YUMMIES FOR YEAST

MAKING YOUR YEAST WORK BETTER

by Graham Sanders

The art of making beer is as old as civilization itself. It’s a wonderful, simple process that has been in place since the beginning of time. Just sprout some grain (usually barley), crush it, add water, extract the sugary liquid, add any flavouring (hops, herbs etc) and let the yeast do the work. For thousands of years this process has worked great. Beer styles have evolved and matured without people knowing much about yeast and specifically how these cells work. So why is it that everyone seems concerned about yeast? All of a sudden, everyone is saying yeast needs this and that or your beer will not be right. Have we really neglected this area, or is a lot of what is being said to be taken with a grain of salt?

This article will look at the needs of yeast, and in particular what a craftbrewer needs to know to look after their yeast and get the best from these wonderful creatures. This article will not get bogged down in science and jargon, but give you down to earth advice on what you need to know.

Why has yeast health become so important?

It is not that long ago we had no real idea about yeast. Fermentation and mashing was a magical process. The march of science in the late 19th and early 20th Century changed all that. By the 1950’s, brewers had an excellent understanding of the complete brewing process. It is, after all, just a series of chemical reactions. Every aspect of malting, mashing, boiling, fermentation and lagering could be rationally explained away by various chemical reactions. That knowledge holds true today as it did all those years ago. The practices of today, from the smallest craft-brewer to the largest commercial brewers still rely on this knowledge. Even with the advances of the large commercial breweries to produce more with less, the basic understanding of the brewing process has not changed.

There was one area that still was mysterious, and that was how cells worked. It is only recently that science has been able to understand the internal workings of a cell, and it has turned out far more complex than they have ever imagined. When you look at the brewing process, there are only two areas where living cells are involved. First is when barley is germinated to make malt, and the second is fermentation. The advances in science have unlocked many of the secrets of how these cells operate. As you would expect, it would be the commercial interests that would be driving this, wanting to use the knowledge to either cut costs, or to get a better efficiency and yield.

It is not surprising therefore, that the main advances in brewing in recent years have been in malting and yeast performance. Malts can now be made to EXACTLY the brewer’s requirements, the understanding of the process is that good. That is because while living cells are involved, malting is still a relative simple process. We can see the results of this research in the malts now available. Yeast and fermentation on the other hand is far more complicated. Science has now given us a good understanding of how yeast cells work, and the commercial brewers are using this information to get better performance out of their yeast, even if its only a fraction better. When I talk about a fraction, I mean even if it’s an extremely small improvement in efficiency, it benefits the large commercial brewers by the sheer weight of volume they push through.

Does a craftbrewer have to worry about their yeast?

As I have said, the commercial interests are driving a lot of this research, mainly to increase profits, and many of these improvements are extremely small, but significant if you mass-produce a product. The first thing you must realise is most of what they do will not affect the average craftbrewer and how they brew. On the same note, there is also information that does relate to craftbrewers that can be taken on board.

What a craftbrewer needs to remember is to look at the whole process, and not consider things in isolation. Remember that beer making has been performed for thousands of years with no worries. When you hear about the commercial brewers doing this and that, it may not apply in any way to a craftbrewer. This is the mistake many make. They take data from a commercial application and try to apply it to a small application, without looking at the whole process. In many cases, what applies to someone making thousands of litres at a go just doesn’t apply when you make 38 litres of beer.

So what is important to a craftbrewer with their yeast?

The craft-brewer should have two aims with the yeast they use
– There should be plenty of them
– They need to be in good condition

Now there are many ways of achieving this, and no one way is the best. All have advantages and disadvantages, but if you obey this golden rule you won’t go wrong.

Plenty of them means the numbers should be high in the wort when fermentation starts – the sheer number of them. Many people talk about optimum pitching rates but that is misleading, as yeast will continue to grow on the wort. What I am talking about here is the final number of yeasts fermenting the beer. The number they talk about (which you can forget as quick as I tell you) is about 10 million cells per ml of wort. The two ways to achieve this are to either allow the yeast to breed up to that number (by supplying everything yeast needs to grow in the wort), or actually pitch near that number in the wort. Either way will work fine.

