One of the promised premise of this site is to try and promote recipes from the ground up that represent the area in which they’re developed. The first step in doing so is sourcing quality ingredients and one of the ingredients that strikes me as being completely under rated in beer is sugar. Most brewers, at best, treat sugar as a necessary evil for use in bottle carbing and conditioning. At worse, it’s a four letter word ingredient. Some of that fear probably stems from historical fears of arsenic poisoning and more recent tribal knowledge passed around by homebrewers that too much sugar makes your beer cidery. That last notion seems to have been started by Charlie Papazian without any real justification or sources cited. I find it hard to believe that sugar is bad when it played such a prominent role in British brewing history and is still an integral part of many Belgian beers. The “best beer in the world”, Westvleteren 12, gets approximately 20% of its gravity from sugar. There’s a disconnect there between the lore and reality. That said, the first quest I’m undertaking is trying to source local sugars for coloring and flavor.
Where to source sugar though?
Well … we have a lot of sweet potatoes.
It’s fairly well documented that sweet potatoes contain lots of amylase (1,2, among other sources) , although the amount of conversion power they contain isn't clearly defined and seems to depend on the specific variety. Is it possible to convert the starch in sweet potatoes enough to produce a reasonable substitute for candi sugar? I endeavored to find out.
I searched out information on mashing straight sweet potatoes, but all the information found seemed to either add malt or alpha amylase powder to convert the starch in the potatoes. Clearly, that’s not how we do things around here, so I proceeded as follows.
I shred the potatoes and measured out 2 lbs. The shreddings were added to 2 gal of water. This mixture was heated up to about 140F and a pH reading was taken. It was a bit high (6.1) so I adjusted it down to 5.4 using 1 mL of 88% lactic acid. I brought the mash up to 146° F for a β-amylase rest. Here’s where things went a bit astray. I have a wireless thermometer sitting in the mash that I set to go off if the temperature rises about 148° F. I left the flame on it's lowest setting and I set a timer for 30 minutes. I went off to attend to a few other things. 10 minutes go by and I go back to the kitchen to check in on things. The wireless probe says the mash is only 147° F, but I can immediately tell that something isn’t right as the pot seems too warm. I check the wireless thermometer against my Thermapen and find that the temperature was at least 158° F. I immediately pull the pot off the stove, cover it, and start spraying it down with cold water in the sink. By doing this and gently rocking the pot to induce some circular movement inside the pot, I get the temperature down to 120° F within a minute or two.
Not wanting to scrap the entire experiment even though I knew the β-amylase would be degraded at this point, I pressed on. I got the mash back up to 146° F, turned off the heat, and conducted a triple decoction. Each decoction was approximately 75% of the solid matter and just enough liquid to keep things from getting scorched. It took approximately 10 minutes to get to boil each time and I boiled for 10 minutes. I added the decoction back to the original pot slowly, stirring the whole time, to keep the temperature at approximately 146° F. Variations in the mash pot were measured to be as low as 136° F (before adding the decoction) to 148° F (overshoot from decoction). The mash was then allowed to rest for 20 minutes until the next decoction was performed.
The measured sugar after the first decoction was 1 Brix, 1.5 Brix after the second, and 2 Brix after the third. Clearly, not all the amylase was lost, but my solid matter was blanched out at this point and I was getting tired of decocting. I pulled the solid matter out of the pot and gave it a thorough pressing to capture all the juice. This was added back to the pot. The pot was then covered and set in an ice bath overnight to chill. In the morning, the liquid was decanted to separate the sugar water from the starch and refrigerated until I could work with it that evening.
The solution tasted sweet, albeit a bit watery and starchy at this point. To concentrate the liquid without caramelizing it, I distributed it into several 9” x 13” pans and let it sit in the oven overnight at 150° F. This went on for 3 nights and reduced the liquid by roughly half each night. Every morning, the liquid was transferred back to a container and refrigerated until the next evaporation cycle. This refrigeration process allowed more of the starch to settle out of solution. Between evaporation, loss to decanting starch, and loss in transferring between vessels, I ended up with approximately 32 oz of liquid that measured just over 8 Brix. This liquid was orange in color, only a shade or two darker than the inside of the original cut potato you see above. The taste had greatly improved. It was much sweeter and cleaner than the original 2 Brix liquid. It only faintly tasted of sweet potato at this point and there was no hint of starch. I think the term “candied sweet potato” would most accurately describe this. I decided to end the experiment at this point in order to post the results and research future methods. I’m also holding the sweet potato syrup refrigerated at this point to test shelf stability and longevity. Despite the initial overcooking catastrophe, I consider the experiment a success because it resulted in a liquid approximately the equivalent of 2 lbs of candi syrup at equal volume and gravity.
What would I do different next time? For one, temperatures would be monitored more closely and held more consistently. I would also try to increase the volume of sweet potatoes used, probably up to 2 lbs / gal. so that the evaporative concentration step doesn’t take as long. Post mash, I think it would be wise to hold the liquid in refrigeration for several days to allow as much of the starch to settle out as possible before starting evaporation. This would reduce losses due to decanting. I also need to find a better way to reduce the liquid. Three nights in the oven is beyond ridiculous. Finally, and possibly most interesting, was a paper I found while researching amylase in sweet potatoes. What it says in a nutshell is that starches will break down and convert to sugars if the sweet potato is held in cold storage (<50° F) for 10 days. As a brewer, this cold conversion kind of blows my mind, but it would certainly make things a lot easier on the extraction side, especially seeing as the sugar concentration appeared to increase by up to 10 times the original amount!
This definitely seems like a viable candi syrup substitute once the process gets refined a bit. I’ll post results of future trials soon, along with information on shelf life, and results of trying to darken the syrup. If you happen to try this, drop me a line and let me know how it worked for you!