A subject came up between myself and another homebrewer regarding the difference between using steeping grains in an extract brew and the use of a partial grain or mini-mash regimen and why one is called a partial mash and one doesn't deserve that much credit. It was more like an argument not a subject, but the subject was about the definition.
I suggested that the primary difference begins with the brewers intention when using either process.
Is the intention to extract sugar from the grains? Or, is the intention to simply provide additional color and flavor to your extract brew?
If the answer is that the intention is to not only add color and flavor but sugar, then a certain percentage of base malt (2-row or 6-row) needs to be included in the grist in order to provide the diastatic power (enzymes) to convert the starches of your mix of grain to simple sugars. Thus, allowing for a measurable extraction. On the other hand, base malt is not needed nor usually included in the mix of specialty grains when only steeping, since the purpose or intention is solely to gain flavor and color.
So, you could conclude that the addition of 2-row or 6-row malt is the distinguishing factor that defines the difference between a mini-mash and brewing an extract batch of beer including steeping grains. However, this isn't always true because (as is the case with me), adding some 2-row to the steeping grains may be included specifically to impact the color and flavor that your looking for. Consequently, an addition of sugar will be incurred because of this practice, regardless of any interest in that result. Naturally, this rules out the above idea of base malt being the critical factor.
Another element of this argument is the quantitative (measurable) factor. When using steeping grains, I don't consider the extract potential and do not factor it into my recipe formulation. Any sugar extraction is inconsequential in this brewing process. On the other hand, with a partial mash brew, I'm careful to consider the extract potential and the efficiency of my system to calculate the available sugars from the mash and how it will determine the amount of extract to be used in the recipe and ultimately the effect on the resulting original sugar gravity. Again, I believe that it comes down to the intention of the brewing that defines the difference between the two techniques.
But wait, there's more.
While I'm on the subject I thought I'd add some more opinion about partial mash brewing. Even though I teach to my students the techniques and show the equipment for doing a mini-mash, I strongly recommend that they jump right into all-grain brewing when they are ready to advance from using malt extract for brewing.
I've read in on-line forums and in brewing books that the mini-mash set up is smaller and thus more convenient for those that don't have the space for an all-grain set up. Those same voices also suggest that working under these limiting conditions may prevent you from achieving the efficiencies that are required by just using grain, but I disagree. If you live in an apartment or have to work in the kitchen or a confined space, you can just as easily use the same system you use for a mini-mash as you would for an all-grain batch of beer.
I have successfully brewed an all-grain batch with a single large boil pot and a mash tun made from a food grade plastic bucket. In fact, during the instructions for the last partial mash class I just recently taught, we were able to extract 80% of the available sugars with a plastic bucket mash tun. The same thing can be done on the stove top using all-grain to make a five gallon batch. Now, ten gallons, that may be more difficult but I'm willing to bet it can be done in the same space.
I'd be interested in hearing from anyone that does all-grain brewing on the stove, especially those that brew ten gallons that way. Cheers!