Sweet Potato Syrup

There is an old article from 1920 about the existence of diastase in sweet potatoes. Diastase is a general term, referring to enzymes that facilitate the decomposition of various polysaccharide starches into maltose, a disaccharide. As I understand it, sweet potatoes contain a substantial amount of beta amylase, and a small amount of alpha amylase (the content increases with aging). The above linked article refers to an older process determined by the US Department of Agriculture (I have not been able to find the article on their site) for making sweet potato syrup using malt, for its alpha amylase content. This is my second foray into sweet potato syrup production, and it went much better than my last time.

I started with 10 large sweet potatoes, acquired from my local grocery store. I am not sure of the weight of the purchase, as I was going to record it from my receipt from the grocery store, but I managed to misplace the receipt. I took each potato and cut it into four pieces length-wise, and then cut those wedges into individual quarter-slices, as shown below.

In the 1920 article, they talked about using shredded sweet potatoes or sweet potato flour. While I think that the shredded sweet potato would have worked well (as well as saving my poor thumb, the tip of which I cut off while I was chopping sweet potatoes), I think that flour would have made filtering an enormous hassle. The quarter-slices worked well, however, yielding two large bowls of slices.

Endeavoring to follow the paper's steps somewhat, I decided to roughly follow their process, which seemed to work pretty well. I added the sweet potatoes to the pan with water to cover (I added a little extra water, which I'll comment on later in the article), and started heating up the mixture over medium heat.

After fifteen minutes of heating, I checked the temperature on the electronic thermometer, for which you can see a bit of the sensor and its wire in the above picture. The temperatures were recorded during heating as follows:

Temperature Fahrenheit (T) at time elapsed in minutes (t) with stove heat setting (Q)

Mins.	°F	Heat
15	157	5
30	160	1
70	180	3
80	189	5
90	197	5
100	208	5
105	210	5
110	212	5

After boiling, I mashed the sweet potatoes using a potato masher. This was complicated by the fact that there was more water than I had intended, which meant that it was harder to be sure that all of the chunks were processed (many weren't). I used a four step process to extract liquor from the pulp, and it still wasn't as effective as I had hoped. My steps were, in order, to sieve using a chinois and pestle, to filter through a metal mesh coffee filter, to filter through paper coffee filters and then to reduce in the oven overnight, as illustrated.

It's not easy to see in the last picture, but the syrup that has been filtered through paper coffee filters still has a fairly small amount of sweet potato pulp in it. I set the oven at 220°F and let the syrup sit for about 8 hours. When I checked it, it was much sweeter, and had reduced by about half.

I intend to improve the process in the future, and I have some very specific ideas for how to do so. Instead of integrating the enzymatic action and heating to boiling, I'll use water to cover heated to about 170°F. After adding the sweet potatoes, I'll put the whole concoction in my (now temperature controlled, more about that in a later post) light bulb oven, and let it convert for about 2 hours. I'll then boil it on the stove and, instead of mashing the pulp and sieving it, I'll remove most the water by straining, and then use a hand blender to process it into a paste. I'll then create a large filter by putting a piece of wool felt over the top of a bucket, with the cut off top of another bucket holding it in place. As I understand it, wool is better for filtering syrup, as it doesn't clog as easily and filters more thoroughly.

Next time I do this, I'll likely also use malt in the mixture. My intention is to make a simple sweet potato beer and, since sweet potato doesn't contain the necessary amino compounds to support yeast fermentation, malt is necessary for the conversion into beer. This sweet potato and malt beer will then be converted into vinegar, to be used as the base for a Worcestershire-like sauce, possibly tomato-based, that I'll be making.

Black Garlic Sauce Update

Since my buddy was in town for the weekend, and I was sending some of the sauce back with him, I went ahead and further processed the sauce. I added some onion powder, garlic powder, black pepper, dried mushrooms and anchovy paste, and then used a hand blender to puree the mixture. After blending, I let it sit for a couple of hours, at which time the mixture had gotten smoother and sludgy. I added 1/2 C of good Fish Sauce, 1/2 C of passable Dark Soy Sauce, and a further 1/2 cup of Michiu. I intend to give the sauce about a month to age, at which point I will see how it has turned out.

If you noticed that the level should be higher, that's because it should be. I sent my buddy back with about 10 oz. of the sauce in a Topo Chico bottle, which he has been instructed to cover with foil instead of the cap with which I sealed it.

The Black Garlic Oven Post

Today, I harvested my last batch of black garlic and, having pre-harvested one head and having taken four for the sauce base, processed the last seven heads, cutting half up to dry for making black garlic powder. The remaining half will be used at-will for recipes, etc., with some going to a buddy of mine. I've also started a new batch for next month, which I'll cover later in this post. First, I'd like to explain how I made my light bulb powered black garlic oven, using the Huskee Tough Guy EPS (expanded polystyrene) cooler, as somewhat pictured below.

