How many yeast cells do I have?

[arnobg]

Harvested this from a 1500ml starter that I poured 500ml from into a mason jar, mixed with distilled water, decanted, added more water. The remaining 1000ml starter went in wort.

Trying to figure out how many cells to start with for calculating a new starter. (First time doing this).

This is a 1 quart mason jar.

 

[arnobg]

Anyone? Trying to estimate starting cell count for starters.

 

[brewshki]

What was your estimation for the starter it came from? If you haven't changed anything about it, isn't it just 1/3 of what you had?

 

[arnobg]

What was your estimation for the starter it came from? If you haven't changed anything about it, isn't it just 1/3 of what you had?

That's a good point. Would that be a fair assumption to make even with possible dead cells in there?

everything I am seeing says estimate 1-8 billion cells per ml. That's quite a range.

I would like to make a starter with this and harvest off of the starter to store it again like last time but I don't want to overpitch.

 

[brewshki]

That's a good point. Would that be a fair assumption to make even with possible dead cells in there?

everything I am seeing says estimate 1-8 billion cells per ml. That's quite a range.

I would like to make a starter with this and harvest off of the starter to store it again like last time but I don't want to overpitch.

i really have no idea. i know there are some good calculators out there. unless you are stepping up and using a stir plate, i doubt you are going to over pitch

 

[BigTerp]

I've been trying to step up my yeast game lately and have done a bunch of reading. The consensus seems to be 4.5 billion cells per/ml for tightly compacted and clean (no trub) yeast. What I do is estimate 4-4.5 billion cells per/ml if the yeast I have on hand is from a starter, and has been chilled long enough to be tightly compacted like yours is. If it is washed yeast I estimate about 3 billion cells per/ml, plus or minus, depending on how much visable trub is in with the yeast. Based on this I plug those numbers into yeastcalc and set the production date (which estimates viability) which will give me a total number of yeast in billions. Not sure how accurate this makes my pitching rates, but I have been trying to stay consistent with this approach my last several batches. Good results so far.

 

[MagicMatt]

The consensus seems to be 4.5 billion cells per/ml for tightly compacted and clean (no trub) yeast.

That's waaaay more than I believe is correct. And you can't just slap a number on it like that without first talking about the percentage of yeast to liquid.

The accepted number I've read is 1-2 billion/ml for a 50% sample. Check out this article by Wyeast for more in depth explanations: https://www.wyeastlab.com/com-yeast-harvest.cfm.

As you can see by the image provided, you can expect 1.0-1.4 billion/ml when you have between 40-55% dense yeast cake. This means given any volume of slurry with 50% being fully settled yeast, you'll average about 1.2 billion/ml.

Your mason jar is too large to accurately determine the volume of yeast cells, so I would recommend transferring to something in which you can more accurately gauge the level. Then you decant until the liquid level is twice the volume of cells, and this should yield about 1.2 billion/ml. Or you can just eyeball it this time and get it into a better vessel the next.

From my eyes in that last picture, I'd say you're close to 25ml of dense yeast.

So in this case, you'd decant until you have only 25 ml of liquid on top of it (50ml total), and then you can estimate 1.2 billion/ml x 50ml = 60 billion yeast cells.

Next, you'd enter that information into a yeast calculator (I like HomebrewDad's) with the date that the yeast was harvested, and it will estimate the viability of the sample. Then you continue on building a new starter using those numbers and your target pitch count. The reason I like HomebrewDad's is it has a specific field for overbuilding (to harvest), so I don't forget.

P.S. There is no need to decant and replace with distilled water. You're only risking possible contamination or infection by messing with the yeast more than necessary. Pour your reserved portion of the starter into the mason jar, then seal it up and put it in the fridge for a week. Once everything is settled out, you can decant to equal parts yeast cake and liquid, and possibly transfer to a smaller container (I use 50ml test tubes).

 

[BigTerp]

Matt - good explanation. I understand what you are saying and have read both Wyeast and White Labs write up on yeast. But I do not measure the entire contents of the slurry when estimating yeast numbers. I only measure the tightly compacted yeast at the bottom of the jar. So in a 500ml mason jar, I may have only 15ml of compacted yeast at the bottom. For me, measuring the compacted yeast is easier than figuring out percentage of yeast in the total slurry. If that makes sense? So if you use Wyeasts numbers and have 20ml of slurry with 50% compact yeast. If you figure 2 billion/per thats 40 billion. If you take the measurement of the yeast cake only at 10ml and figure 4 billion/per ml you also end up with 40 billion cells. I think we both have the same idea, just different ways to skin a cat.

