Layered yogurts and dairy desserts are very popular in Europe. Many varieties and brands are available on the supermarket shelves there. In the US, there isn’t nearly the amount of fresh dairy desserts as are available in Europe, but there is a variety of fruit on the bottom (FOB) yogurts, especially Greek yogurts. Each type of layered product brings with it certain challenges that must be overcome. Making a layered dairy product takes a certain knack and there are some potential problems that must first be considered.
One reason for the rise in popularity of fruit on the bottom yogurts in the US in the last few years is the growth of Greek yogurts. Many fruit preps, if stirred into a Greek white mass, will disturb the white mass and thin the very heavy body or texture that is desired and expected. Although the prep is not distributed throughout the white mass in fruit on the bottom products, there is still an interaction between the components. Fruit on the bottom yogurts are a two-phase system where both components seek to find an equilibrium over the shelf life of the finished product. Water will migrate between the layers and water-soluble components of each phase such as color, calcium, sugar, and flavor will diffuse from one layer into the other, changing the characteristics of both layers. This migration can have a negative impact on the finished product and there are a few considerations to keep in mind during product formulation.
Color bleeding is one side effect of the water migration that can be considered a defect in layered products. There are a couple of approaches that can be taken in product formulation to address the problem. Different colors will migrate at different speeds due to their molecular size, with smaller molecules migrating faster than larger ones. Working with a color manufacturer to pick a color that will migrate more slowly is one approach. Another important factor to slow migration of color is to match the Brix (or sugar content) of the white mass to the Brix of the fruit prep as closely as possible. This again will slow the migration but neither technique can stop it completely. The stabilizers used in the fruit and/or white mass do not influence the migration of water and cannot slow or stop the diffusion of color between the phases. It is not possible to keep the color segregated in the prep during the typical shelf life of commercial yogurts.
Stabilizers do need to be considered in formulation of fruit preps for FOB yogurts or other layered products. Calcium present in the white mass will diffuse into the fruit prep and if an ion sensitive hydrocolloid such as a low ester pectin is used to stabilize the fruit prep, the calcium and pectin have the potential to interact over the shelf life. DuPont™ Danisco® GRINDSTED® Pectin YF 310 is designed to prevent interfacial gelation in this application. It is reactive enough to stabilize the fruit in the prep and give it a texture that will prevent mixing with the white mass as it is deposited on top of the fruit, but will not continue to gel with the diffusion of calcium over time. If a pectin or other hydrocolloid is used in the prep that is too calcium reactive, a gelled fruit layer will form on the bottom of the cup (sometimes referred to as a “hockey puck”) that will not be able to be stirred into the white mass when eaten by the consumer. However, if a gelled texture for one layer is desired as a contrast for a unique dual textured dessert or yogurt, this interaction can be exploited to gain that effect.
In addition to being beneficial to slow the migration of color between the phases, matching the Brix of the white mass and prep also helps maintain the texture of both phases over the shelf life. In applications such as Greek yogurts, it is common to use a high Brix fruit prep to sweeten the finished product because the white mass does not contain any sugar. However, the great solids differential will cause more moisture to be pulled out of the white mass that can cause cracking and drying of the white mass as well as thinning of the fruit prep. A “leather-like” texture may even develop at the interface. Depending on the degree of difference between the layers, the finished product may have a completely different texture at the beginning of the shelf life than at the end of shelf-life.
Although it may be desirable to have a product with two distinct layers on the shelf, the effect of water migration is always at work in these two-phase systems. Keeping this in mind during formulation and choosing colors that will migrate slower, aiming to match solids of the two phases, and using stabilizers in the prep that will not interact with the calcium in the white mass are the keys to minimizing the impact of this phenomenon and creating a successful commercial product.