Melting cheese

The home cook has limited options for melting a non-melty cheese (e.g. cheddar), often resorting instead to a ‘manufactured cheese-like substance’ that has been engineered to melt (something from the Kraft line, for example).

What is the problem with melting cheese? It doesn’t melt as an emulsion, it separates into a tough protein lump surrounded by a pool of fat/water. Nice. It helps to understand a bit more about milk and cheese if you want to melt it.

Milk

Milk is essentially fats, proteins and serum (water and other nutrients). There are a lot of different milk proteins but they can be divided into two groups – curd and whey. The main proteins in the curd group are the caseins and, in the whey group, (unsurprisingly) whey proteins. It is the caseins that matter for this topic (there are a number of types of casein protein, but I will mostly refer to them as ‘casein’).

Casein proteins clump to form a larger amalgamation (micelle, from the Latin for crumb) that is held together by calcium (most of milk’s calcium is in the micelles). As micelles reach a certain size, a specialised casein protein (kappa-casein) attaches to the outer surface and orients itself so that the micelle acquires a surface-negative charge (i.e. it caps the micelle). This repels other casein proteins and micelles and stops the micelle from growing further. The protein-calcium micelles are thus separated and dispersed in the milk. To make cheese, we want to clump them together.

Cheese

Our ancestors might have stored milk in ‘containers’ made of a calf stomach, and may have found that when stored in the fourth stomach of an unweaned calf (<8-10 months), something surprising happened – it separated into a solid substance and a watery liquid – the curds and whey. The curds kept better than the milk itself, and could be stored to improve food availability. Cheese was born.

1816801922_35cf19a3bc_qThe calf stomach was the key. It contains a specialised enzyme (chymosin, more commonly referred to as rennet) that coagulates milk. Calves effectively make cheese in their 4th stomach. Stomach acidity also helps, as acids curdle milk too. Presumably this evolved to help with milk digestion for the calf. As the calf weans, production of chymosin stops (it is no longer needed). About 2-3,000 years ago, shepherds learned that extracting the brine from the calf’s stomach gave them a ready supply of milk-curdling rennet and long-lasting cheese. There were also implications for lactose intolerant individuals. These days, refined chymosin is typically produced by non-animal fermentation (bacteria/fungi).

While most protein enzymes randomly break up proteins, chymosin is explicit – it removes just the part of the kappa-casein protein that projects outward from the micelles, thus stripping the micelles of their outer negative charge. The micelles then clump together in much larger conglomerates, squeezing out the whey and forming a protein curd. Milk fat globules are large compared to micelles and get trapped in the curd as the protein matrix forms.

Acids work by neutralising the micelle’s surface negative charge with their free H+ ions. However, curds made mostly by acid (e.g. ricotta, cottage cheese) are weaker than with rennet because the acid also releases the calcium that binds the casein micelles together. Some of the freed casein can then be lost to the serum – fewer are available to rebind and they do so loosely. Acidity also destroys other bacteria that can contribute to flavour, so acid-cheeses are bland compared to rennet-cheeses.

Perhaps surprisingly, that about describes most cheeses (as far as I can tell). Most cheese labels will read: Milk, starter culture, enzyme (rennet) and salt. The culture (bacteria) has the job of converting milk sugar (lactose) to lactic acid, thereby adding an acid to the mix (aiming for a pH of 5.2 – 6.3, raw milk is 6.6) that neutralises the kappa-casein which is further broken down by the rennet. The seemingly infinite varieties of cheese results from the subtle manipulation of these processes and the inclusion of flavour-creating bacteria or fungi that develop the cheese’s flavour as it ages.

Melting cheese

At last I’ve got to the point of this post. Unless designed to, most cheese doesn’t melt in desirable ways. When heated to the melting point, the fats liquify and the casein curd starts to break up, releasing the fat. With continued heating and stirring, residual water in the curd evaporates and the remaining curd rebonds more tightly. The result is a tough, tasteless protein lump surrounded by a pool of leaked fat.

In this picture, 50g of generic cheddar cheese and 50g of water were heated to melting:

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The protein lump in the centre was semi-solid:

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And set into something tough and tasteless weighing 21g:

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Ways around the problem

Perhaps the most famous method for melting cheese is the Swiss Fondue. The ‘secret ingredient’ is the addition of a tart white wine, although the alcohol plays no role in melting. Wine contains a number of naturally-occurring acids from grapes, such as citric acid and salts of this acid (citrates).