I also like to error on the side of over estimating the initial yeast I have on hand as other reading I have done suggests off flavors, while minimal, are more noticeable from overpitching rather than underpitching.

 

[arnobg]

That's waaaay more than I believe is correct. And you can't just slap a number on it like that without first talking about the percentage of yeast to liquid.

The accepted number I've read is 1-2 billion/ml for a 50% sample. Check out this article by Wyeast for more in depth explanations: https://www.wyeastlab.com/com-yeast-harvest.cfm.

As you can see by the image provided, you can expect 1.0-1.4 billion/ml when you have between 40-55% dense yeast cake. This means given any volume of slurry with 50% being fully settled yeast, you'll average about 1.2 billion/ml.

Your mason jar is too large to accurately determine the volume of yeast cells, so I would recommend transferring to something in which you can more accurately gauge the level. Then you decant until the liquid level is twice the volume of cells, and this should yield about 1.2 billion/ml. Or you can just eyeball it this time and get it into a better vessel the next.

From my eyes in that last picture, I'd say you're close to 25ml of dense yeast.

So in this case, you'd decant until you have only 25 ml of liquid on top of it (50ml total), and then you can estimate 1.2 billion/ml x 50ml = 60 billion yeast cells.

Next, you'd enter that information into a yeast calculator (I like HomebrewDad's) with the date that the yeast was harvested, and it will estimate the viability of the sample. Then you continue on building a new starter using those numbers and your target pitch count. The reason I like HomebrewDad's is it has a specific field for overbuilding (to harvest), so I don't forget.

P.S. There is no need to decant and replace with distilled water. You're only risking possible contamination or infection by messing with the yeast more than necessary. Pour your reserved portion of the starter into the mason jar, then seal it up and put it in the fridge for a week. Once everything is settled out, you can decant to equal parts yeast cake and liquid, and possibly transfer to a smaller container (I use 50ml test tubes).

Interesting. I used this tutorial which has some conflicting opinions.

https://www.homebrewtalk.com/yeast-harvesting-novel-approach.html

This one does as well, a stickied post from the yeast forum.

https://www.homebrewtalk.com/showthread.php?t=519995


What is right and wrong, that I don't know but that's why I am the one asking for help so I appreciate all advice and opinions. Just trying to look at all sides and possibilities.

 

[MagicMatt]

Matt - good explanation. I understand what you are saying and have read both Wyeast and White Labs write up on yeast. But I do not measure the entire contents of the slurry when estimating yeast numbers. I only measure the tightly compacted yeast at the bottom of the jar. So in a 500ml mason jar, I may have only 15ml of compacted yeast at the bottom. For me, measuring the compacted yeast is easier than figuring out percentage of yeast in the total slurry. If that makes sense? So if you use Wyeasts numbers and have 20ml of slurry with 50% compact yeast. If you figure 2 billion/per thats 40 billion. If you take the measurement of the yeast cake only at 10ml and figure 4 billion/per ml you also end up with 40 billion cells. I think we both have the same idea, just different ways to skin a cat.

Yes you are mostly correct, those number would be fairly equal estimates. However, as stated the number of cells per ml for a 50% sample is closer to 1.2B (you could even go as high as 1.4B, but not much more than that, definitely not as high as 2B). Thus, in your example you'd have roughly 30B cells estimating by percentage of yeast to liquid, which means the compacted yeast is roughly 3B/ml by itself, not 4B. Again, I still consider this to be a "best case" scenario, or at least one closer to the upper extreme. Getting 4 billion cells settled into a ml's volume is not very easily achievable without some sort of vacuum or compression.

Interesting. I used this tutorial which has some conflicting opinions.

https://www.homebrewtalk.com/yeast-harvesting-novel-approach.html

This one does as well, a stickied post from the yeast forum.

https://www.homebrewtalk.com/showthread.php?t=519995


What is right and wrong, that I don't know but that's why I am the one asking for help so I appreciate all advice and opinions. Just trying to look at all sides and possibilities.

