What does it mean in plant extraction when there is an acid fraction and a neutral fraction? How are these fractions obtained? Also when a journal article says The acid fraction was separated following the usual workup and decolorization, how are these things done?
An acid fraction is one obtained by use of an acid to change the pH. For example, alkaloids are often more easily extracted with an acidic solution. Neutral fractions have no modifier added.
As for the "usual workup", we need more context to give a good answer. Decolorizing is usually done with charcoal.
Extraction could be done by 2 ways , the first one is to selectively extract a specific class of compounds based on their physicochemical properties, such as alkaloids which is extracted using acid base method, also fatty acids which can be isolated by reaction with alkali in a process called saponification, the second way is using a method that extract all compounds and start to do fractionation according their polarity using liquid-liquid fractionation by consecutive extraction using solvent with different polarity such as hexane, methylene chloride, ethyl acetate, methanol, sometimes you need to do some work up before extraction to remove interfering substances such as fatty acid, waxes (a process called defatting using petroleum ether) , tannins (by precipitation using lead acetate) or pigments such as chlorophyll and carotene ( by passing the extract through activated charcoal) you could find more information in this book (https://www.springer.com/gb/book/9780412749100)
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