What causes rancidity, how to treat it and how to prevent it .



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How to treat and prevent coolant rancidity.

November 2021

Every machinist loves the unpleasant, lingering smell of rotten eggs, locker rooms, and sweaty workout clothes, right? Wrong. This is typically what rancid coolant sumps smell like, and it’s one of the most common reasons for operator dissatisfaction in machine shops. Anyone who’s worked in a machine shop long enough has probably encountered a rancid coolant odor. We’re here to help you GO BEYOND living with the stink, and understand what causes it, how to treat it, and how you can prevent it.


Rancid coolant is a clear sign that your coolant needs maintenance or replacement. Improper coolant maintenance can decrease the working life of a machine by 10% each year. The foul odor coming from your coolant is bacteria and fungus taking over your sump. There are two main types of bacteria that can be found in metalworking mixes – aerobic and anaerobic. Aerobic bacteria thrives in the presence of oxygen, but It’s usually not responsible for coolant rancidity. Anaerobic bacteria thrives in the absence of oxygen – this is the type of bacteria that’s typically responsible for rancidity. Commonly known as “Monday morning odor”,  perfect example of this is coolant sumps that are allowed to “rest” over weekends in a sump. Tramp oil – oils from hydraulic leaks, way lubes, spindle oils and oils and soils carried in with the work piece float to the top of the sump when machines are idle.  This layer of oil seals the sump from the air, creating a perfect place for anaerobic bacteria to thrive. Turn the machine back on on Monday morning and “phew”.

Coolant that is idle for too long creates the perfect conditions for bacterial and fungal contamination. As if the smell isn’t bad enough, Bacteria and fungus also cause production headaches such as clogged filters, destabilized coolant, and an excessive amount of machine downtime needed to clean out and recharge sumps.  Additionally, a by-product of bacterial growth is lowered pH in the sump, which causes the machine to rust.


Treating rancid tramp oil and bacteria filled coolant sumps is much more time consuming and expensive than simply maintaining your coolant regularly. If your coolant has developed a rancid odor, cleaning out your sump can remove odor and prevent it from returning. Here are the steps you should take to clean a machine sump.

  1. First, use Ashburn Final Rinse in the machine at about 10% concentration.  Run your machine normally for one to two shifts.  This will clean all of the fluid passageways in the machine.
  2.  Next, remove the coolant and thoroughly clean out all chips, sludge, coolant and residue from the machine.
  3. Next, circulate Ashburn Machine Cleaner Plus through the machine for at least 20 minutes to rid the system of any bacteria, fungus, or other residues.
  4. Next, add 2% of coolant concentrate and circulate the mixture for 15 more minutes.
  5. Once this mixture has finished circulating, drain the sump and wipe the machine down thoroughly.Make sure to remove all of the remaining liquid from the system. If too much machine cleaner is left in the machine, foaming and coolant emulsification may occur.
  6. Lastly, pre-mix new coolant and recharge the machine.


    Preventing coolant rancidity is much easier than treating it. Three great ways to prevent rancidity are aeration, machine cleaning, and fluid maintenance.Aeration equipment allows oxygen to continuously circulate through metalworking fluids. Oxygen circulation prevents the growth of the anaerobic bacteria that causes odor.Periodically cleaning your machines as a preventative maintenance practice helps to prevent the growth of bacteria altogether.Lastly, proper fluid maintenance practices can be made to prevent the growth of bacteria. Some metalworking fluids contain strong microbial inhibitors. These fluids are designed to control bacterial growth as long as the solution stays within the recommended PH and concentration ranges. Regular fluid checks and adjustments can ensure a long lasting, worry free sump life.

    If you need help troubleshooting fluid problems, or want more information about how to improve your fluid and tool life, contact your Ashburn metalworking specialist about FLUID360 products and services. Start protecting your production today.

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