The first one you linked doesn't seem to touch on cell counts at all, unless I'm missing something. Also, it is a tad old in terms of the washing with distilled water. See this article by Brulosopher regarding modern conventions:
http://brulosophy.com/methods/yeast-harvesting/

The 2nd link you posted says "A safe estimate is normally 1.5 billion cells per ml of thick settled slurry". This is exactly in line with what I posted, so I'm not sure where the discrepancy lies. Perhaps it's the 4B number in the chart he posted, but you should note that is for "professionally cultured yeast, not using malt extract". This means in a yeast lab, not in your kitchen. The more closely applicable number would be the 2B/ml of "thick, settled slurry", which is definitely a "safe estimate", but likely a tad lower than is necessary to estimate.


So basically there's 2 ways to estimate: based on the settled, compacted yeast alone, or by comparing the percentage of compacted yeast to liquid.

Using the former, it is safe to assume 2 billion/mL of settled yeast (meaning do not count the volume of the liquid at all). Just keep in mind that in this case, if there is 20 mL of yeast, that means there are ~40 billion yeast cells total, regardless of how much liquid is on top.

Using the latter, you would decant until the total volume was twice the volume of settled yeast, and then you would know you have roughly 1.2 billion cells per mL of total volume. Using the same scenario, this would mean 20mL of yeast, so decant to a total volume of 40mL, and then you'd have about 1.2B x 40 = 48 billion cells.

Either way you end up with roughly the same number. Sure, 8 billion cells is a bit of a discrepancy, but you'll be hard pressed to find 2 methods of cell estimation that yield the same exact number. I, for one, trust Wyeast. They seem to know what they're doing.


I'd like to add that when I find discrepancies such as this, I generally go with the smaller number to be safe. I'd rather overbuild slightly than underbuild and put unwanted stress on the yeast. There hasn't been any indication that overbuilding in a homebrew setup can cause any ill effects. There's been a few exBEERiments to this extent.

 

[BigTerp]

All of this is interesting to me. I've been trying to find ways to increase the quality of my beer. Getting pitching rates figured out is one of them. I've VERY open to varying opinions on yeast cell density per ml. So it seems I may have been overestimating my starting cell count. Without lab equipment I know it is still pretty much a guestimate, but I'd like to find a somewhat accepted number and start stick to it. With that being said, for each ml of tightly compacted yeast only (directly from a starter) what would you estimate the concentration to be? Somewhere between 2-3 billion per ml? What if it is washed yeast? 1-2 billion, depending on trub contents? That would be basically doubling what Wyeast says for a yeast slurry, correct?

Like I said, I'd rather error on the side of underpitching. I cannot remember the source, but I found a fairly arbitrary "study" where someone underpitched, overpitched and had a control pitch for a few different batches. Each time fellow homebrewers picked out the overpitched sample as the one with any noticeable off flavors. Interesting stuff!!

 

[arnobg]

Don't have time to write the other questions I have to your long helpful post, but I cited that link because I'm not dealing with slurry I am dealing with yeast harvested straight from a starter.

I harvested that yeast from a brand new smack pack. I made a 1.5L starter and poured off 0.5L and stored it just as in that link, so since I am not dealing with slurry 1.5 billion/ml would not be accurate....

 

[MagicMatt]

All of this is interesting to me. I've been trying to find ways to increase the quality of my beer. Getting pitching rates figured out is one of them. I've VERY open to varying opinions on yeast cell density per ml. So it seems I may have been overestimating my starting cell count. Without lab equipment I know it is still pretty much a guestimate, but I'd like to find a somewhat accepted number and start stick to it.

Check out the xBmt's on the Brulosopher site. They have done quite a few different experiments with yeast, including under and over pitching, creating a starter vs direct pitching, etc. I'm sure you'd be surprised that after all is said and done, there likely won't be any perceivable differences in the finished product. One of the main advantages to building a starter is a quicker start to ferment, or to rouse very old yeast (>6 months) where viability might be too low to finish the beer. I'm not an expert on yeast and I don't want to mislead y'all into thinking these things I say are gospel, but just trying to share some of the more trusted sources for these things, as one can find quite a bit of contradictory information out there.

With that being said, for each ml of tightly compacted yeast only (directly from a starter) what would you estimate the concentration to be? Somewhere between 2-3 billion per ml? What if it is washed yeast? 1-2 billion, depending on trub contents? That would be basically doubling what Wyeast says for a yeast slurry, correct?

