If you look at the reactions in the conversion of glucose to alcohol and CO2 (Glycolysis pathway), you will see that phosphopyruvic acid is converted into acetaldehyde (a Glycolysis intermediate) which again is converted into ethanol. An all malt wort with OG 1048 which is used in the production of a standard beer will produce 7mg/l acetaldehyde as a normal fermentation secondary product.
Acetaldehyde is easily detectable in beer which has not completed the maturation or conditioning stage, which could be as short as 48 hours in beers not of the lager type. Such beer is often called “green beer” not referring to its colour, but to the fact that the flavour is not fully mature. Together with diacetyl and H2S, acetaldehyde is the most noticeable green beer flavour. Acetaldehyde contributes an apple-like taste which if detected is considered an undesirable off-flavour. As the beer matures most of the acetaldehyde is taken back into the yeast cell and reduced to ethanol. Keto acids (of which there is a very small amount in beer) are formed during carbohydrate catabolism. Their importance is rather as intermediates in the formation of the related amino acids, aldehydes, acids, esters and alcohols. These aldehydes (like acetaldehyde) are reduced to the corresponding alcohols like isoamyl alcohol, 2-phenylethanol and the ester ethyl acetate – all important beer flavour contributors. A very important factor in the removal of green beer flavours is that it depends on having enough live, well adapted, active, yeast present in suspension to take the compounds into cells and reduce them towards the end of primary fermentation and the beginning of secondary fermentation.
Krausening will increase maturation time because it creates a new peak of fermentation byproducts which will then have to be reduced again. Flavour problems are caused by too early separation (flocculation) of the yeast, because the oxo compounds mentioned earlier can then not be removed. Abnormal fermentations like long lag phases, yeast that “run out of steam” before the required amount of fermentables are used (hung fermentation) or trailing fermentations all cause flavour problems. Abnormal fermentations can also be caused by bacterial contaminants such as Zymomonas and non compliant appy’s, but the pathway to form acetaldehyde stays the same. Which brings me to the conclusion that 1st and 2nd batch fermentations with the same yeast, freshly propagated from smackpacks, vials or dried yeast, (even if a large starter was made) is not normal and ferments a beer either of “non-matching” quality or with flavour problems. To avoid this, the brewers rule is to add at least a quarter of previous (old) generation yeast together with the freshly propagated yeast starter. Practical implications to home brewers is to stick to the same beerstyle at least 3 times in short succession with serial re-pitching to give the new yeast time to adapt to their specific wort makeup, brewing setup and fermentation conditions, before changing to a new beerstyle.