Pineapple Tarts

February 04, 2022

Welcome back to the Soft Matter Kitchen and happy Lunar New Year! Things have been eventful for me lately and I’m happy to say that I’ll soon be starting a new job here in Singapore that I look forward to sharing more about. This month I tried making a classic festive dish for the Lunar New Year, pineapple tarts.

 

What are pineapple tarts?

Pineapple tarts are a dessert that consists of a buttery pastry dough and pineapple jam. They are very common in Southeast Asia and I often had them when visiting my relatives in Malaysia. In Singapore they are especially ubiquitous as a festive treat during the Lunar New Year holiday which just took place. Researching why this might be the case, I found that the name for pineapples in Hokkien and Cantonese can be translated to something like "fortune come", making them a good fit for Chinese New Year. Overall, I found this dish to be fairly simple to make but much more labor intensive than I was expecting. The jam is formed by concentrating a pineapple purée via evaporation over several hours and mixing in sugar towards the end of this process; the dough is a simple mixture of mostly butter, flour, and egg.


Making the pineapple tarts

Blending the pineapples into a foamy purée resulted in a material with very slight shape-retention ability

There are several common morphologies of pineapple tarts, I chose to make the type which consist of a ball of pineapple jam encapsulated in the pastry; the recipe I followed can be found here. Using a store-bought pre-skinned pineapple, I blended it into a foamy purée. At this stage the purée did have a slight yield stress and could hold its shape; this seemed to largely be due to the large number of air bubbles jamming against each other. Shortly into simmering the purée, the air bubbles rose out of the solution, resulting in a much lower viscosity and no more apparent yield-stress behavior. Gradually--over the course of a couple of hours--the purée concentrated and became more viscous and somewhat moldable, but prior to adding the sugar, any formed shapes would easily fracture apart. After the addition of the sugar and further cooking of the mixture, nearly all the water seemed to be removed and I was left with a very sticky pineapple jam paste. When discussing Dalgona coffee foam, I mentioned how the addition of sugar in that recipe greatly changed the texture and stability of the foam by viscosifying the continuous phase. For this pineapple jam which is a suspension of jammed pineapple particles in a sugar solution, stability is not necessarily an issue since there is no tendency for these particles to merge with one another and rise out of the material the way air would. However, the texture was greatly affected by the dissolving and concentration of the sugar, and the material came to resemble a moldable soft candy which was important for rolling it into spheres; I would estimate that the yield stress of this material was several thousand Pascals, between chewing gum and Play-Doh.

The pineapple jam is not the only yield-stress fluid in this recipe; prior to cooking, the pastry dough is also a granular yield-stress fluid consisting of flour granules wet by butter and egg and it is significantly weaker and softer than the pineapple jam--likely on the order of one hundred Pascals. Encapsulating the pineapple jam in the dough reminded me of some ongoing research I have related to how two different yield-stress materials may interact. When making this style of pineapple tart, it is important that the yield stress of the inner material--the jam--is significantly larger than that of the outer material--the dough. If the stress necessary to deform the dough were higher than that of the jam, it would be very easy to lose the nice spherical shape of the pineapple orb while attempting to mold the dough around it. I suspect that for other varieties of pineapple tart the rheological requirements of the jam are quite different. For the style of tart that consists of a cookie with a flat dollop of jam on top, it is likely desirable that the jam have a much lower yield stress--perhaps even lower than the dough--to enable it to smoothly spread on the surface. This is something that I'll need to investigate further another time.

 
 

Regarding the eating experience, the tarts were indeed very tasty but I didn't find them markedly different from store-bought varieties and the labor required certainly weighed on me. If I were to make these again, I would probably try to find some way of automating the mixing of the pineapple purée during the evaporation process.