Why do you have to rehydrate yeast? Why follow all of the meticulous steps to rehydrate yeast? After all, you could just chuck it into solution and it would work. But that’s not the ideal situation.
When you make yeast, you grow it in solution “in vivo” and it’s fermenting. Then in order to package it as dry yeast, you have to get it to shut down and put itself into suspended animation to preserve itself for later use. You have to tell the yeast to go to sleep and wait for another more advantageous time for it to live again. The reason you rehydrate yeast and do these steps is so that when you wake up the yeast, you give it the best conditions possible for the yeast to be successful doing what it is that you want it to do.
For example, if you’re going to run a marathon or compete in a decathlon, you don’t party with your friends until 3 a.m., drink a bunch of sodas, eat some snack cakes and then hit the track. What the rehydration step does is wakes the yeast up gently in a non-stressful situation and it feeds the yeast what it needs to get the biological processes started in the right way.
After the rehydration process, you’re taking the yeast and dumping it into your juice and/or grape must, which is actually a relatively inhospitable solution for many reasons. Throwing a bunch of yeast into a bunch of cold juice or freshly sulfited crushed grapes is kind of like throwing someone who can halfway swim out into the middle of the lake. It’s not a kind thing to do.
As previously stated, the reason for rehydrating yeast and following the rehydration protocols is to give yourself the highest likelihood for perfect fermentation success. The greatest concern of winemaking is when the yeast doesn’t work as planned. When you have a bad outcome from fermentation issues from stressed yeast these issues are difficult to fix. Whether it is a stuck fermentation or H2S production, these yeast fermentation issues are very time-consuming and expensive to try and fix while also maintaining wine quality. If you want to increase your likelihood of success you should rehydrate your yeast. If you’re going to rehydrate your yeast, you should take the time to do it right to ensure your maximum likelihood of success!
Each step of the rehydration process is done for a reason. There are reasons why you do each step. For example, if you are too aggressive with a pump or shear mixer when mixing up the inoculum you can create too much shear force, you can start fracturing the yeast cells and they start to die, and you don’t get the same impact or inoculum you get with a bunch of healthy, live yeast cells doing their thing.
To create a successful fermentation, you need to have a certain inoculum range (yeast cells and water you put into the fermentation), or cells per milliliter. And it is in the millions of cells per milliliter. You’d have a hard time even wrapping your head around it. Let’s just say it’s a lot. What happens is, yeast will grow and grow, but it needs to hit a level of concentration before it starts to ferment. When you measure out yeast in an inoculation rate, generally you use somewhere between 1 pound to 2 pounds of yeast per 1,000 gallons of juice or must.
Back to inoculation rates. 1 pound to 2 pounds of yeast per 1,000 gallons of juice or must is a big range. And there are cost implications to consider, right? Or are there, really? So how much cost difference are we really talking about?
Let’s say your yeast costs $22 per pound and you are making 1000 gallons, (just to make the math easy):
- The yeast cost to inoculate your wine at 2#/1000 gallons= $44.00 divided across 1000 gallons = $0.44 per gallon or $0.09 per bottle rounded up
- The yeast cost to inoculate your wine of 1#/1000 gallons=$22.00 divided across 1000 gallons = $0.22 per gallon or $0.05 per bottle rounded up
- So, the cost for the “insurance” to make it less likely to have problematic or stuck fermentations is about $0.04/bottle or $22.00 for a 1,000 gallon batch, does that sound expensive relative to the volume??? How much time and effort does it take to try to “fix” a problematic fermentation? In our experience it is quite a lot of time and effort which equals lost profit.
Let us break some things down as a comparative reference for our same 1,000 gallon batch of wine:
- A $1,000/ton grape price will set you back roughly $1.18 to $1.33 / bottle. (Move the decimal place 1 spot to the right for high end Napa fruit)
- A French oak barrel to age the wine in will cost you roughly $2.50 to $5.00 a bottle depending on 50% or 100% new oak, and some other assumptions
- A “nice” cork to seal your bottle, can be around $0.35 to $0.55 per bottle
- And the bottle itself . . . around $1.50 to $3.50 per bottle depending on the shape and weight of the glass
- So, how does $0.04/bottle of yeast “insurance” or even the full price of $0.09/ bottle stack up in the TOTAL cost of your bottle of wine? With only the things we have mentioned we are at $5.58/bottle to $10.47/ bottle and the yeast cost is not even 1% of the cost at either inoculation rate. We wish our car insurance was so cheap relatively!
So, a winery can go toward the skinny end of the inoculation rate for cost reasons, but they may have a lot of problems because they did not get enough yeast into the beginning of the fermentation process and allowed other bacteria and other wild yeasts to get established and do part of the fermentation with variable results. So, one of the things you’re doing with a higher dosage rate is ensuring a level of culture yeast concentration that’s going to outcompete the undesirable yeast and bacteria and helping the yeast inoculum survive the inhospitable conditions and risk factors.
In other words, when you crush grapes, there is yeast all over it from the vineyard. Yeast is around us everywhere. Some of it is bread yeast. Some of it is wine yeast. Some of it is yeast that who knows where it came from and what it is or what it will do. Most of it will die out, but some of it is strong enough and will start to grow and ferment in. So, when you’re doing your inoculum, you want to make it a certain concentration level so that the yeast that you put in there outcompetes the other yeasts and bacteria.
How does it outcompete? When the yeast is getting going in the beginning of fermentation, it starts by grabbing much of the nutrients out of the juice. The way yeast works is it brings the nutrition it needs into the cell. When it is time for the yeast to divide and multiply the cell shares what it has between the mother and daughter cell and divides in two.
So, if the culture yeast is grabbing all of the nutrition and good stuff, the other bad things don’t have the ability to get started. So, make sure you have a high enough inoculum, make sure your fermentation starts on time and make sure what yeast you’re adding is going to actually ferment what you’re adding it to.
To wrap up, measuring the inoculation rate is extremely important. And the more difficult the situation, the more you should add. There are a lot of risk factors – if your fermentation is going to be really cold or hot, if your fermentation is going to be particularly high-alcohol content, if you are going to try and use a yeast that is a weak fermenter, if there is a lot of mold, mildew or grape damage, if you add a lot of SO2 at the crusher for various reasons, etc. There are probably eight to 12 risk factors for fermentations, and a good winemaker should adjust their strain dosage based on those risk factors. If you start adding risk factors up and you are coming up with more than a few, you should be sure to rehydrate your yeast properly and you should use more inoculum to ensure your fermentation success.
Contact Ravago Chemicals North America (RCNA) to learn more about rehydrating yeasts and our full line of Maurivin products from AB Biotek.
