Hop Utilization: Problem?
Often people have 5-10 gallon recipes they want to scale down to one gallon recipes. Common advice is to scale the recipe down by dividing the ingredients by whatever number of gallons the original recipe is by how many gallons you want in your version. What often comes up is a question about hop utilization and whether you need to increase the amount of hops due to the lower boil volume. There is division on the issue. Some people say that lower boil volume means lower utilization, therefore you need more hops to reach the same IBUs and flavor. On the other side, people say if there is a difference it is negligible and indistinguishable from the same recipe produced at five or ten gallons. My take is: it depends on many factors.
One issue is how big the original recipe is. If you are scaling a recipe from five to one, the difference is likely insignificant. However, if you are scaling from fifteen gallons to one gallon or maybe a commercial-size recipe to one gallon, the difference may be noticeable. Since the boil contents are proportionally the same, there should be no loss of flavor or aroma from the hops but when the boil volume decreases it is more likely to reduce the isomerization of hop acids which will affect the bitterness from the hops. Personally I do not concern myself with hop utilization for this reason but if you make a lot of hoppy beers this may be a concern.
Another issue is how you convert the recipe. If the recipe is a partial boil and you want to do a full boil one gallon batch, there is going to be a significant difference in hop utilization because you are changing the size of the boil to the proportion of ingredients in it, which will affect all aspects of hopping. If you are adjusting a partial boil to a full boil, you need to rebuild the boil schedule rather than just dividing by five.
Third, any flaws in your process at the one gallon level may affect hop utilization. If you have too much or too little boil volume due to trying to operate on a smaller scale can affect utilization. I'll talk about the challenges of small batches below but because the batch is smaller, the margin of error grows and this can affect your final product.
Once you have a handle on your small batch process, as long as you are scaling fairly normal recipes from regular homebrew sizes to smaller sizes, hop utilization will probably not be an issue. However, if you are highly technical about your brewing, you may want to do some math to figure out if you need to make an adjustment. I've found some of the brewing software, likely due to rounding issues, will boost hop volumes at lower batch sizes.
Challenges of the Small Batch
Since smaller batches have smaller mash mass, you will lose heat more quickly than a large mass that can retain heat. You need to keep this in mind for your mash. You don't want to shoot for a 150F mash and end up spending half the mash time in the 140s or 130s. You may want to think about using a smaller container than your cooler mash tun or working on the stove to better control heat. The more surface area you give the mash the faster it will cool.
On the other hand, since the mash mas is lower, it is easier to overshoot temperatures and quickly overheat the mash to the point you denature the enzymes and cannot convert the starches. This is especially concerning while any direct heat source is heating the mash. If you infuse water and then heat there is a chance to overshoot as the heat source is heating the mash. If you are keeping a heat source active to try to maintain a certain temperature it's also easy for the temperature to start to climb. Unlike a ten pound mash, a two pound mash can heat very quickly while you are not looking at the thermometer. So if the heat is on, watch the thermometer closely.
Also consider how your efficiency, boil volume, etc. will change if you are using a different mash technique on smaller batches. I have a cooler I use for large mashes but for a gallon batch I go BIAB on the stove. With BIAB I get better efficiency than I do with the cooler and batch sparging even though I have the same crush using the same mill. So that's something that has to be considered in scaling the recipe up. You may find it too hard to control temperatures in a cooler (unless you have a very small one) at the one gallon level.
Adjustments to the heat source and boil kettle may be appropriate to maintain consistency. If you boil on a system set up for larger batches, such as a turkey fryer, you are going to apply a lot of heat to a small amount of liquid. You increase the chance of added carmelization due to the smaller boil volume and that can affect the flavor.Additionally, you need to contemplate changes to boil off. With a smaller volume in a pot designed for larger batches you will end up with a larger surface area than normal. This increases evaporation.
Using your regular fermenters (buckets and carboys) can leave a lot of headspace. While I believe the headspace is a non-issue if you are fermenting for a few weeks, if you move your fermenter a lot of let it sit for months the CO2 layer created by fermentation will start to let oxygen diffuse into the beer and you might get some oxidation or acetobacter growth. Consider getting some smaller fermenters. Some people have found 1-3 gallon HDPE buckets. I like to use five liter wine jugs. They are easy to work with and leave some headspace for krausen. With a gallon of beer I get a little less than a liter of headspace. Sometimes I do need a blow off tube.