Good condition means they are healthy. You would think that any yeast in wort would be healthy, having all the nutrients its needs to grow but this is not so. The example I would give is grass growing on your lawn. As long as you water, weed and mow it, you get a good lawn. But the grass is not growing ideally. The grass can grow better. We all know the lawn will perform far better if we fertilize it, or top-dress it. So it is with our yeast. They are doing a great job whatever way we use them, but there are ways to make them work better by having them in perfect condition, or if you like, fertilized.

So let’s look at the two common ways yeast is pitched into the wort, dry and liquid, and apply these rules.

DRY YEAST

This is an almost foolproof method of pitching yeast, and it is for this reason that dry yeast is sold with brew kits the world over. Nothing can be simpler than just ripping a packet open and pouring the contents into a fermenter. This is the way the kit manufacturers want it to be. The kit people have basically got it right when you add a kit with a kilogram of sugar and pour the dry yeast on top. It does the job nicely, fermenting out the sugars, and leaving just enough body leftover in the final product to make a beer. This is not by accident, as the yeast pitched by this method has been grown specifically to do this job, but more importantly, the numbers supplied are just right for this procedure and ingredients.

The craftbrewer strikes immediate problems the moment they stray from the ‘kit and kilo technique’. The most common problem is when they substitute malt for sugar, and find the beer hasn’t fermented properly, tends to be sweet, or does not carbonate the bottles. That’s because basically the yeast can’t handle the extra workload of all the complex sugars malt has. Why?

Yeast manufacturers go to great lengths to get their dry yeast right. Each yeast cell has been grown in ideal conditions before it is dried. In fact, the yeast in a dried yeast pack is about as healthy as you can get it. The yeast is in such good condition that it will reproduce in plain water, such are the reserves of nutrients packed into each dry cell in the packet. Lets look at the two conditions a craftbrewer should be concerned with to understand why they would have difficulty when they add extra malt.

- There should be plenty of them

There is a lot of yeast in a dry yeast packet. In fact, for a 20 litre batch with just a simple kit and kilogram of sugar there is just enough to do the job. The numbers are just right in a 5-gram packet to be able to ferment this type of wort, but nothing left in reserve. The yeast is basically exhausted when it finishes, yet is able to ferment these simple sugars, but in a wort that contains more malt and less refined sugar you need more yeast. There are extra stresses placed on a yeast in this environment, including osmotic pressure (from having a more concentrated environment caused by the malt, this puts pressure on the cell wall) and the fact they have to deal with more complex sugars which require the yeast to work a lot harder to convert it to alcohol. What this means is one packet of yeast of about 5 grams wont be able to ferment the beer properly. The final yeast numbers are not high enough to deal with the extra stress. So what yeast you do pitch from one packet falls short and tires out and dies before the job is completely done, if you like they just get plain exhausted. To get the ideal numbers you actually need to pitch more than one 5 gram packet of yeast.

When you have an all malt beer, you should pitch for a 20-litre batch the following amount to get good numbers of yeast

Ale Yeast in average wort – 10-15 grams of dried yeast (2 average packets)
Lager yeast in average wort – 20 – 30 grams of dried yeast (4 average packets)
Beers with higher than 5.5% v/v final alcohol– double the rate

- They need to be in good condition

There are two areas the craftbrewer should be concerned with, the condition of the packet of yeast and helping the yeast become healthy again from their deep sleep. Even though the yeast is dried, it is still alive and like anything will die if its not treated right. The yeast you buy should not have been mistreated. It should be stored at as low as temperature as possible, and should be as fresh as possible. A craftbrewer should always check with his local shop how they treat their yeast, and how old it is. Dried yeast can lose 50% of its viability within six months if treated wrongly. Good brew stores ensure they have fresh yeast in stock, check and get rid of old yeast from their stock when its gets too old. Just ask your local store how long they keep their yeast for. It may surprise you the answer you get.

You know yourself that if you are suddenly woken up from a deep sleep it takes a while to gather your thoughts. In a way it’s the same for yeast. Yeast need time to get it together as they come back into life. You may have that morning cup of coffee to get going, but for yeast their coffee is a good drink of water to reawaken themselves. The craftbrewer should give their yeast their morning cup of coffee before you pitch – it’s called rehydrating your yeast. Instead of throwing your dried yeast on top of the beer, you rehydrate it first. This ensures you get the maximum number of living yeasts to pitch and they are healthy.

