Where to start??
seing as how its been brought up with a single word answer, Nitrogen. Nitrogen, or a mix of co2 and n2,
will give a creamy head, its due to the N2 in atmosphere being at high levels, N2 is less willing to come out of solution in order to come to equilibrium with atmospheric levels. But now on to something more useful....
K&K, generally, has a higher proportion of simple sugars than AG, partial, or extract brewing. Simple sugars form alcohol, but contribute nothing to body. (in fact they degrade body, as alcohol is less dense than water.) So, if the k&k brew has a high proportion of simple sugar in the mix (ie dextrose, glucose, or god forbid sucrose), the head will suffer as a result.
Final gravity is a contributing factor; the lower the fg, the lower the body, the harder it is for co2 to remain in solution forming a tight head. This again, comes down to simple sugars in the mix. With a 'thicker' liquid, it is harder for co2 to come out of solution.
The level of hopping...The more isomerised alpha acids, the more bitter the beer. The alpha acids do contribute to head retention. Kit beers are for the most part, quite under-hopped (which is why for most of them you cannot use all malt, or they will be too sweet.)
The yeast makes a difference. Some yeasts, at least from what I've seen, are more likely to give good head than others.
The amount of dextrins in the wort....unfortunately, with k&k, you don't do the mash, so you can't even really begin to think about what percentages of dextrins are in the wort. And many AG brewers don't even go that deep into it, they just know that hotter/and or stiffer = more dextrinous and leave it at that. But at least an AG brewer has the opportunity to take it into consideration, should they choose to do so.
How it was carbonated, and how long it has been on the gas....Many people, myself included, force carbonate, due to time constraints. This does tend to cause coarser carbonation, with bigger bubbles, and therefore the head isn't as good. The 'let er sit' method tends to give finer bubbles and better head. However, most times when force carbing, it is going to sit and calm itself down anyway, and will have better head after a couple of days on the gas. Unless you drain the keg in one sitting.
So, after much typing and refilling of my glass, what can a K&K brewer do to make it better? (without beer snobs saying "go all grain")
Firstly, replace all the simple fermentables. Ditch the dex. Go all malt. If you do this, you will need to boil some hops to counteract the additional sweetness. But that actually kills 2 birds with one stone, because it also means that you will have more isomorised alpha acid for nucleation.
Secondly, add some dextrins into the mix. One way is the use of maltodextrin ( :angry: ) which contributes nothing other than body. A much better way is to steep some specialty grain, which will give body, colour, flavour, and flavour. (yes, I mentioned it twice. Its worth mentioning twice.) Carapils, crystal, cara-anything, chocolate. Somewhere in the region of 3-10%.
Thirdly, add some longer chain proteins, such as can be found from wheat. Substitute some of your malt for wheat malt. The protein chain from wheat malt is longer than from barley malt, which is why wheat beers have a head on them that can substitute for a childrens bouncy castle. Up to 10%.
Just a note on percentages....when talking percentages, what I mean is percentage of
extract weight. Which for all intents and purposes, is the dry weight. For liquid malts (including kit tins), that is 80% of the actual weight. For steeped grain, it is about 30% of the weight of the grain.
For a typical k&k ale, you could have, as an example,
1x 1.7kg kit (52.4%)
1 kg ldm (37.3%)
0.25kg dry wheat malt extract (7.5%)
0.25 kg crystal steeped. (2.8%)
23L
OG 1044
FG (predicted, on average) 1012.
You would then need to boil some hops, or it would be too sweet. 20-30g at 15minutes should be enogh to re-balance, depending on what kit you are doing in the first place, and what hops you are using. You would need about 8-10 IBU extra.
Carbonate this to 2.5vol co2, with a balanced system, and this should give you a head, even with a kit, that you could put a tie on.
Hopefully I have been sober enough in this post to not be too confusing. Btw, and OT, beer does not seem to alleviate a 104 fever.
