Essentials of Thermal Processing. Gary Tucker S.

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СКАЧАТЬ 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.

1.3 PACKAGING FOR HEAT PRESERVED FOODS

      Nicolas Appert's first products were packed in glass. Soon after his discovery was published, Peter Durand, a British merchant patented the idea of preserving food using tin cans. The patent (No 3372) was granted on 25 August 1810 by King George III of England. After receiving the patent, Durand did not make any canned food himself, but in 1812 sold his patent to two other Englishmen, Bryan Donkin, and John Hall, for £1000. Donkin was involved with tinning of iron sheets from 1808 and was keen to expand it to the food industry. Donkin and Hall set up a commercial canning factory and by 1813 were producing their first canned goods for the British army. In 1818, Durand introduced tin cans in the United States by re‐patenting his British patent in the United States of America (http://www.wikipedia.org).

      The first ‘canisters’ were made from iron plates that were dipped into molten tin to stop it from corroding. The ends were soldered closed with molten lead. The metal was thick and the cans were heavy and strong. The cans often weighed more than the food that was in them. They had to be cut open with a hammer and chisel.

Appert also started using cans. He made them himself in his cannery. They had the capacity of between 4 and 45 pounds and could be reused. He also added handles to some of them so that they could be used as cooking pots once opened. Although he preferred round cans, he made oval and rectangular ones too, at the request of his customers (Goldblith 1971a).

      Improvements to the can came when steel was invented, and this allowed for a much thinner metal which had the same strength to be used. In 1888, the hermetic double seam was invented by Max Ams. This paved the way for automated can lines to be made, whereas before about 6 cans per hour were handmade, the first automated can lines could make about 60 cans per hour. Can making lines today can run as fast as of 1500 cans per minute.

      Tin is an expensive metal. In the 1930s, hot dipping of tinplate was replaced by electroplating, where much less tin can be used to perform the same job. Improvements in steel making technology have resulted in even lighter weight cans. Single reduced tinplate of 0.19–0.21 mm thickness and double reduced (DR) tinplate as thin as 0.10–0.15 mm are now used to make cans all over the world.

      Although improvements in can design (e.g. by beading the body walls) can compensate to some extent for the loss of strength due to the thinner metal, many of the down‐gauging improvements are possible only due to improvements in can handling.

      1.3.1 Convenience – the can opener is invented

      Only when thinner steel cans came into use could the can opener be invented. Before then, canned food used to come with the written instructions: ‘Cut round the top near the outer edge with a chisel and hammer’. The first can openers were primitive claw‐shaped or ‘lever‐type’ design. Robert Yeates, a cutlery and surgical instrument maker patented the first can opener in Great Britain in 1855. It was a wooden handled leaver‐type cutting blade. Three years later, in the USA, Ezra Warner patented another design (Warner et al. 1858). His looked like a bent bayonet. The large curved blade was driven into a can's rim and then forcibly worked around its edge. This first type of can opener was deemed to be too dangerous for ordinary people to use it, and the store assistants opened each can before it was taken away (http://www.wikipedia.org).

Another can opener was invented by William Lyman of the United States in 1870. It had to be pierced into the centre of the end and had a cutting wheel that rolled around the rim of the can end. It was difficult to operate as it had to be adjusted to the size of the end. A breakthrough came in 1925 when a second, serrated wheel was added to hold the cutting wheel on the rim of the can. The basic principle of this opener was the same as is used on the modern can openers. The first electric can opener was introduced in December 1931.

      The easy opening end is the ultimate in convenience as far as can openers go. In the 1960s, a pull tab was patented by Ermal C. Fraze owner of the Dayton Reliable Tool Company in Ohio, USA, for aluminium ends for beverage cans. A lot of work then went into the development of an easy opening tinplate end that could withstand the requirements of being retortable and had a good shelf life. By the1980s, these ends were available commercially.

      1.3.2 Other forms of packing for ‘canned foods’

      Other forms of packaging for ‘canned foods’ have also become popular and practical; including glass, various plastics, and composites. Although glass was not new – being the packaging that Appert used to develop his method, it was difficult to seal adequately and cans soon became the packaging of choice. Developments in the closures for glass, starting with the Mason Jar in 1858, resulted in glass becoming a popular alternate to cans. Improvements in glass making technology have resulted in bottles that don't break easily under the high heat and pressure conditions experienced during thermal processing. Improvements in lids, caps, and closures that form hermetic seals are easy to open and reclose and have tamper evident features such as buttons that ‘pop’ on first opening have helped to make glass a viable alternate.

      Developments СКАЧАТЬ