Expanded polystyrene is recommended for applications below 160 degrees Fahrenheit, with this one coming in about 10 degrees below that mark. I started by covering the walls with aluminum foil, reflective side inward-facing, securing it with normal Elmer's Glue-All. I then drilled a 1/2" hole in the bottom center of the container, through which to pass the wires for the light bulb. The tiles were positioned in a diamond shape in the bottom, as depicted here:

The tiles have small felt feet on them in the form of a flattened "X", with top and bottom one inch in from the edge of the tiles. This keeps them from scratching the aluminum foil, while providing a solid base upon which to place ball jars, with enclosed garlic. While I have used ball jar canning lids in the past, I have decided to switch to aluminum foil as the exclusive lid, as it doesn't need to be a perfect seal and I'm concerned about the BPA content of the canning lids. I have placed the reflective side up, as below:

Note the stone filled jars from the last post. They provide IR absorption and thermal mass for the system. As a strongly IR absorbing material, stone also emits IR, bolstered and buffered by the high specific heat of the water. In my other experiences with fermentation, I have understood that light exposure can be detrimental to the fermentation process. Googling "Light effect on fermentation" seems to provide a number of articles, blog entries, &c. that suggest that both visible light and UV radiation can have a negative effect on fermentation. I genuinely don't know if they apply at 140°F-150°F--a perceived consensus indicates this range for black garlic (per this instructable and this make article)--but this mitigates the issue, while the dry heat resists mold and non-beneficial bacterial growth.

Each head is placed in an individual 8 oz. ball mason jar. Some of those that I have are normal and some are quilted, but I don't think it really matters which one is used, save the utility of having the measurements on the side of the normal ball jar. I leave the heads sealed in the jars for 30 days, at which time I remove the lids and let the garlic dry. This results in cloves that are dark, textures and have shrunk away from the skins of the garlic. Some in the last batch were drier than others, with some reducing to a powder when pinched between the fingers, while others were leathery and pliable. The harder examples were cut up and reserved for additional drying, for making black garlic powder.

To measure temperature, I punched a hole in the side of the cooler using a small meat thermometer, calibrated in boiling water (e.g. This Taylor product) around the middle of the height of the jars. Using this to monitor temperature, I used a number of different light bulb wattages.

 I found that I had to use 40W bulbs during higher Central Texas summer garage temperatures, and a 30W bulb during lower temperatures. The incandescent bulb needed will vary by ambient temperature, but I would wager that 40W is a reasonable base starting point for most room temperatures. I conducted primary research with a nominal quantity of water in the system (and no garlic) monitored by a webcam. I will provide the records from this experimentation in an update to this post.

For reference, I used a ceramic light socket I got from a cheap lamp I bought from Lowe's hardware store. There is one very near my home, and I found that they have ad hoc porcelain light sockets for between $3 and $4. I used a cord from another, broken appliance, and soldered some fork/U-shaped terminal connectors to the end of its wires. I bought the cooler at Academy, and I can't easily find the price, though I believe it was no higher than $30. Altogether, I remember calculating (some time ago) that the apparatus, plus electricity, with initial cost of garlic was less than the cost of twelve heads of black garlic online. For the record, this is an excellent use of incandescent light bulbs, as they put off far more heat and IR radiation than light. In the month it takes to do this, the oven costs on the order of $5-$8 per batch, including electricity and garlic, but not including the cost of manufacture.

Black Garlic Sauce Base

Early this morning, I chopped up some black garlic into a mix of 1 cup of Kikkoman light soy sauce and and 1/2 cup of Michiu, as seen below.

I used four heads' worth of black garlic cloves in the sauce. My intention is to store the sauce in the refrigerator for a week, and then blend up the sauce base, while adding some onion, garlic, anchovies, dark soy sauce, fish sauce and (possibly) ginger. I plan to then store the sauce in a sealed container in the refrigerator for a further few weeks, at which time I will test the sauce to see how it tastes. 

The sauce is meant as a cooking ingredient or base for another sauce. I fully expect that it will impart significant savoriness, or umami, to the dishes in which it is added. 

I also put some rocks in jars tonight. In my current black garlic oven, I have four pint jars full of water between the light bulb and the garlic, to provide the system with some thermal mass.  In thinking that I'd rather the heat be a little more even, I reckoned that stone absorbs IR radiation, so the addition of rocks with  water should deliver more even heat throughout the oven while protecting the garlic from direct light. The jars are pictured below.

I'll next post a detailed guide to the structure and setup of the black garlic oven, when I load it with garlic this Saturday, following the harvest of the black garlic currently finishing its drying process.

Intro and Black Garlic December Harvest

I'm starting this blog as a record of my hobbies. As a DIY enthusiast, I have a number of esoteric interests. I'm going to use this as a way to publicly record my pursuits, and as a basic host for various guides and project details.

That said, I have a December 1st harvest date for the most recent batch of black garlic from my cooler and light bulb based black garlic oven. The above picture is a pre-harvest sample that I took (after mistakenly believing this Wednesday to be my harvest date), including some unpeeled cloves from near the middle of a head, and the peeled cloves from the outside of the head. They have a good texture, and the aroma is great.

While I'll be posting the details for my extremely cheap DIY black garlic oven at a later date, my basic process is pretty simple. I keep the garlic in the oven for 30 days between 140°F and 150°F , adjusting the light bulb used as the outside temperature changes. At 30 days, I remove the lid for the jars the garlic is aged in, to allow for the final drying of the heads, which takes 10 days. My roommate has had the idea of using a higher wattage bulb (probably 60W) in combination with a lower wattage bulb (maybe 30W or 40W, depending on ambient temperature) for heating and maintaining, respectively. The two would be hooked up to a relay, controlled by a circuit with at least one thermocouple as its input.

I plan to chop and dry about four of the heads, after which I'll make black garlic powder. Another four will be soaked in some light soy sauce and blended up with some dark soy and fish sauce, to be used for cooking. The last four will be used as-is. I'll be making 24 heads in the next batch, instead of the 12 that I made in this batch, some of which I'll use in a tomato-based fermented sauce. I might further double up the capacity, since I think the cooler could be more densely packed and work well.