Why should you do this? To understand this you have to imagine the yeast cell is like a balloon with the balloon being the cell wall. The cell wall not only stops the inside of the cell spilling out, but it controls what goes in, and out, of the cell. Now the dried yeast is like a balloon still in its packet, its ready to be blown up, but the cell wall is not working yet. If you put the dried yeast in your beer directly, the yeast will start to blow up (absorb water) but the cell wall wont be able yet to control what goes in the cell, and what goes out. Many yeast cells will absorb water and other compounds uncontrollably. They will be damaged from the osmotic pressure of the sugars in the wort (the cell ruptures) and as a lot of sugar enters the cell uncontrolled and minerals leave the cell just as rapidly, the yeast will die. Many other cells are damaged and will not perform properly. The yeast manufacturers realise this of course, and give you an extra yeast packet to compensate for this. This is acceptable as long as you are just adding this to a kit and kilo of sugar. The beer you make with this is still quite watery. This sort of damage becomes far more evident with all malt brews.

To avoid this all yeast should be rehydrated. If you put the yeast in ordinary tap water, it is not under the same stress, and can slowly blow up and be perfectly healthy to pitch. Every yeast manufacturer will have specific temperatures to use, and you should check with your brew shop, but generally you should add your yeast to a cup of water that is about 35C, stir, wait 20 minutes, then pitch this.

Yeast Nutrients

There is little need to worry about nutrients with dried yeast. They are packed up with everything you need. The manufacturer has seen to this. However they do benefit in their growth if a little oxygen is in the wort at pitching. A good stirring with a large spoon to get oxygen in the wort will help the yeast grow better. Other than that, nothing else is needed as the yeast manufacturer has added all the essential nutrients in the packet and the wort has the rest.

Dried Yeast Summary

All you need to worry about is the following:
1. Pitch a lot more yeast.
2. Rehydrate your yeast.
3. Make sure the yeast is fresh and looked after.
4. Add some oxygen to the beer before pitching.

LIQUID YEAST

Liquid yeast is the opposite to dry yeast. They are hydrated (alive) and therefore have to be in some form of liquid or gel to stay alive. The reason there is a market for this yeast is simply that the dried yeast manufacturers do not supply the full range of yeasts that a brewer can use. The day that happens I can see the liquid yeast market dry up (sorry about that joke).

The first thing that strikes someone about liquid yeast is that there seems no right method of using it. Everyone does something different. Some make big starters and pitch, some reuse the yeast from previous batches, some ferment out a starter and decant the liquid off to pitch the slurry, a few breed up fresh batches. Yes it’s very confusing and everyone will tell you that their method is right. Well that’s correct in a way, but other people’s methods are also right. First lesson of liquid yeast is - THERE IS NO ONE METHOD OF USING LIQUID YEAST THAT IS THE BEST.

Again lets look at the two areas a craftbrewer needs to worry about.

- There should be plenty of them

The difficulty here of course is counting how much yeast you have. There is no way most of us can do that with any certainty. But this has been covered in other yeast articles of this web page so please read these to get an idea how to get the right numbers. When you read these articles, you can see that there are two ways to get the right number of yeasts. You either breed them up in a starter culture, stepping up until you get the numbers right, or, you already have the numbers present to pitch, for example, in the yeast slurry from a previous brew. You can also buy liquid yeasts that are in pitchable quantities where the manufacturer has got the right number in the vial or container. All these ways will achieve the same result, that is the right number of yeast. The biggest problem craftbrewers face in using liquid yeast is that they under-pitch the number of yeast they need

But there is more. Remember again, we need to look at the whole process, the bigger picture. So let’s look at the next area.

- They need to be in good condition

This is the area that a lot of people find difficult to understand when using liquid yeast. Yeast cells are a lot like us in a way, they are born, grow, reproduce, age and die. A yeast cell will have between 10 and 30 divisions (if you like, have 10 to 30 babies) before it eventually dies of old age. Yes, they die and do not live forever. In any population of yeast there are young, medium and old yeast cells. And like people, they behave like us. 10 young people can do the work of 30 old people. 10 healthy people can do the work of 100 starving people. So it is with yeast. So when you look at someone’s method of using yeast, you should look at the yeast dynamics of the population. So when I say the yeast should be in good condition, I mean look at the population as a whole.

