verb

• (transitive) To make slightly or moderately acid; to acidify.
• (transitive) To make sour in a moderate degree; to sour somewhat.
  "Twenty leeches were ordered to be applied round the nose every two days; frequent emollient fomentations; the local vapour bath; general bathing; stimulating pedilavia; a strict regimen; vegetables, milk, white meats; demulcent or acidulated drinks; avoidance of exposure to the sun or to cold air; flannel waistcoat and trousers. This treatment, being strictly observed for two months, caused all the inflammatory symptoms to disappear, […]"
• (transitive) To use an acidic catalyst, with the chemical change being emphasised over the importance of the change in pH. Used in the processing of biodiesel co-products.