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is backset or sour mash = "spent mash" or "spent wash"


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I am a bit confused about how the terms spent grain, spent mash, spent wash and stillage are used  which is why i do not completely understand the term sour mash (as used in much of bourbon production)

As i understand it

Spent grain (or draff) moist caked leftover grain after lautering off the sweet mash liquid and may be sold and used as cattle feed

My real confusion arrives when it comes to spent mash and spent wash .....  are they the same term ?

Mash = wort = unfermented liquid having fermentable sugars from grain (made fermentable by diastatic action)

Is spent mash the mash after fermentation and before distillation (which i know as WASH) or is it the brownish liquid left over after vapor stripping the wash in a still which means it is the stillage

So is the backset used in bourbons actually the SPENT WASH obtained from the still after distilling the top product (ethanol) out?

So does that make sour mash = dunder (sometimes used in rum production)

Please correct any or all the above statements so that i may understand the actual content of sour mash and from where it is taken 

Thank you adi members

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The difference in a lot of the terms you are mentioned is whether grain is in or grain in out, but really, these terms often mean multiple things, so context is important.

Brewers spent grain (aka spent grain) - Spent grain separated when lautering.  Generally sweet, spoils (ferments) very quickly).

Distillers spent grain (aka spent grain, DSG) - Spent grain separated from stillage post distillation.  Generally sour.

Beer - In the case of distilling, this is the fermented mash.

Wash - Same as beer, although sometimes this is used to indicate the liquid-only version of beer.  You tend to hear this in Rum, where it's confusingly called both wash and 'rum beer'.

Stillage - the remaining liquid in the still, in the case of an American style whiskey, this contains the spent grains pre-separation (sometimes called whole stillage), in the case of rum, there are no solids.  

Mash - Mixture of grain and water, pre-fermentation.  

Spent mash - Grain in stillage.  Could be processed post-distillation into:  DSG, Spent Wash (mostly discarded), and Backset (spent wash reserved for the next batch).

Spent wash - Liquid-only stillage.

Backset - Typically liquid-only stillage that is reused in the next mash to acidify (sour) the whiskey mash.

Dunder - Spent wash for rum, and yes, this is similar to using backset in whiskey.  Dunder may be aged/ripened, but doesn't need to be.  In the case of Jamaican rums, dunder and "muck" are often confused.

Sour Mash - Two types of sour mash, both have a similar end goal, which is reducing the pH of the mash by introducing acid.  Backset is one option, the other is bacterial, using lactic acid bacteria (or other bacteria that may be present in the equipment, for example, living in the walls of a wooden fermenter).

Worth noting, backset and dunder are sometimes used to reduce the water required to mash, or even to reduce the nutrient required - goes beyond only being acidification.  

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  • 2 weeks later...
On 11/9/2023 at 11:38 AM, perfection said:


Much clearer but am still not clear about spent mash and DSG

Spent mash is grain in stillage which is already post distillation to what does "processed post-distillation into DSG etc...."


Spent mash is what comes out of the still from a Grain in Distillation. You remove the "Water" from the Mash and you get "Distillers grains" in the Ethanol Industry. This "Wet Cake" product is about like cake batter.  This can be further dried into "Dried Distillers Grains" or DDG's.

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So spent mash in Bourbon and other spirits that practice fermentation 'on the grain: = (full) stillage = spent grain and liquid sans alcohol coming out as waste from a distillery operation. The spent grain in this may be processed into DSG

Silk City Distillers - If backset is more usually the thin stillage (as opposed to full stillage) why call sour mash sspent mash ---why not spent wash?  (my original question)?


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This doesn’t follow strict rules like math or logic, these terms often only have a specific meaning only when used in a specific context.  

Sour mash describes the technique, the process of souring (acidifying) the mash, either through bacteria or backset.

Sour mash is also sometimes, but not always, used to describe the resulting whiskey that was made using the sour mash process.

I suppose someone could point to a fermenter and mutter the words sour mash, and we’d understand that too.  Even though it’s not mash, it’s wash.

Afaik, nobody is using the words sour mash to describe the backset, thin stillage, or spent mash.  None of this makes perfect sense, but that especially doesn’t make sense.


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The distilling industry sucks for consistent terminology. Each region, spirit type and language has its own nuance. Without a reference book to define and delineate terminology there's never going to be consistency.  

For what you're asking SCD nailed it for what most people mean when they use those terms. 

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