Читать книгу Essentials of Thermal Processing - Gary Tucker S., Gary S. Tucker - Страница 13

In 1859, Louis Pasteur demonstrated the role of micro‐organisms in food spoilage. In his experiment, he heated broth in long swan necked jars to sterilize them. The jars either had filters on them, or very long necks that allowed only air, but not dust and other particles through. Nothing grew in the broths unless the flasks were broken open. He therefore correctly concluded that the living organisms that grew came from outside, as spores on dust, rather than spontaneously generated within the broth or from the air. He showed that the growth of micro‐organisms was responsible for spoiling products like beer, wine, and milk.

He invented a process in which milk was heated to kill most bacteria and moulds already present. This process was soon afterwards known as pasteurization (http://www.wikipedia.org). A little known fact about Pasteur is that he, because of his status as a respected scientist, served on the board of Public Hygiene and Sanitation, in France. One of his responsibilities was food laws. As a result of his discoveries regarding the cause of food spoilage, various methods of preserving foods were researched by many others. Some of these involved adding chemicals to the food. In 1870, Pasteur advocated that the public had ‘the right to know’ what was being put into the food and stated that all ingredients and additives must be declared on the label (Goldblith 1971a) (Fig. 1.3).

Microbiology was in its infancy in the nineteenth century. Many people contributed to developing it into the science it is today. A few of these scientists who made significant contributions to the understanding of the science of canning are mentioned below (Goldblith 1972).

Joseph Lister (1827–1912) was an English surgeon who made the important development of antiseptic surgical practice for medicine and microbiology (Fig. 1.4). Lister searched for a way to keep bacteria out of wounds and incisions made by surgeons, as death from sepsis was frequent at the time. He read a paper by Louis Pasteur that described the growth of anaerobic bacteria and suggested killing them by heat, filtering, or with chemicals. Carbolic acid (phenol) was known to deodorize sewage, so Lister soaked surgical dressings, and sprayed carbolic acid into the air of operating theatres. The protected wounds did not become infected, and the patients recovered quickly. The success of this technique was so remarkable that aseptic surgical practice was soon established worldwide.

Fig. 1.3 Louis Pasteur, painting by A. Edelfeldt (1885).

Fig. 1.4 Joseph Lister, painting by unknown.

Robert Koch (1843–1910) isolated the Bacillus bacteria responsible for tuberculosis and the one for anthrax. He also isolated the bacteria responsible for cholera, Vibrio cholera. In his work with anthrax, he noticed that when the bacteria were exposed to unfavourable conditions, they went into a dormant state, forming internal spores that could survive for extended periods in the soil, causing outbreaks of the disease at opportune times. More of Koch's contributions to microbiology were in the areas of culturing bacteria and examining them. He developed solid culture media by adding gelatine and other solidifying agents to liquid media in order to obtain isolated growths of micro‐organisms. These isolated growths were called ‘colonies’ and were found to contain millions of individual micro‐organisms packed tightly together. He noted that the colonies were visible to the naked eye, whereas the individual cells were not. Koch also found that, by adding dyes to micro‐organisms smeared onto a glass slide, individual cells could be seen more clearly with a microscope.

All canners had losses due to sporadic swelling and spoilage, but the actual causes for it were unknown, and the canners did not know what they should do to overcome this problem.

In about 1860, Isaac Solomon, a canner in Baltimore in the USA, added calcium chloride to the cooking water enabling it to ‘boil’ at 116 °C instead of 100 °C and thus drastically reducing the cooking times that had been in use.

In 1895, Dr Harry L. Russell published a paper describing swelling spoilage in canned peas. He did experiments where he processed peas at higher temperatures and longer times and showed that the percent spoilage was significantly reduced.

Two important advances in food science and canning were the invention of the steam pressure retort by A.K. Shriver in 1875 and publication of the time and temperature requirements of canned foods by Underwood and Prescott. William Lyman Underwood was a canner who wanted to understand what caused canned food spoilage (Fig. 1.5). Together with Samuel Cate Prescott, they studied many cases of food spoilage over many years. They demonstrated that spoilage could be caused by inoculation and that the proper time and temperature combinations could prevent spoilage. They showed that bacteria were the causative agents of canned food spoilage, the importance of heat penetration in processing canned foods, the importance of proper cooling of canned foods, and they were also the first to recommend incubation testing of canned foods. The significance of their work cannot be over‐emphasized; canning passed from an individual skill to a scientific discipline.

Fig. 1.5 Underwood and Prescott.

Courtesy of the MIT Museum.

There are many excellent scientists who have contributed invaluable insight into the field of thermal processing, but most of the foundation work was done by those mentioned above.

The early bacteriological studies on spore death kinetics were done by different researchers at various temperatures. This work was done between 1921 and 1948. As could be expected, the lower the temperature, the slower the rate of kill. Stumbo took this information and calculated the z‐value of 18 °F (10 °C) and an F‐value of 2.78 minutes for Clostridium botulinum spores on a thermal death time curve that passed through 250 °F (121.1 °C) at 2.78 minutes (Tucker 2008). This temperature was appropriate for practical cooking times as well as being safely achievable in the processing vessels of the day. This was the basis of the Fo3, at 250 °F concept which is still applicable today.

The General Method for calculating scheduled process times was originally described by Bigelow et al in 1920, but contributions by Ball in 1928 and O.T Schulz and F. C. W. Olson in 1940 resulted in a much improved General Method. M. Patashnik published his improvements which are the most widely used today in 1953 (Patashnik 1953; Park 1996).

The invention of a standardized reliable thermocouple probe for measurement of real time heat penetration temperatures was an important contribution made by Ecklund (1949).

J.R. Manson, A.A. Teixeira, and K. Purohit were three of Stumbo's graduate students who also contributed significantly to the field of thermal processing. They were engineers, and the first to apply engineering mathematics to simulate the coupling of heat transfer with thermal inactivation kinetics in thermal processing of canned foods. Teixeira used this approach to find optimum retort the time and temperature combinations that would maximize quality retention while delivering specified target lethality. Manson carried Teixeira's work further by improving the mathematical model to simulate convection as well as conduction heat transfer. Working together, Teixeira and Manson demonstrated how such models could be employed in real time online control of batch retorts by automatically extending process time to precisely compensate for unexpected process deviations (Teixeira et al. 1969). The research mentioned above took place largely in the USA.

In the United Kingdom, T.G. Gillespy and his team did valuable work on processing times and temperatures of a large range of foods at Campden Experimental Factory (now Campden BRI) (Tucker 2008). In France, H. Cheftel at Carnaud Research did extensive work on canned foods and published Principles and methods for establishing thermal processes for canned foods in French. In South Africa, G.G. Knock did much to increase the understanding and hence reduce the incidence of thermophilic ‘flat sour’ spoilage in canned peas (Knock 1954). These researchers and others collaborated from all over the world to improve the understanding of the mechanism of canned food spoilage and the requirements necessary to ensure that safe canned food is produced. Improvements in product safety, quality, and energy usage are still continuously being made. G.S. Tucker at Campden BRI further developed numerical techniques for thermal process calculations and a computer program to recalculate process deviations in real time.