The question of 'why can't we just leave jars in the canner after processing' has come up a few times. I just saw a good cite in Canning2 and want to share it here under it's own topic so we can find it easily when it comes up again.

dede

http://www.emlab.com/s/samplin…..our_spores

Flat Sour Spores

Two species of Bacillus comprise the flat sour spore group. These are Bacillus
coagulans (Bacillus thermoacidurans) and Bacillus stearothermophilus. These
bacteria are thermophilic aerobic spore formers and are found in acidic food
products such as canned tomato and dairy products. If the conditions are
favorable these organisms may germinate and lead to food spoilage. Flat sour
organisms are a big concern for individuals involved in processing canned foods.
Growth of flat sour organism may cause the pH to drop to between 0.3 and 0.5.
The organisms may grow in cans, yet the ends of the cans will remain flat,
compared to other organisms such as Clostridum thermosaccharolyticum that will
swell the ends of the cans when spoiled. The spoilage of the food is due to a
drop in pH from the growth of the organisms.

There are 3 categories of thermophilic sporeformers that cause food spoilage:

Those that produce flat-sour spoilage such as Bacillus stearothermophilus and
Bacillus coagulans.

Those that produce gas but not hydrogen sulfide such as the thermophilic
anaerobe Clostridium thermosaccharolyticum.

Those that produce hydrogen sulfide spoilage such as Desulfotomaculum
nigrificans.

Bacillus stearothermophilus is relatively common in the environment and can be
found in soil, hot springs, desert sand, arctic waters, ocean sediments, food,
and compost. It grows very well at 65°C and has a limited tolerance to acid. It
is a Gram-positive rod shaped bacteria.

Bacillus coagulans is found rarely in soil but may be found in many food
products. They have been found to multiply very rapidly in acidic foods such as
canned tomato juice, silage, medicated creams, and antacids. B. coagulans is
also a Gram positive rod.

The history of controlling flat sour spores goes as far back as the 1930's when
the National Canners Association developed a standard media to isolate flat
spores from spoiled canned foods. Being able to isolate and identify the
organisms changed the industry and increased the understanding of flat sour
spores.

Flat sour spores are not directly pathogenic to people. These organisms only
cause spoilage in canned foods if the product is not cooled properly. If not
cooled properly, the organism has the potential to grow and decrease the shelf
life of the canned foods. A food spoiled by flat sour organisms will produce a
sour smell and taste. Flat sour spore testing is of most interest for food
processors that want to ensure a long shelf life of a product and to prevent
early spoilage in sensitive tomatoes and dairy food products.

That, or one of them tasted flat sour. Once is all it takes