So for an example lets look at two ways of using liquid yeast. Fresh yeast starter verses pitching on the slurry of a previous brew. The population dynamics are -

Now both will give the same result, a great beer, but the yeast dynamics are totally different. Both are in good condition, but in different ways. The fresh yeast starter makes up for not having ideal numbers of yeast by having a young hard-working population, while the slurry may have a much older sluggish population but makes up for it by sheer weight of numbers of yeast, a lot doing less work. The end result is both combinations are healthy enough to ferment the wort properly, even though they are so different.

So now let’s look at using the yeast from an old bottle of beer, where you make a very small starter.

This is not good because the yeast is old and in very low numbers. It will take a long time to breed up, the lag time will be extensive, and the chances of infection greatly increased. Mutation of the yeast is a real possibility. There is absolutely nothing going for the yeast here. It needs a lot of work to convert at least one of the factors to a more satisfactory level. For instance, a larger starter will have more nutrients, a progressively younger population and more numbers. This is the essence of using liquid yeast, to control one or more of these factors to improve the yeast dynamics.

If you can get one or more of these conditions very high, it can compensate for other conditions. If you read my article on yeast culturing, you can see I use yeast nutrients and yeast cultures from agar plates.
My yeast population dynamics are with my starters.

My initial pitch will be lower than normal, but I compensate greatly in other areas. I set up the ideal conditions for rapid growth and start from a position of strength. By the time the yeast is at numbers to ferment, it is near perfect condition, even if the pitching rate may be lower than most people recommend. Again the point is look at the whole picture with liquid yeast and your brewing in general.

So when using liquid yeast look at the big picture. NO ONE WAY IS BEST, but there are wrong ways.

Yeast Nutrients

I gave the example earlier that while grass grows with plenty of water, it will grow better with some fertilizer. So it is with yeast. So one way of making the population dynamics of your yeast better is to feed your yeast. Now what I mean here is feeding them above the level of what the wort provides.The wort, even from a total grain mash, does not provide the ideal level of all the nutrients yeast requires to grow. Many people are of the mistaken belief that the wort is the ideal medium for yeast to grow. The wort will provide all the nutrients for yeast to grow, but that doesn’t mean its perfect. There are some critical areas we must consider when looking at yeast nutrition. Lets look at some of the important ones.

Oxygen

This has been beaten to death by almost everyone in every book, forum or web site, so there is no real need to cover this in depth. The only thing to add here is to consider a secondary hit of oxygen/air as the yeast is growing, or even better, regular small doses of oxygen during the yeast growth phase. The level of oxygen the yeast absorbs is one of the determining factors in the final number of yeast cells and their health in the wort. The yeast in its growing phase has the capacity to remove all the oxygen from the wort in as little as 20 minutes. It is highly recommended to keep adding oxygen while the yeast needs it, before the yeast switches to anaerobic respiration (before you see bubble rising up).

Multiple small injections are preferred as it is feeding the yeast exactly what it wants without putting stress on the yeast or oxidising the wort with excessive oxygen. This is more important with starters than the final pitch, as the yeast is in its maximum growth rate. It is better to have the starter in good condition than to rely on the wort to recover yeast numbers. It is also more practical with starters. It is more important to get the starter populations up, than under-pitching the wort and trying to build it up after the final pitch. The key here is to introduce either a higher than normal level of oxygen by using pure oxygen, or keep the oxygen up to the yeast by using a pump on a regular basis. Making starters in plastic PET bottles and squeezing fresh air in every hour does wonders and is a cheap and easy way to do this.

Zinc

In the last few years one nutrient above all other has come to be very important to yeast growth. There was never a problem with this nutrient until recent years when brewers switched from copper boilers to stainless steel and noticed poor yeast performance. Lack of zinc was identified as the cause. It was discovered that copper has extremely low levels of zinc in it, while stainless steel has none, and the copper boilers were actually adding zinc to the wort at very low levels. Further zinc was being added in zinc coated metals used in the brewing process. As breweries used more and more stainless steel, these sources of zinc disappeared, with zinc deficiency becoming a problem.

To understand this further, zinc is very important to yeast growth and its internal functions, especially enzymes. It is an essential micronutrient. Now being a micronutrient, the levels required are extremely low, 0.4-0.6 ppm available zinc in the wort is considered the optimum level for healthy yeast growth and performance. The level of zinc and other micronutrients are extremely high in grain and malted grains, levels quoted are easily 100 times and more to what a yeast needs to grow (as well as other micronutrients, like copper, manganese and magnesium). This makes sense because the potential young plant also needs zinc to grow. So why doesn’t this transfer over to the wort?