No, not correct. Remember, Wyeast's numbers are based on percentages, not volume of slurry. So according to them, the number of cells in a sample where, when settled, 50% is yeast, will be about 1.2 billion cells per mL. To figure how many cells are in the settled yeast cake alone, simply multiply by 2 (since 1/2 of the sample is not yeast) and you'd get about 2.4 billion cells per mL of settled yeast, regardless of total volume. So that's the number I'd use if you only want to look at the yeast cake and not the total volume. A 50mL sample with half (25mL) being yeast would mean 1.2B x 50 = 60 billion cells total, and therefore that same sample could be read as having 25mL of yeast at 2.4B/mL = 60 billion cells total.

I've never washed yeast so I can't comment on that aspect, but I'd imagine there would have to be less cells/mL than from a starter with minimal trub.

Like I said, I'd rather error on the side of underpitching. I cannot remember the source, but I found a fairly arbitrary "study" where someone underpitched, overpitched and had a control pitch for a few different batches. Each time fellow homebrewers picked out the overpitched sample as the one with any noticeable off flavors. Interesting stuff!!

I haven't noticed this, and in fact have noticed the opposite - that if you underpitch, you'll end up with a different tasting beer. Mostly because it under attenuates and leaves a slightly more malty/sweet taste. I've never had any side effects of over pitching other than an incredibly quick start to fermentation (within 2 hours).

Here's a quote from one of these exBEERiments:

"...the one thing I cannot stand is an under-attenuated, sticky-sweet mess. Call it what you want, but as a new brewer I learned the key to avoiding a cloying disaster was to mash low and, perhaps more importantly, pitch a ton of yeast."

--http://brulosophy.com/2015/04/20/yeast-pitch-rate-single-vial-vs-yeast-starter-exbeeriment-results/

Here's another link that talks about over pitching vs under pitching.

Don't have time to write the other questions I have to your long helpful post, but I cited that link because I'm not dealing with slurry I am dealing with yeast harvested straight from a starter.

I harvested that yeast from a brand new smack pack. I made a 1.5L starter and poured off 0.5L and stored it just as in that link, so since I am not dealing with slurry 1.5 billion/ml would not be accurate....

Quite the contrary: the 1.2 billion/mL is ONLY accurate for yeast harvested from starters as you have done.

I believe you're misunderstanding the terminology. A 'slurry' is simply a homogeneous sample of liquid yeast. Where you get the yeast from does not make it (or not make it) a slurry. When you decant a portion of your starter into a mason jar to reserve, you now have a mason jar of slurry. Once the yeast settle out to form a cake, it's no longer a slurry since it's not homogeneous. If you were to shake the ever-living ish out of the jar, then it would become a slurry again.

Harvesting yeast from starters is a very good approach, and it's what I practice as well. The best part of it is that you should know before you even make the starter how many cells you will harvest. If you make a 1.5L starter that reaches an estimated 300 billion cell count, and harvest 500mL of that starter, you've just collected ~100 billion cells, no need to measure (although it definitely helps to verify). Regardless of how much you do harvest, if you let it settle and then decant the liquid until the total volume is twice that of the yeast, you're looking at about 1.2 billion/mL. This only counts for starter-harvested yeast since yeast collected from the bottom of a bucket or carboy will have numerous other byproducts of brewing and fermentation in there (trub), so there will be less yeast/mL.

 

[BeardedBrews]

There are 41,215,175,000 viable yeast cells left in that jar. You will need 1825mL of 1.037 sg wort spun at exactly 133rpm at 72°F for 31 hours to grow the minimum mandatory quantity of yeast: precisely 300,000,000 cells.



#science



Chris and Jamil estimate low, Kai estimates high, Ray and Marshal point out that the average beer connoisseur is very unlikely to distinguish between high and low in a finished beer.

Personally, I would pretend that's an old vial of yeast, pitch it in a 2L starter, and figure I have about 300B cells when it finishes.

 

[QuadConPana]

Personally, I would pretend that's an old vial of yeast, pitch it in a 2L starter, and figure I have about 300B cells when it finishes.

I did the pitch rate calculator on Brewer's Friend and found that 20 billion in a 1.5l starter will end with 3/4 the number of cells that 100 billion would. If you were to do 2 steps, the second step would provide almost 95% as many cells.

Apparently, the amount of food in the starter is a bigger determinant than number of starting cells.

Of course that's just how many cells. No idea if there would be a substantial difference in yeast health.

