Update, December, 2022. My monitoring of the viability of this yeast product over time revealed very good news. The packaging of this yeast (manufactured December 2019 and purchased April 2020) said "Best before the end of December, 2021", and I now have great evidence that this is quite a conservative estimate. today (December 8, 2022) I tested two batches of yeast using the method described below, and compared the results with what I obtained when the yeast were newly purchased in April of 2020, nearly two years ago. First, I found that a batch of yeast stored since April, 2020 vacuum-sealed as described tested at 110% compared to the April, 2020 value (see left-hand picture above). (Obviously the yeast did not get better with age; I attribute the extra 10% to a slight change in experimental protocol leading to a slightly higher temperature in the test. My bad as an experimentalist, but minor in this case, in my view.) Second, yeast that had been stored in the -20 freezer for one year vacuum sealed, then stored for another year in an closed jar in the refrigerator at 40 degrees, scored at 85% in the test (right hand picture, above). Since this test was run under the conditions where the vacuum sealed yeast scored 110%, I think it is reasonable to say that storing these yeast even for one year in the fridge cost only about 25% of their viability. Thus, if you stored the yeast that long in the fridge, the most you would have to do to compensate for degradation is to use 33% more yeast than the recipe calls for.(And, just for the record, previously I used yeast that had been stored in the fridge for 9 months - and had scored at 100% in the "champagne flute test" described here - and used it in the normal amount to make pizza dough. The dough rose as normal, so this method of measuring CO2 release by monitoring the amount of foam produced in 10 minutes seems a good measure of potency in getting dough to rise.)I will continue to monitor and report on this yeast batch over the coming months and years.-----Original Review:I did a test with these "instant" yeast right out of the bag, based on a clever idea described in an excellent previous user review (Steve Thomas, Jan. 11, 2015), which I modified slightly (see below). The yeast were quite potent, based on their ability to convert sugar into CO2 (sorry, earth!). I've used the yeast to make several loaves of bread, and they work fine. Any problems I might have had I attribute to my inexperience. But I'm learning, and the bread gets better and better. I ordered this huge, one pound bag of yeast only because I was finding it impossible to get yeast in the little packets at a grocery store. Impossible. So now I have what may well be (see update above) a lifetime supply.Some user reviews of this product say that these yeast on arrival were not very active, but I wonder whether this might not have been due to the "middleman" who is handling the product. If you look on Amazon, you will see the same product available from many distributors, of all sorts of products, and the distributors seem to change from month to month (for example, the supplier of the bag I received no longer has this yeast available). How long was the bag on the shelf? How well was it stored by the distributor? These are more likely to determine the quality of the yeast you receive, rather than some product irregularity or failure in quality control by the manufacturer. So, as a first time buyer from a particular seller, you are in the tough spot of trying to assess the track record of the distributor (is the majority of their product line foodstuffs that require some care about storage conditions and expiration dates? What is THEIR record according to Amazon shoppers?). Then, once you get your stuff, check the packaging to make sure it is not over or close to its expiration date (mine on arrival was vacuum sealed and only about 5 months old, with a year and a half left on its stated lifetime), then test it to see if it is up to snuff. If it fails to deliver, you might have a chance to get a refund from a responsible re-seller.Once it arrives, portion most of it out for long term storage and put the rest in a jar in the fridge. Use standard test conditions so you can occasionally test the yeast to see if they are still potent. I plan to do this at least every time I open one of my freezer bags of stored yeast from this batch I just received. Since the yeast arrived vacuum sealed, I used my vacuum sealer to prepare the portions destined for the freezer. I weighed out portions of a couple of ounces into coffee filters (to keep the yeast from getting sucked into the vacuum tube), carefully wrapped them, put each one into a plastic vacuum bag, and vacuum sealed them. Regarding storage in the fridge or freezer, you want to avoid giving the yeast an opportunity to pick up moisture. The best way to do that is to allow the fridge jar or freezer bag to warm up to room temperature before opening it. If the yeast is cold when you open the container, it will act as a condenser for any moisture that is in the air.You might argue that you don't need to do some kind of lab test to check your yeast, just see how the bread comes out. Personally, I would rather do the standard test described below. Bread baking requires various ingredients and conditions, and it seems to me if a batch fails there might be a number of reasons for failure. Furthermore, why commit a lot of time and materials to making bread, only to find out later that the yeast had lost their punch? Testing ahead of time avoids that risk, and is simple and quick.I modified the Steve Thomas protocol in order to get by with less yeast for each test. I used a cheap glass champagne flute and added a quarter cup of 115-120 degree water and immediately dissolved a half teaspoon of sugar. I stirred to dissolve the sugar and monitored the temperature till it had cooled to as close to 110 degrees F as I could get it. Then I stirred in a teaspoon of instant yeast, making sure to distribute it evenly in the water, including rescuing any clumps that are clinging to the side of the flute or floating on top. Let stand 10 minutes, which I timed from the moment I added the yeast, so the 10 minutes includes the time I stirred to disperse the yeast. I put a mark on the glass at the level of the originally added quarter cup of water, then a mark for where the "foam" had reached after 10 minutes. With my yeast, on arrival, the foam at 10 minutes had gone up the flute 1.5 inches from the water mark. Now, in a year's time, let's say, when I thaw out the next portion of vacuum sealed yeast, the first thing I'll do it repeat the test, in the saved marked flute, and see how the thawed yeast perform.I used the champagne flute because it is narrow, and I thought that would provide better accuracy, especially since I was intent on cutting down on the yeast (and therefore water) being used in the test. If you don't have a champagne flute or other piece of narrow glassware, you might want to stick to the Steve Thomas protocol using a cup measure. The point is that you can use a previously described test, or devise your own test. Either way, use it on yeast you know to be good quality, then in the future you can use exactly the same conditions to see how the yeast currently being tested measure up to the "standard". If it's beginning to lose it's punch, it's time to use more in the recipe, and/or re-order.