Start here: two bacterial pathways, not one butter switch
Build the basic mechanism before drawing conclusions.
The argument
What this chapter will give you
Connect malic-to-lactic acid conversion with bacterial citrate metabolism and diacetyl.
This is a reading chapter, not a definition enlarged to fill a screen. Its sections build the idea from mechanism and method to the language you can use in front of an actual glass.
Start here: two bacterial pathways, not one butter switch
Malolactic conversion changes malic acid into lactic acid and carbon dioxide through bacterial activity. It is distinct from yeast converting sugar into alcohol. The change commonly reduces acidity and alters the wine’s acid balance. Diacetyl is an aroma compound that can smell buttery; its formation is associated with bacterial processing of citrate, rather than being the direct product of malic acid becoming lactic acid. The processes can occur in the same microbial setting without being the same reaction. Malolactic conversion may overlap alcoholic fermentation or follow it.
What is happening
After alcoholic fermentation, bacteria can convert sharper malic acid into softer lactic acid. Bacterial citrate metabolism can also produce diacetyl.
Bring it back to the glass
Three checks before the conclusion
- Name the process or growth stage before explaining its consequence.
- Use the worked example to distinguish an observation from its possible cause.
- Identify the missing evidence before applying the explanation to a real bottle.
The language bench
Solid things you can actually say
Use these as sentence shapes, not answers to copy. Replace the details with what your glass can defend.
Start here: two bacterial pathways, not one butter switch
- “Malolactic conversion changes malic acid into lactic acid and carbon dioxide through bacterial activity.”
Worked example: conversion without obvious butter
- “Imagine two wines that both complete malolactic conversion.”
What can the explanation support?
- “Malolactic conversion can contribute buttery aroma, but neither process nor butter guarantees the other.”
Make the sentence earn its keep
“That one clue proves the whole story.”
“Malolactic conversion can contribute buttery aroma, but neither process nor butter guarantees the other.”
Keep the mechanism specific and match confidence to the available evidence.
Try it now
A five-minute field exercise
- Does no butter mean no malolactic conversion?
- Write your answer and its reason before reading the explanation.
- Answer: No. Acid conversion and perceptible diacetyl are different endpoints. Draw one arrow from malic acid to lactic acid plus carbon dioxide, and a separate pathway from citrate metabolism toward diacetyl. Explain why tasting softness or smelling butter cannot substitute for measurements when confirming completion.
The line worth keepingMalolactic conversion can contribute buttery aroma, but neither process nor butter guarantees the other.