The most unfortunate thing for the chemists was the fact that Pasteur who was a chemist by training first established that “fermentation processes are result of microbial activity”. Actually a local brewery industry was facing a problem. Instead of alcohol, fermentation of sugar was producing lactic acid in their vats. They asked Pasteur to solve the problem. Pasteur found microscopically that instead of yeast the vat material contained some rod and spherical shaped small objects. He transferred the objects (vat infusion) into new sugar a solution, which results in further development of the same organisms in the solution & also lactic acids instead of alcohol, was produced. Further transformations in fresh sugar solution gave similar results. Pasteur thus named those small organisms “New Yeasts” that were actually bacteria and not yeast. But in Pasteur’s time microorganisms were not classified.

Chemistry had made great advance in early decades of nineteenth century. Urea was synthesized from inorganic source in 1828 which lead to the belief that all organic compounds can be synthesized chemically (earlier it was thought that organic compounds can only be synthesized by living organisms). This was the reason why chemists (J.J. Berzelius, J. Lubig & F. Wohler etc.) thought that all the biological processes are chemical process and has no relation with organisms. The actual truth was something different though. All the biological processes are indeed chemical process, but are controlled by living organisms only through actions of their unique enzymes. So when in 1837 C. Caniard-Latour, Th. Schann & F. Kützing independently proposed that fermentation of sugars to ethyl alcohol and CO₂ was carried out by yeasts, no chemists believed it & opposed the idea. They thought a simple conversion of sugar to alcohol must be a chemical process and has no link with presence of any organisms in the infusion.

Fermentation and Putrefaction

Now there may be a long controversy over spontaneous generation theory but one thing was certain that the presence of microorganisms in organic infusions bring about some chemical changes in the organic infusions itself. These changes primarily are of two kinds: -
i) Fermentation & ii) Putrefaction.

i) Fermentation: - When the organic infusions are of plant origin (i.e. mainly composed of carbohydrates) then their breakdown results in the formation of alcohols & organic acids and are known as fermentation process (in a crude way).
ii) Putrefaction: - When the organic infusions are of animal origin (i.e. mainly composed of proteins) then their breakdown produces ill-smelling compounds and are known as putrefaction process.

Pasteur’s and Tyndall’s experiment proved that microorganisms do not arise spontaneously in properly sterile organic infusions. Thus the doctrine of “Spontaneous generation” during eighteenth - nineteenth century was discarded. But one must keep in mind the fact that primary origin of life in earth did involve a kind of spontaneous generation. Though it cannot be compared with the false doctrine as that was much more gradual & too delicate to describe.

Tyndallization/Discontinuous heating

Tyndall that started thinking about finding a process that will not only kill the bacteria but also their spores during sterilization. This resulted in the development of the process of discontinuous heating which was later termed as Tyndallization. In this process organic infusions were 1st heated for few minutes to kill the bacteria present in it. Then the infusion was kept for sometime in room temperature. This would result in the germination of the spores (heat stable) of the bacteria. Then the infusion was again heated for few minutes to kill the germinated spores (not heat stable). Then again the infusion was kept at room temperature for germination of the spores still left (late germinating spores). After following this cycle for several times one could destroy all the bacteria including their spores present in the organic infusion. Tyndall optimized that boiling for 1 minute for successive 5 cycles (with rest in between) would make the infusion sterile whereas non stop boiling even for 1 hour would not have that effect. But there was a drawback in this system of sterilization as germination time & heat resistance of bacteria vary from species to species and hence one must have a thorough understanding of the bacteria present in the infusion which is very difficult.