Brew day opportunity, last minute. No time to build a starter. using Wyeast 1968 in a 15 gallon batch @ OG 1.045 est. I only have 2 packages of yeast. Fermenting at 72f.
I will definitely be stressing the yeast by under pitching.
How detrimental, if any, can this be?
Will it increase esters/phenolics positively or make for an off flavor dud?
If the packs swell like they should, I think you’ll be fine flavor wise. I find 1968 to be a rather slow starter comparatively, so you might want to prepare yourself for that.
Hope your brew day went well if today was the day.
You will be underpitching, but in a 1.045 wort it is unlikely to ruin the beer. Two packs of Wyeast 1968 London ESB Ale provide far fewer cells than standard pitching rate recommendations, so the yeast will need to reproduce more in the fermenter. That growth phase is what drives higher ester production, so you can expect a more pronounced fruity profile, which is normal for this strain.
The tradeoff is that stressed yeast are more likely to produce excess diacetyl and may attenuate less efficiently. This is especially relevant with this strain because it is highly flocculent and can settle out before fully finishing fermentation if conditions are not ideal.
If the wort is well oxygenated and fermentation is kept under control early on, it should still produce a solid beer. The result will likely be a bit more estery and possibly slightly sweeter than intended, but not inherently flawed.
Agreed on all counts. The key would be the health of the yeast .. if it’s good I think the beer will be great. I would also use some pure O2 in the fermenter when you pitch. I know many brewers have abandoned O2 but I still use it in starters and in the fermenter. Finally .. 1968 is notorious for diacetyl as Jordan mentioned so I would absolutely make sure to get the fermenter into a warmer spot towards the end of fermentation and maybe try to get a sample before packaging the beer. I am very sensitive to diacetyl and I hate, hate, hate it. Keep us posted .. curious how it came out.
EDIT: The floccing mentioned by Jordan is wild too .. 1968 flocs like crazy and when I use it I have to keep rousing it to make sure the beer is done fermenting. I once went to harvest the 1968 to use in another beer and the yeast was like a solid pancake on the bottom of the fermenter. I felt like I could pick it up and frisbee it across the backyard! 
They were nice and activated after about 4 hours. Actually my first time using 1968 so was unaware of the potential for a slow start. Also the first time making a Fullers london Pride clone and missed my OG by 5. It was supposed to be 1.045 but only hit 1.040. I think it was my crush. Looked at it after milling and thought to myself “I better mill this twice” Nahhh..ohh well, not concerned about it. As mentioned, I did and always give my liquid yeasts a good dose of O2. 2 minutes on a 1/4 psi.
I am definitely going to collect and re-pitch this yeast, quite possibly in a barleywine that I wanted to make with a fruitcake theme to it.
To touch on the potential diacetyl issue, I really don’t think I know what it tastes like. I always do a rest with my pils/lagers or anything sub 60* otherwise there shouldn’t be that issue or I’m just not able to taste it. Sort of like cilantro and the soap taste some people get…I don’t get that taste either.
1968 is what I use most often. I don’t know how to describe the flavor maybe a mineral/malt flavor. I just recognize it as the flavor of a cask London Pride.
edit and yes it is a slow starter
Yeah .. bready and minerally. I really love 1968 although I don’t make many ales anymore and when I do it’s often with S-04 or Omega West Coast. The S-04 has a similar, unique “bready” and “english” character that I really like and I will do bitters, ambers, pale ales, blondes, reds, etc. I think you will really like 1968. It flocs like crazy. It has low(er) attenuation. It will produce diacetyl for sure unless you’re careful. But great character.
This beer has been in the fermenter since 4/1 and not once was I able to pour a taste from the sampling port…this yeast is definitely great at flocculating. Tmrrw is keg day. Will collect some yeast for future use. Can’t wait to taste this 1st iteration of a Fullers London Pride attempt. 