 

[MagicMatt]

I did the pitch rate calculator on Brewer's Friend and found that 20 billion in a 1.5l starter will end with 3/4 the number of cells that 100 billion would. If you were to do 2 steps, the second step would provide almost 95% as many cells.

Apparently, the amount of food in the starter is a bigger determinant than number of starting cells.

Of course that's just how many cells. No idea if there would be a substantial difference in yeast health.

Great point, as vitality seems to be becoming a bigger factor in determining starter health than viability. See another nice xBmt regarding this exact topic:

http://brulosophy.com/2015/06/29/yeast-pitch-rate-pt-2-viable-cell-count-vs-vitality-exbeeriment-results/

Also, don't forget the (somewhat) importance of inoculation rate - it's not just how much 'food' there is, but rather how that amount relates to the number of viable yeast cells. There seems to be some doubt as to how important this actually is now, but I'd still recommend following the guidelines of trying to keep the inoculation rate between 25 and 100 million/mL for optimal growth.

 

[BigTerp]

Thanks again for the in depth explanations. It's been helpful.

So, I'm now building up a starter for a 20 gallon batch that is planned for next week. I'm using YeastCalc to determine what steps I need and how big each step needs to be. I'm making my first step at 1 liter and then splitting that into 500 ml each to split with my buddy for his 10 gallons. Using the 2.4 billion per ml number that first step will give me 183 billion cells. So I can safely assume I now have a 500ml slurry of about 90 billion cells. I plan to build that into another 1 liter starter which will give me 226 billion cells. I want to also split that in half to save for a future batch. So I should be able to label that container as having 113 billion cells, correct? And not have to fuss with figuring out slurry amounts and concentrations per ml? The remaining 500ml slurry will go into a 2 liter starter to give me my final amount for 10 gallons. Using these starter sizes each step is keeping the inoculation rate within the range that YeastCalc reccommends (25-100 millions cells per ml) which will help with the vitality and health of my yeast. This is using 1.037 OG starter wort with each step. Should the OG remain the same for each step? It will be going into 10 gallons of a 1.054 wort.

 

[BeardedBrews]

So I should be able to label that container as having 113 billion cells, correct?

Your numbers look correct to me.

Should the OG remain the same for each step?

Stick with the same 1.037-1.040 for all of the steps, the yeast won't mind the change in the actual batch.

 

[MagicMatt]

Thanks again for the in depth explanations. It's been helpful.

So, I'm now building up a starter for a 20 gallon batch that is planned for next week. I'm using YeastCalc to determine what steps I need and how big each step needs to be. I'm making my first step at 1 liter and then splitting that into 500 ml each to split with my buddy for his 10 gallons. Using the 2.4 billion per ml number that first step will give me 183 billion cells. So I can safely assume I now have a 500ml slurry of about 90 billion cells. I plan to build that into another 1 liter starter which will give me 226 billion cells. I want to also split that in half to save for a future batch. So I should be able to label that container as having 113 billion cells, correct? And not have to fuss with figuring out slurry amounts and concentrations per ml? The remaining 500ml slurry will go into a 2 liter starter to give me my final amount for 10 gallons. Using these starter sizes each step is keeping the inoculation rate within the range that YeastCalc reccommends (25-100 millions cells per ml) which will help with the vitality and health of my yeast. This is using 1.037 OG starter wort with each step. Should the OG remain the same for each step? It will be going into 10 gallons of a 1.054 wort.

I disagree with BeardedBrews - those numbers don't quite add up to me. I'm basing this on some guesses as there is missing information needed in order to correctly calculate (like how much yeast you're starting with, and what date it was harvested). The reason I say I think something is amiss is because even if you use the freshest smackpack available (containing 100 billion viable cells), with an initial 1L starter you'd only end up with about 148 billion cells, not the 183B you proposed.


So let's figure this out correctly. Can you provide the following:

- How many cells are you starting with?
- What is the harvest/mfg date on the cells you are starting with?
- Is it fresh from a smackpack or vial, or is it pre-harvested?
- Stir plate or no stir plate?

 

[BeardedBrews]

I disagree with BeardedBrews - those numbers don't quite add up to me.

I made the assumption that he had 50B viable, and was using a stir plate.

Chris and Jamil say he would have 123B in the 1L, and Kai suggests 194B. For the purposes of this exercise I have to think 180 is close enough.

