(Level of) Modification: A measure of the length of the acrospire in relation to the size of the kernel.
Under-modified: Acrospire grows to < 75% of the length of the kernel
Well Modified: Acrospire grows to >= 75% and <= 100% of the length of the kernel
Overmodified: Acrospire grows to > 100% of the length of the kernel (never sold or targeted)
In each batch of malt a certain percentage will be had of each level of modification. In a well controlled process the largest percent is the target percentage.
Step Mash: A mashing procedure whereby the temperature of the mash is raised in timed steps to activate various enzymes or release acids contained in malted barley.
Decoction: A mashing procedure whereby a thick portion of the mash is boiled and added back to a thin portion in order to raise the temperature. The temperature is normally raised in timed steps just as a step mash, though only two or three steps is common.
What do the terms "highly modified", "modern", "hot" mean?
They refer to varieties of barley bred and malted to contain large amounts of enzymes such that they convert fast in the mash. They also refer to "well modified" malt as defined above. Convert (or conversion) meaning the act of the enzymes converting starches to sugars in the mash.
My purpose in defining these terms is to try and find an association between the level of modification and the mash type.
What benefit does a step mash have on each of these types of malt: under-modified, well modified and over modified malt?
Step mashing has nothing to do with malt modification but rather making use of certain properties of the malt at each temperature. Ferulic acid rest, Protein rest, Beta rest, Alpha rest, etc... None of those care about malt modification, save the speed at which the conversion occurs. Under-modified malts and/or recipes whose Degrees of Lintner values aren't high enough may require more time for each step to complete or convert.
What benefit does a decoction mash have on each of these types of malt: under-modified, well modified and over modified malt?
Decoction mashing is normally used on under-modified malts where it is helpful to burst the cell walls to release additional starches and enzymes. It also has the same effect that step mashing has when the rest temperatures are stepped up.
The only association that I can find between level of modification and step/decoction mashing is the time assigned to each step and in a very general sense the step times may increase with under-modified malts but would require experimentation in any case. Are there any other associations?