Purple Sweet Potato Poppy Seed Whole Wheat Sourdough Hokkaido Milk Bread

I had a bit of mashed sweet potato leftover from a previous bake and wanted to use it up. I decided to try adding it to a Hokkaido Milk Bread done with 30% whole wheat and this is what I came up with.

For 1 loaf in a 9”x4”x4” Pullman pan

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Instructions
Mashed Purple Sweet Potato
To prepare the purple sweet potato, peel and cut into 1.5-2 cm cubes and place in a small metal bowl. Pour 1 cup of water into the Instant Pot, then place the small bowl containing the purple sweet potato onto the trivet in the Instant Pot. Pressure cook 18 mins normal. Quick release the pressure. Remove from the Instant Pot and allow to cool until safe to handle. Mash the purple sweet potato.

Levain
Mix the levain ingredients in a jar or pyrex container with space for at least 300% growth.
Press down with your knuckles or silicone spatula to create a uniform surface and to push out air.
At a temperature of 76-78ºF, it typically takes up to 10-12 hours for this sweet stiff levain to be at peak. For my starter I typically see 3-3.5 times increase in size at peak. The levain will smell sweet with only a mild tang.
Tangzhong
In a sauce pan set on medium heat, stir the milk and flour until blended. Then cook for several minutes until well thickened, stirring regularly with a spoon or heat-resistant spatula. Let cool in the pan or, for faster results, in a new bowl. Theoretically it should reach 65ºC (149ºF) but I don’t find I need to measure the temperature as the tangzhong gelatinizes at this temperature. You can prepare this the night before and refrigerate it, ensure that it is covered to prevent it from drying out.

If you plan on using a stand mixer to mix this dough, set up a Bain Marie and use your stand mixer’s bowl to prepare the tangzhong.

Dough
In the bowl of a stand mixer, add the milk (consider holding back 10 g of milk and adding later if this is the first time you’re making this), egg, tangzhong, salt, sugar, diastatic malt (optional) and levain. Mix and then break up the levain into many smaller pieces. Next add the flour and vital wheat gluten. I like to use my spatula to mix until there aren’t many dry areas. Allow the flour to hydrate (fermentolyse) for 20-30 minutes. Mix on low speed and then medium speed until moderate gluten development this may take 5-10 mins. You may want to scrape the sides of the bowl during the first 5 minutes of mixing. Next add room temperature butter one pat at a time. The dough may come apart, be patient, continue to mix until it comes together before adding in more butter. Once all the butter has been added and incorporated increase the speed gradually to medium. Mix at medium speed until the gluten is well developed, approximately 10 mins. You will want to check gluten development by windowpane during this time and stop mixing when you get a good windowpane. You should be able to pull a good windowpane, not quite as good as a white flour because the bran will interrupt the windowpane somewhat. Next add the mashed sweet potato, that way it does not interfere with the gluten development. Mix until well incorporated in the dough.

On the counter, shape the dough into a tight ball, cover in the bowl and ferment for 2 - 3 hours at 82ºF. There may be some rise visible at this stage.

You can next place the dough into the fridge to chill the dough for about 1.5 hours, this makes rolling the dough easier to shape. Remember, if you do so the final proof will take longer. Alternatively, you can do a cold retard in the fridge overnight, however, you may find that this increases the tang in your bread.

Prepare your pans by greasing them with butter or line with parchment paper.

Lightly flour the top of the dough. Scrape the dough out onto a clean counter top. Shape into a batard and place in your prepared loaf pan.

Cover and let proof for 6-8 hours at a warm temperature. I proof at 82°F. In general, I find the addition of mashed purple sweet potato slows fermentation so despite using 20% prefermented flour, this is a slow final proof so be patient. You will need longer than 6-8 hours if you chilled your dough for shaping. I proof until the top of the dough comes to within 1 cm of the top edge of the pan.

Preheat the oven to 350F and brush the dough with the egg-milk wash. Just prior to baking brush with the egg-milk wash again then sprinkle poppy seeds on the crust. Finally score the top of the dough.

Bake the loaves for 50 minutes or until the internal temperature is at least 190ºF, rotating as needed to get even browning. Shield your loaf if it gets brown early in the baking process. After 50 mins remove the bread from the pan and bake a further 10 mins by placing the loaf directly in the oven on the rack with the oven turned down to 325ºF. You can brush the top of the loaf with butter if you wish at this point while the bread is still hot to keep the top crust soft.

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