Memoir: Memoir on Alcoholic Fermentation

Context and purpose
Louis Pasteur approached the problem of alcoholic fermentation at a time when chemistry and biology offered competing explanations. Prevailing chemical theories treated fermentation as a spontaneous chemical decomposition of sugars, independent of life. Pasteur sought to determine whether living organisms were essential agents of fermentation and to clarify why different fermentations produced distinct products.

Experimental strategy
A combination of microscopic observation, careful control of air exposure, and manipulations that separated particulate matter from liquids framed the experimental program. Pasteur compared clear, sterilized media with samples containing visible cells, tracked the behavior of yeast under varying conditions, and observed how contamination altered outcomes. Filtration, heating, and isolation methods tested whether fermentation persisted when visible living material was removed.

Main observations
Yeast were shown to be living cells that multiply during fermentation rather than inert chemical catalysts. Samples that contained yeast cells reliably produced alcohol and carbon dioxide, while liquids from which cells had been removed by filtration or prevented from receiving microbial inocula failed to ferment. Exposure to specific contaminating organisms led to alternate products such as lactic acid, linking particular micro-organisms to particular types of fermentation.

Key conclusions
Fermentation was established as an act of life: specific living organisms carry out specific fermentative transformations. The production of alcohol from sugar is not a purely chemical decomposition but a biological process performed by yeast. Different fermentations arise from the activity of different microbes, and preventing microbial contamination prevents unintended fermentations.

Methodological and theoretical implications
The work emphasized empirical control, the use of microscopy to connect cellular life with chemical change, and the importance of eliminating extraneous variables. By displacing a purely physico-chemical account, the investigation shifted the explanatory framework toward microbiology and physiology. It also suggested practical methods for controlling fermentation by managing the presence and identity of microorganisms.

Practical and industrial impact
Demonstrating the biological basis of fermentation had immediate relevance for brewing, winemaking, and other industries that relied on controlled fermentation. Identifying the causes of spoilage and showing how specific microbes produced undesirable products suggested routes to better sanitation and process control. These insights led to improved reliability and quality in fermented products.

Legacy and later refinements
The identification of microorganisms as agents of fermentation laid groundwork for the germ theory of disease and the development of microbiology as a discipline. Later discoveries refined the picture: enzymatic activity within cells can catalyze fermentation reactions outside intact cells, showing a chemical mechanism nested within a biological context. Nonetheless, the central idea that living organisms determine the course of fermentation remained transformative.

Overall significance
The investigations reoriented thinking about chemical transformations in biological settings by showing that life, and not merely chemistry in isolation, governs many such processes. The findings bridged microscopic observation and macroscopic chemical change, reshaped industrial practice, and helped open the modern era of microbiology and biochemistry.