If we take the lowest estimates on each step, that would be 60B into the next starter to grow to 170B, then 85B into two liters to grow to 280B. It would be a 0.56M/mL/P pitch which is a thin, but not unreasonable.

If you like the Kai numbers then he'd have about 390B in the final two liters, which is right on target at 0.78M/mL/P.

If they were my starters I would do 2L on each step and then decant down to 1L before splitting. The extra starter wort is only $2.00 so I'd err on the high side instead of rolling the dice on 10 gallons of wort.

 

[MagicMatt]

I made the assumption that he had 50B viable, and was using a stir plate.

Chris and Jamil say he would have 123B in the 1L, and Kai suggests 194B. For the purposes of this exercise I have to think 180 is close enough.

If we take the lowest estimates on each step, that would be 60B into the next starter to grow to 170B, then 85B into two liters to grow to 280B. It would be a 0.56M/mL/P pitch which is a thin, but not unreasonable.

If you like the Kai numbers then he'd have about 390B in the final two liters, which is right on target at 0.78M/mL/P.

If they were my starters I would do 2L on each step and then decant down to 1L before splitting. The extra starter wort is only $2.00 so I'd err on the high side instead of rolling the dice on 10 gallons of wort.

Ah, agreed now sorry. I was using YeastCalc and forgot to select the stir plate option (I'm used to using HomebrewDad's where it defaults to stir plate).

Carry on.

 

[BeardedBrews]

(I'm used to using HomebrewDad's where it defaults to stir plate)

That's my go-to source too. Quick and easy, and probably close enough

For all of this math I really think that Terp owes us both a pint of this magical 20-gallon batch!

 

[BigTerp]

I disagree with BeardedBrews - those numbers don't quite add up to me. I'm basing this on some guesses as there is missing information needed in order to correctly calculate (like how much yeast you're starting with, and what date it was harvested). The reason I say I think something is amiss is because even if you use the freshest smackpack available (containing 100 billion viable cells), with an initial 1L starter you'd only end up with about 148 billion cells, not the 183B you proposed.


So let's figure this out correctly. Can you provide the following:

- How many cells are you starting with?
- What is the harvest/mfg date on the cells you are starting with?
- Is it fresh from a smackpack or vial, or is it pre-harvested?
- Stir plate or no stir plate?

Thanks for the response.

I had a little over 25ml of compacted yeast cake from a previous starter that was completed on 12/6/15. Using the 2.4 billion per ml I end up with 60 billion initial cells. Punching that into yeast calc, along with harvest date, that gets me 42 billion viable cells. I use a stir plate and using Kai's calculations that gets me 182 billion after a 1L starter.

Correct me if I'm wrong, but the reading I've done suggests that Kai's research is based on equations that were produced from stirred starters. Jamil's used the scaling of stirred starters to come up with his data.

So, to me, it seems that Kai's research MAY be more accurate when it comes to figuring out what you end up with when using a stir plate for starters. Which is why I've been using the data that Kai's equations produces.

 

[BigTerp]

For all of this math I really think that Terp owes us both a pint of this magical 20-gallon batch!

HA!! I certainly do appreciate the help!! I've really been trying to dial in different parameters of my brew process to, hopefully, step my finished product up to something I can consistently produce. Yeast pitching rates in general are, for the most part, straight forward. But figuring out what you have on hand and how to get to that optimal pitching rate is a bit of guess work. I'm hoping to find a consistent approach I can stick with. The converstaions in this thread has really cleared things up for me.

I'm also brewing 5 gallons of a Belgian Wheat (blonde), my wifes favorite, this weekend to enter into a homebrew competition in March. It will be 5.25 gallons using a fresh vial of WLP400. The numbers for that batch will be a piece of cake to obtain now. :rockin:

 

[BugHunter]

I haven't noticed this, and in fact have noticed the opposite - that if you underpitch, you'll end up with a different tasting beer. Mostly because it under attenuates and leaves a slightly more malty/sweet taste. I've never had any side effects of over pitching other than an incredibly quick start to fermentation (within 2 hours).

Overpitching can lead to a depressed ester production rate. If you're brewing a clean beer that's not a bad thing, but if you're brewing something like a Belgian that relies on yeast esters as a core flavor component it can definitely be picked out. The beer would, very likely, end up tasting bland and watered down. Between over and under pitching it's definitely the less serious issue, but it's still one to watch out for if you're trying to pin down why a batch turned out less than stellar.