NOVELTY - An esterification pretreatment for reducing a high free fatty acid feedstock (1) comprises forcing a reaction mixture (10) comprising feedstock, alcohol (2), and acid catalyst (3), through greater than or equal to 2 controlled flow cavitation apparatuses (12) at a processing pressure capable of generating a hydrodynamic cavitation zone in each cavitation apparatus; maintaining a pressure differential across each controlled flow cavitation apparatus at 50-250 pounds/square inch (psi); and holding the reaction mixture exiting the controlled flow cavitation apparatus in a holding tank (13) for greater than or equal to 10 minutes at greater than or equal to 100 degrees C.    USE - Method of esterification pretreatment for reducing a high free fatty acid feedstock i.e. soybean oil, corn oil, cottonseed oil, olive oil, palm oil, palm kernel oil, peanut oil, linseed oil, safflower oil, canola oil, rapeseed oil, sesame oil, coconut oil, Jatropha oil, mustard seed oil, castor oil, tall oil, algae oil, soap stock, beef tallow, lard, brown grease, white grease, yellow grease, fish oils, bacterial oils, or fungal oils, producing esterified fatty acids and triglycerides useful for transesterification reaction to produce biodiesel fuel (all claimed).    ADVANTAGE - The free fatty acid content of the feedstock is reduced to less than 2, preferably less than 0.5 wt.%. The method is cost effective. Reduction of free fatty acid content in the feedstock is completed within 20 minutes. By lowering the free fatty acid content before the base-catalyzed transesterification reaction to product biodiesel, the soap produced in the subsequent transesterification reaction is reduced, biodiesel production efficiency is improved, and purification and/or separation steps necessitated by high free fatty acid content in the feedstock are eliminated. The controlled flow cavitation zone acts to provide a high level of energy to the reaction, which increases the rate of esterification reaction. This high level of energy is used to introduce alcohol to the reaction sites of the free fatty acids at a faster rate than conventional methods. The holding tanks provide dwell times for further introduction of alcohol to the reaction sites and for further completion of esterification reactions. By using controlled cavitation in combination holding tanks to pretreat high free fatty acid feedstock sources, reaction times, production costs, and soap by-products are reduced in downstream biodiesel production involving transesterification reactions to product fatty acid alkyl esters.    DETAILED DESCRIPTION - An esterification pretreatment for reducing a high free fatty acid feedstock (1) comprises providing a feedstock comprising a free fatty acid content of 3-30 wt.%; providing an alcohol (2) and an acid catalyst (3); combining the feedstock with alcohol and acid catalyst to form a reaction mixture (10); forcing the reaction mixture through greater than or equal to 2 controlled flow cavitation apparatuses (12) at a processing pressure capable of generating a hydrodynamic cavitation zone in each controlled flow cavitation apparatus; maintaining a pressure differential across each controlled flow cavitation apparatus at 50-250 psi; and holding the reaction mixture exiting the controlled flow cavitation apparatus in a holding tank (13) for greater than or equal to 10 minutes at greater than or equal to 100 degrees C.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view illustrating the method of reducing free fatty acid content of a feedstock stream using controlled flow cavitation apparatus(es) in combination with holding tank(s).    High free fatty acid feedstock (1)    Alcohol (2)    Acid catalyst (3)    Reaction mixture (10)    Controlled flow cavitation apparatuses (12)    Holding tank (13)