How to get more lactic acid vs acetic acid in sourdough?
This is a theoretical answer based on several publications and it would require actual experimental tests. The most probable tip seems to maintain your sourdough at 32 °C as it will affect facultatively heterofermentative lactobacilli present in your sourdough.

A lot of spread knowledge provides, without explanation, several tips such as: feed your starter more regularly, use more starter, add baking soda, use white flour, add baker's yeast, rest your dough at cooler temperature, etc.

Wikipedia currently cites a post by Debra Wink to assert:


  Conversely, a wetter and warmer starter has more bacterial activity and less yeast growth, with more lactic acid relative to acetic acid.


Debra Wink's post is a quite detailed theory of what's going on with lactic and acetic acid productions in sourdough based on several research papers.

The temperature argument is not so clear and seems to be based on so-called baker's rule that low temperatures (20–26 °C) are better for yeast growth. Indeed, this paper argues that you will need a warm temperature (90 °F or 32 °C) and enough humidity as these are the optimal conditions for the growth of lactobacilli, and you need to slow the growth of yeast. While some yeast species (such as Candida milleri) grow faster within 20–26 °C range, the optimum temperature for growth of Saccharomyces cerevisiae, the species of baker's yeast, is supposed to be 30–35 °C.

And more importantly, yeast does not produce acetic acid or lactic acid, it only produces ethanol (alcohol) and CO2. All acids are produced by bacteria.

Lactic acid is produced by lactobacilli and to increase the lactic acid to acetic acid ratio, it seems you will indeed need to favor lactobacilli growth, hence the temperature argument.

However, details are more complex. There are three kinds of lactobacilli:


obligately homofermentative, producing two lactic acid molecules per 6-carbon  sugars;
obligately heterofermentative, producing one lactic acid molecule, and one ethanol molecule and one CO2 molecule per 6-carbon sugars (or just lactic acid and ethanol for lactobacilli able to ferment pentoses) ;
facultatively heterofermentative, chosing either path depending on availability of sugars and required energy.


Facultatively heterofermentative lactobacili will apparently choose the homofermentative path (produce only lactic acid) at higher temperature.

Acetic acid is a by-product of the metabolism of heterofermentative path: whether you end up with ethanol (CH3CH2OH) or acetic acid (CH3COOH) depends on the availability of co-substrates.

Obviously, you might consider having only homofermentative lactobacilli, or facultatively heterofermentative lactobacilli in a warmer environment, yet you might not be able to choose your starter with this precision in your kitchen.

Indeed, the issue seems to concern obligatively heterofermentative lactobacilli including Lactobacillus sanfranciscensis, which has been extensively studied as it is part of common sourdough. According to the post mentioned above, it will produce acetic acid by co-metabolizing fructose with maltose. However, Henry Ng, in a 1972 paper referenced by this post, found that limiting oxygen will limit the amount of acetic acid in a sourdough composed of Lactobacillus sanfranciscensis. In fact, this paper established Lactobacillus sanfranciscensis as an obligatively heterofermentative lactobacilli.

Furthermore, according to Michael Gänzle, another author being referred to, oxygen is the co-substrate for acetic acid, Lactobacillus sanfranciscensis will reduce fructose to mannitol, not to acetic acid, and the amount of oxygen is limited anyway (unless you shake it as Henry Ng's team did). Consequently, the production of lactic acid appears to be the principal factor of the lactic to acetic acid ratio.

Please note that this author stresses that you do need acetic acid in your sourdough:


  Spicher says that a ratio of 20 acetate to 80 lactate is optimal. […] The acetic acid is furthermore important as growth of spoilage organisms such as molds or rope causing bacilli (Bacillus subtilis) is inhibited by high acetic acid concentrations.