These micronutrients are easily extracted from the grain into the wort and into the boiler. Before the boil the level of zinc and other micronutrients in the wort are very high, more than enough for yeast to use. When you boil the wort a strange chemical reaction takes place. What scientists have found is when you boil the wort most of the micronutrients combine with the break material (proteins and tannins) and precipitates out in the hot and cold break, so is left behind. The micronutrients are stripped out of the wort. For instance, copper levels drop some 80% and manganese levels drop by 60%. Most micronutrients will be reduced significantly in the wort as they relocate to the break material, but the important fact is there is still plenty left in the wort for the yeast to absorb, use and grow. So in general the micronutrient level of the wort after a boil is not a worry for a craftbrewer.

But there is one micronutrient that is the exception - Zinc!!!!!!!!!!. Over 99% of the available zinc drops out in the break. The typical level of zinc at the end of the boil is only 0.1ppm zinc in the wort. It is at the barest minimum level. Yeast will use all this zinc and more if it is available. Yeast requires 0.4-0.6 ppm ideally, and will perform far better if they can get it. The addition of a tiny amount of zinc has a dramatic effect on yeast growth, no different to adding fertilizer to your lawn. But like fertilizer too much will have detrimental effects. This includes increased medium chained fatty acids, producing off flavours such as stale, goaty, rancid and soap tastes, increased acetaldehyde, esters and higher alcohols. So you must add exactly the right amount of nutrient to help the yeast along. See the end of this article for some recommended rates.

On a side note, extract brewers face an interesting dilemma. If they don’t boil their wort there is enough zinc still in the extract because most extracts aren’t boiled as such, they are rapidly dehydrated. Most extracts when you boil them will form hot and cold breaks, just like normal worts. So the zinc level may/or may not be already ok in the extract they are using, it depends on how it’s made. Regardless of the extract, when they boil their extract and cause a break to form, they are also dropping out essential micronutrients out of their beers. The longer they boil the more that drops out. The problem they have is there is no real way of knowing how much zinc is in their extract and extract companies are reluctant to tell brewers this information. This is why many extract brewers sometimes see no benefit in adding a small amount of zinc, while other do see benefits. It depends how much zinc they remove from their extract and that depends on the initial zinc level of the extract, and how long they boil the extract for.

Calcium

There has been a lot written about the need for calcium in mash water to acidify the mash properly. But what is not so well known is calcium is a vital nutrient for yeast growth. 10% of a yeast cell is calcium based compounds, and the yeast requires calcium to grow and reproduce. Normally a good masher will make sure their calcium levels are high enough for the mash, a side effect being plenty of calcium would carry over to the wort for the yeast to use. Extract brewers and mashers who do not check their water properly may strike troubles with calcium deficiency with their yeast cultures.

Australia has generally very soft water with very low levels of calcium. An addition of ¼ teaspoon of calcium sulphate (plaster of Paris) certainly would help liquid yeast grow. But as I have said, generally most grain brewers and malt extract manufacturers ensure there is sufficient calcium when they mash. But if you do mash and your yeast is sluggish, check your calcium levels and consider a calcium addition. The level quoted for mashers is minimum 50ppm Ca, but a lot of that is lost during the mashing process where it is precipitated out. So mashers should aim for this figure, but extract brewers should look at having at least 20 ppm Ca in their water for healthy yeast growth and performance.

(pH)

Ok, pH is not a nutrient but relates closely to calcium and yeast nutrition. It was recommended using calcium sulphate as a source for calcium as it has a side effect of lowering the pH of the wort. This section applies equally to extract brewers here as mash brewers, even though mash brewers normally have a built in pH adjustment with mashing. Australian water is generally soft, with little buffering capacity and slightly alkaline (see pH article). Chemically, yeast is a big bunch of amino acids that produce enzymes to break proteins down and rearrange everything. Without enzymes, proteins are actually very stable and cannot be rearranged without either a lot of heat or some kind of acid. Its one of the reason living creatures are made of proteins. Enzymes are in all living creatures, it's what keeps us alive and enzymes are very pH dependent. In fact enzymes can stop working all together at the wrong pH.

Extract brewers in particular should consider the pH of their beers to ensure their yeast is in the best environment to work properly. Extract brewers will get far better yeast performance, less off flavours by adjusting the pH of their wort to about 5.2 before pitching their yeast.

