the first time, instead of wood based fuel, resulting in a much more efficient source of energy. Advances in mining techniques and metal‐working, especially iron founding, resulted in many new uses for metals like iron, copper, and tin (http://www.wikipedia.org).
Towards the end of the seventeenth century, a chain of observations, experiments, and arguments began that disproved the belief that life could be generated from non‐life. Microbiology, as a science, can be said to have begun with the development of the microscope. In the mid‐seventeenth century, although not the inventor of the microscope, Antonie van Leeuwenhoek (1632–1723) (Fig. 1.1), a Dutch draper, ground his own lenses and made small microscopes that could magnify up to 500 times. He had exceptional attention to detail and was the first to provide proper descriptions of his observations, which included protozoans from the guts of animals and bacteria from teeth scrapings. His descriptions and drawings were excellent, and he conveyed his findings in a series of letters to the British Royal Society during the mid‐1670s. Although the observation of his ‘animalcules’ stimulated much interest, it remained as an oddity until the eighteenth century.
In France, the French Revolution (1789–1799) took place, largely as a result of growing dissatisfaction due to poverty and a shortage of food and increasing malnutrition. The Napoleonic Wars (1803–1815) further influenced the economy and developments in Europe.
It was in this time that Nicolas Appert was working as a confectioner. He was born on 17 November 1749 at Charlon‐sur‐Marne. His family were not wealthy, and young Nicolas acquired an education largely through personal effort. He was trained as a chef and worked and experimented with different kinds of food processes (fermenting, distilling, and preserving) for his own interest all of his life (http://www.appert‐aina.com).
Fig. 1.1 Antonie van Leeuwenhoek, painting by Jan Verkolje.
The French Army was busy with many military campaigns, and a major concern to the French Government was that they were losing more troops to diseases like scurvy and malnutrition, than to battle casualties. In 1795, they offered a prize of 12 000 Francs (a lot of money at that time) to anyone who could find a way to safely preserve food. This offer caught the attention of Nicolas Appert. He had noticed that the sugar syrups that he used for his confectionary, kept almost indefinitely when heated, and stored in stoppered glass bottles. He began experimenting with preserving other types of food, also by heating them and storing them in stoppered jars. He worked scientifically and had a keen attention to detail. He started with using champagne bottles, but soon had them modified with a wider mouth, so that he could fill them more easily.
By 1804, he felt confident enough to test some product on the French Navy. The test was a huge success. In 1806, more tests were conducted. His invention was assessed, and early in 1810, he was informed that he could claim the award of 12 000 Francs, but had to publish the exact details of his discovery, which he did (Goldblith 1971b).
Appert is known as the Father of Canning. Heat sterilization is also known as Appertization. Although his first products were in glass bottles, he soon started using metal cans at the cannery he built with the prize money he won for his invention. He had to deliver 200 copies of his processing methods (printed at his own expense) to the French Government before they would give him his award. The book was very detailed and described the canning process much as it is still today (Appert 1810). Appert described the process as follows (Featherstone 2012):
Enclose the foods to be preserved in bottles.
Cork the bottles carefully.
Boiling the bottles in water for various lengths of time (depending on the foods).
Remove the bottles and cool them.
In 1811, he has a second edition printed in French as well as English and Swedish, and in 1823 a third, and in 1831 a fourth. The later editions were translated into many other languages (Bitting 1937).
Although Appert's methods worked, no one at the time understood why. Appert thought it was due to the heating and exclusion of air. At that time, it was widely believed that
