Berry Bhapa Doi

March 31, 2021

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What is yogurt?

Yogurt is a type of fermented milk product. The word, “yogurt” comes from the Turkish root meaning “thick”. There are at least two steps involved in making yogurts from milk: heating, followed by fermentation at a lower temperature via the addition of bacteria. There are two standardized bacteria used in industrial yogurt fermentation, Lactobacillus bulgaricus, and Streptococcus thermophilus. As the bacteria digest the lactose in the milk, they acidify their surroundings as well as produce flavor compounds that give yogurt its sour flavor.

In my entry on ricotta cheese, I briefly discussed the composition of milk and how it contains free-floating protein micelles that fall apart and reform into network structures when acidified. The key differentiators of yogurt from ricotta cheese are the method and timescale of acidification. Ricotta is typically acidified rapidly using vinegar or lemon juice, whereas yogurts may take up to 18 or so hours to acidify depending on the temperature of fermentation. It is known that slower fermentation leads to a finer gel network structure, very different from the grainy texture of ricotta. In addition to the casein proteins which compose the micelles, milk also contains individual suspended whey proteins which are unaffected by acidic conditions. However, the initial heating step for yogurt (which is also often present for ricotta) does denature some of the whey proteins which causes them to go to the surface of the casein micelles. This reduces the density of the acidified casein network, modifying the texture of the gel network.

Making the Bhapa Doi

The recipe for berry bhapa doi in the cookbook that I mentioned was quite simple; for flavor I chose strawberries as the “berry”. First, I blended the strawberries into whipping cream, then whisked it together with condensed milk and a Greek-style yogurt. The precise distinction between ordinary, Greek, and Greek-style yogurt is a bit unclear to me and different terms are used in different countries. In general, what seems to make a yogurt Greek or Greek-style is that it has been strained to partly remove some of the milk whey. In a traditional case, I would expect that this means that Greek yogurt has much less water content and a higher fat percentage, though this did not seem to consistently be the case with various yogurts I looked at in my grocery store.

Regardless of the particular production method, the yogurt I used had a noticeable yield stress between 30 and 50 Pascals. This was reduced in the final mixture as we would expect since the cream and condensed milk serve to dilute the material. A key indicator that the final mixture still had a finite yield stress were the irregularly shaped pockets of air that were trapped at the walls of the glass pots I used for cooking. If there was no yield stress, we would expect these air pockets to relax into a more spherical shape, though they might still adhere to the glass wall due to surface tension with the slightly rough surface.

Though the prescribed recipe called for baking the covered glass pots in an oven at around 100 C, I wanted to more precisely explore the effect of temperature on the texture of the final dish. Thus, after sealing the glass jars, I submerged them in a water bath controlled by my sous vide machine and cooked them for 30 minutes at either 75 C or 85 C. After cooking, I placed both jars in the fridge overnight and added some chopped fruit.

Rheologically, I was not sure what to expect from cooking the yogurt. Because the jars were sealed and cooked at temperatures lower than boiling, I didn’t expect any concentration of the mixture to occur. Additionally, unlike with cheesecake, I hadn’t added any egg or other gelling agent. However, sampling the dish cooked at 75C, I was struck by how the texture reminded me of a very creamy cheesecake. Cooking at 85C resulted in an even stiffer material that was almost like a soft cheese. I would classify both dishes as yield-stress fluids, though I did see a bit of weakening of the materials after stirring, suggesting that the structure does suffer some irreversible breakdown with agitation. I believe that the explanation for why cooking the yogurt induced this stiffening of the material lies in the presence of whey protein. As I mentioned, heating milk causes whey proteins to unfold; this is taken advantage of to modify the texture of regular yogurt since whey will bond to the casein proteins that compose the primary microstructure. However, it is also possible for whey proteins to bind to each other and form a gel network of their own. To confirm whether that the gelling was due to the yogurt or some aspect of the fruit, cream, or condensed milk, I performed a control experiment by cooking only the yogurt at 85C and found that indeed the texture was very similar to the Bhapa Doi. This was a little surprising based on my reading as I would have thought that after the initial heating to produce yogurt, the whey gets recruited into the casein protein network and wouldn’t’ be free to form a new secondary network. It may be the case that either there is still enough free whey in the yogurt, or that with the second heating cycle the whey is able to detach from the casein to form a network.

Overall, the dish was quite tasty and refreshing. I hesitate to call it something like “Bhapa Doi sous vide”, just because that ignores the literal meaning of the original dish. If you decide to make this dish yourself in a similar style, I recommend the 75C variety. I’m excited to try out additional recipes from this book; many thanks to my loving friend for the great gift.