Mash brewers should check their pH right from the start of their brewing. pH affects all aspects of the brewing process, but I won’t cover that here. A masher should also ensure that his wort at pitching is in the low 5’s as well for optimum yeast performance.

Nitrogen

Yeast is mostly a package of proteins/amino acids which make up over 50% of a yeast cell, and these are made with nitrogen. A term used often is FAN (free amino nitrogen) but I will stick to just nitrogen. Mashers in general have no difficulty here, as the grain will supply more than enough nitrogen and in the right form. We all know about how wheat adds so much it can cloud your beer.

Grain brewers should be aware that some grains do not add enough nitrogen, especially rice and corn. This is not normally a problem to all grain brewers, as these grains are used in relatively small amounts. Extract brewers should be careful though, as some extracts have a high proportion of rice malt in them. The amount of nitrogen in them can be very borderline, especially the “food malts” that are in the market. I could quote that you need nitrogen level at a minimum of 150 mg/L, but most of us have no way of measure that, so for most craftbrewers they need to just be aware of the problem. Similarly any recipe that has a high amount of sugar may also suffer lower than desired nitrogen levels. It can be the cause of poor yeast performance when you use liquid yeast when the nitrogen levels are low.

The yeast starves as it can’t find the stuff (proteins) to reproduce. This causes incomplete fermentation and off flavours from yeast self-digesting itself trying to maintain its nitrogen levels.

Unfortunately there is no real way a craftbrewer can tell if his wort has enough nitrogen. If you are an extract brewer and in doubt the addition of a small amount of yeast nutrient used in the wine industry, will help greatly. Most yeast nutrient that you can buy is basically nitrogen-based compounds supplemented with phosphates and zinc.

YEAST NUTRIENT SOURCES

I recommend anyone using liquid yeasts should at least consider additional nutrients for their yeast. Yeast nutrients packs used in the wine industry can be considered initially. They work fine when used as a supplement. You don’t need to add a full packet, but adding a little will help your yeast along. How much to add will depend on how deficient your wort is, and will require a little trial and error. But a good guide is a small pinch used in cooking. That’s enough that you can hold between two fingers.

A new product on the market is Servomyces, which is yeast that has been breed up with a lot of zinc in it. You add to the boil to increase the zinc level. This kills the yeast, the cells rupture and release zinc into the wort. You add this about 10 minutes before the end of the boil. Any earlier and you will again lose the zinc in your break material. This was designed to get around the German purity laws where you can’t add nutrient directly to the wort. I have used this and it works very well. To give you an idea how small amount you need, the rate you add it is 1 gram per 100 litres of wort. This can be difficult to measure.

I have a simpler solution that works just as well.

Firstly I recommend adding two packets of dried yeast – say 10-15 grams (for every 20 litres of wort) 10 minutes from the end of the boil. As I said earlier, the yeast manufacturers make sure each packet has all the nutrients yeast needs to grow. By throwing this into the wort, the cells rupture, releasing the nutrients that were going to be used by the dried yeast into the wort, so that your liquid yeast can use them instead.

I also like to add some extra zinc.

So I make up a zinc solution and add about 0.4 ppm available zinc to the wort at cut off. This replaces the zinc lost in the break. How do you make this? You don’t need to understand chemical formulae, just follow these couple of examples:

Zinc Solutions

Two common sources of zinc are Zinc Chloride (ZnCl₂) and Zinc Sulphate that normally has water attached to it (ZnSO₄.7H₂0). This hydrated ZnSO₄ is still a crystal, it just absorbs water from the atmosphere. You can get these from school labs or lab suppliers.

In these examples we are looking at adding 0.4ppm of zinc to the final wort.

ZnCl₂
Make a solution of 4.17g ZnCl₂ (dry) per litre of de-ionised or distilled water.
This solution contains 2mg of Zinc per mL.
So for 20L wort you add 4ml to get 0.4 ppm Zn in the wort

ZnSO₄.7H₂0
Make a solution of 35 grams of ZnSO₄ per litre of deionised or distilled water.
This solution contains 8 mg of zinc per mL.
So for 20L wort you add 1ml to get 0.4 ppm Zn in the wort

Try some of these tips to get happier yeast. The results will surprise.

Shout
Graham Sanders