To Fuse or Not to Fuse?

One of the core chemistry principles of the casein proteins in milk is their ability, or tendency, to self-associate, or to stick to one another. This tendency is what drives the formation of the massive casein aggregates found in milk, called the casein micelles. The driving force for self-association is because the caseins tend to be relatively hydrophobic and thus these types of amino acids like to cluster together as much as possible to limit the area that is in contact with water.

In the cheesemaking process, one of the critical first steps is to add rennet to form a gel network. After the gel attains sufficient rigidity, we cut this gel (coagulum) into small pieces to help initiate water loss (whey) through the pores of the curd particles. If we don’t stir those newly formed curd particles, they would very quickly clump back together and reform into a large mass. The ability for curds to re-fuse back together is an often-overlooked characteristic of cheesemaking, and maybe we can utilize this behavior as we look for further innovations in cheesemaking.

John Lucey, Ph.D., is the Owen R. Fennema Professor in Food Chemistry at the University of Wisconsin-Madison’s Center for Dairy Research.

In many of our American-style cheeses, after we drain the whey, we transfer the curds onto a drain table or belt, partly depending on if we are making cheese via the stirred curd or milled curd methods. In the milled curd method, we let the curds fuse back together again and after sufficient acidity has been attained, we run that solid slab, or mat, of curd through a mill, where it is cut into smaller pieces to facilitate effective salt distribution. In the stirred curd process, we keep stirring the curd particles to prevent them from fusing back together. In both curd processes, after salting, we bring the curds back together again using block forms or molds so that the curds can fuse one more time. We then get the final cheese block.

All of these processes rely on the fact that the caseins, the main structural component in cheese, have this ability to stick to one another and reform into larger structures. For example, when we make Mozzarella, before the curd goes into the cooker/stretcher, we cut the curd up into smaller pieces (via either the stirred curd or milled curd process), which allows for faster heat transfer when the curd hits the hot water in the cooker/stretcher. When the Mozzarella is extruded from the cooker/stretcher, the cheese has reformed into one continuous mass — we don’t see the individual particles because they have fused back together.

String cheese is a version of Mozzarella with long protein fibers and we have seen cheese plants produce incredibly long string cheese. In 2021, Weyauwega Star Dairy in Wisconsin broke its own record for the world’s longest (continuous) piece of string cheese at 3,832 feet or about five city blocks.

Improper curd fusion can cause defects in cheese like “seaminess” where the curds don’t properly fuse together and the outlines of some of the curd particles are visible when the cheese is cut open. One condition that is important for proper curd fusion is temperature — if the curd particles are too cold during pressing, they do not fuse well with neighboring curd particles. Of course, pressure helps curd fusion, which is why we put pressure or force on the cheese when it is placed in blocks or forms. This pressure also helps exclude any entrapped air from the cheese, which reduces the risk of mechanical openings in the cheese. High (dry) salt levels on the surface of the curd particles can also be problematic as it can dehydrate the surface of the curds, which negatively impacts the ability of the curds to fuse together.

Another key variable that can impact curd fusion is pH. If pH is too low (acidic), the cheese may not have good curd fusion resulting in a more crumbly and brittle cheese body (think of cheeses like Feta). At low pH values (< 5.0), the casein molecules are not as mobile, once they connect with another casein molecule they stay attached and thus lack the tendency to break and re-form new interactions with other curd particles. Curd fusion works best within an intermediate range (pH 5.2-5.7), where the loss of some insoluble calcium crosslinks within the casein particles enhance their mobility. Curdiness may disappear as the cheese ages because proteolysis will help degrade the casein network and allow more fusion of the curd particles.

The ability of curds to fuse together can be used to create some innovative products. For example, Co-Jack cheese was invented in Arena, Wis., by mixing Colby and Monterey Jack curds before the pressing stage. The resulting cheese is marbled with orange from the colored Colby, and white from the Monterey Jack curd. Researchers have also investigated blending curds made from cheeses of different fat contents so that the blended cheese has an intermediate fat content.

I am also aware of some unique specialty cheeses where some mature (aged) cheese is added to fresh curds, which seems to help accelerate characteristic flavor development. However, the addition of high levels of some types of flavor ingredients to curds can result in poor fusion.

Another interesting area of research and development utilizing curd fusion is the use of technologies that facilitate blending and extrusion, such as equipment like a Vemag. The Center for Dairy Research (CDR) is commissioning a new Vemag unit in its pilot plant. This type of equipment can reduce trim losses that occur with cheese that need to fit into unique molds like a longhorn. Attachments to these types of devices can extrude the cheese into various shapes and portion sizes. Sometimes these types of reformed cheese can suffer from a grainy texture, which is partly due to a lack of complete fusion of the curds after processing. Optimizing the curd chemistry and the use of vacuum during extrusion can help improve the final texture.

Curd fusion is an often overlooked, but an essential part of the cheesemaking process. Dialing in the correct conditions to get proper curd fusion can improve cheese quality as well as facilitate the development of new innovative products. DF