Yesterday I enjoyed a great meal at our local Turkish restaurant, Istanbul. We enjoyed our lentil soup (kirmizi mercibek corbasi) with cacik, their homemade Turkish yogurt with fresh parsley, cucumbers and garlic. The lentil soup is pretty spicy, so my daughter and I add lemon and then fold in cacik, to give an absolutely delectable combination, with just the right amount of heat. (Yes, I have to sip the soup repeatedly as I add cacik to get the level just right!)
Lentil soup at Istanbul Restaurant, Ithaca NY.
What is the yogurt doing for this combination?
First, the parsley, cucumbers and garlic, as well as the lemon, activate the cold receptors—this activation helps to dampen the activity of the hot receptors, so the overall heat is just right.
Second, I was wondering about the fat in their yogurt (which, incidentally, is not very creamy, unlike the Greek yogurt you can buy around here). Technically, fat also helps turn off the hot receptor TRPV1, but yogurt’s sourness/acidity should turn that receptor on. When tasted by itself, this restaurant’s yogurt isn’t particularly sour—maybe the parsley/cucumber/garlic, by turning off TRPV1 are cutting some of the sourness as well. But did fattiness contribute to the decrease in heat?
My attempt to answer this question led me into the maze of literature—most of it fairly old actually—on the sensory properties of yogurts with different percentages of fat.
Not only can yogurt contain varying amounts of fat, it can also contain varying amount of sugar, and dairy proteins, all of which could influence sourness, as of course differences in lactic acid concentration, and citric acid concentration as well. Further, lactic acid has a bitter taste for some people, which may also influence the perception of sour. The result is a complex of taste receptor and trigeminal interactions that are, at least at first blush, difficult to parse.
As described in the most recent paper I could find on the subject, trained panels evaluated the sensory characteristics of a number of commercial yogurts. The panelists learned to recognize the
different possible flavors by tasting solutions with characteristics flavors.
(One example that I, having had children and grandchildren, particularly appreciated is contained in Table 2 of this article:
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Butyric
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An aromatic that is sour and cheesy, reminiscent of baby vomit
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The numbers are indicative of the panelists’ perception of intensity.)
All in all, the authors tested for 25 flavor/taste characteristics and 10 texture, mouthfeel, and mouthcoating attributes.
The correlations among the flavor characteristics were particularly instructive. For example,
“overall sour, lactic, and sour taste were significantly correlated to sharp/bite flavor. Sharp/bite comprises of sour, astringent, and pungent impressions, thus this correlation was understandable.”**
These correlations are all the more understandable if you know that sour, astringent and sharp/bite are all produced by activation of TRPV1. Although I prefer to reserve the word “pungent” for chemicals that activate the cold receptor TRPA1, such as wasabi, this confusion is also understandable as well, as both sharpness and pungency refer to the pain caused by activation of either of these receptors.
But back to fat: overall it is hard to find evidence that the fat in yogurt modifies sourness or sweetness.
In the Brown/Chambers paper, there is no strong correlation between fattiness and sourness or sweetness. Earlier studies seem to indicate either no effect or dampening of sourness by fat.
One study may give us insight into why there are such differences: the authors of this study did not lump all the panelists together, but looked at the effect of individual differences on the relationship between sourness and fattiness. These individual differences turn out to be critical, as some people found no effect and others sis. This research described in this paper was done long before we had DNA analysis, so we cannot say for certainty that these individual differences in perception were caused by individual differences in the gene for TRPV1 or its expression. However, it seems to me that this may be the cause. The genetics of TRPV1 in humans are extremely complex, with possibilities for both gain and loss of function, so that one may readily expect huge variation in response.****
In any case, this particular yogurt from Istanbul restaurant is not very sour to me, but whether it is the cold receptor activators (parsley, cucumber, and garlic) or the fattiness, I cannot say.
** Grammar as in the text.
*** H. Tuorila, et al. Sensory attributes and acceptance of sucrose and fat in strawberry yoghurts. International Journal of Food Science and Technology (1993) 28, 359-369.
**** Ruslan Dorfman, Hubert Tsui, Michael W. Salter, and H.-Michael Dosch. TRPV1 GENETICS. In Vanilloid Receptor TRPV1 in Drug Discovery, Gomtsyan, Arthur and Faltynek, Connie R (eds.) ISBN 9780470175576, pp. 134 - 149.
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