Think of the citrate molecule like a crab with two strong claws. The molecule attracts calcium ions out of the micelles and holds them firmly in a double-pincer arrangement. Citrate is a sequestrant – by binding calcium it stops calcium from having an active role (it also known as a chelating agent).

The effect is to break up the micelles by robbing them of their calcium glue, which frees the caseins. Casein is an emulsifier (it has a fat soluble end and a water soluble end and can help disperse fat in water). So, they hold onto fat while slipping around each other. Acid also helps release the calcium that binds the casein micelles together.

Another traditional method is to make a roux-based sauce (flour, butter, milk) and melt the cheese in that. The starch in the flour expands, gelatinises and gets in the way of micelles clumping up. The disadvantage is that starch masks some of the cheese flavour.

Modernist melting

Heston B suggests an approach for the home cook that combines the fondue and starch methods. He concentrates white wine (reducing 300ml to 30ml) and combines it with stock. Then, grates cheese and dusts it with cornflour, adds the cheese to the liquid and stirs until it’s melted, then stops (further stirring will tighten the curd). He adds some cream cheese at the end to lighten. He also adds a squeeze of lemon juice (presumably for taste, as the wine has already contributed acidity).

The more direct solution (if you have the ingredient) is to melt cheese with a small amount of citrate, such as the salt sodium citrate. About 4% of the weight of the cheese should do it. Adjust the consistency with a liquid of choice (water, milk, stock, beer). An easy cheese sauce for macaroni would be 100g cheese, 100g milk and 4g sodium citrate.

This is the previous cheese/water combination, melted with 2g sodium citrate:

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Cooled and used as a cheese spread:

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Or, use less liquid, pour the melted cheese over a non-stick baking sheet and tip the sheet to spread the sauce out to an even thickness, let it cool and slice into squares/circles for your own melty cheese slice (to include in a burger or a toasted sandwich). Or, set as a log and cut slices from it (see Chefsteps link below).

Other ‘melting salts’ can also be added such as sodium phosphate, sodium hexametaphosphate or calcium phosphate. Melted, then cooled cheese can benefit from added phosphates – for reasons not well understood (by me at least) they reduce a certain graininess that can occur in the cooled and set cheese. A minor point though.

Sodium citrate could already be in your local store. Alternatively, it is easy to buy online (in Australia I use this source). One bag will probably last a lifetime.

Should I be adding sodium citrate to my food?

Sodium citrate is a salt of citric acid (as found in citrus fruits such as lemons). Similarly, sodium chloride (table salt) is a salt of hydrochloric acid.

Citrate is a critical part of our cellular respiration – the process whereby we generate the energy needed for our cells to function. It is so important that the metabolic cycle that generates this energy is sometimes called the ‘Citric Acid Cycle’ (or the Krebs Cycle, after the gentleman who described it and received a Nobel prize for his trouble).

1024px-Citric_acid_cycle_with_aconitate_2.svgEvery cell in our body is constantly executing the citric acid cycle and generating the energy we need to survive. The cycle does not depend on ingested citrate, each cell manufactures and maintains its own supplies. Because this is happening in every cell of our body, it means that the body manufactures astonishing amounts of citrate – a staggering 2 kilograms a day for the average person. Dietary citrate is a drop in this citrate ocean. Regulatory authorities impose no limits on dietary citrate.

It can be found in carbonated beverages, ice-cream, jams, sweets, milk powder and wine. It is a also produced by the action of antacids such as Alka-Seltzer. Sodium citrate has an E-number of E-331. You may also see E-339 (sodium phosphate) and E-341 (calcium phosphate) on some processed cheese labels.

Process Cheese

The discovery that it was acid salts in wine (combined with some acidity) that kept the cheese liquid in a fondue, eventually led to mass-produced processed cheese. In 1916 James Kraft patented a method that combined another melting salt (a phosphate) with an acid, to liquify (and pasteurise) cheese. In 1950 Kraft Foods marketed the first shelf-stable, sliced, processed cheese (or embalmed cheese, as its detractors campaigned for it to be designated). Kraft is now the world’s second-largest food conglomerate.

15101715895_3d28ea41ccThe advantages of processed cheese were: It could be heated sufficiently to pasteurise and then be cooled and set, extending the shelf-life of the cheese without refrigeration; cheese scraps could be melted, mixed and set into a single log of profitable cheese; it had a uniform look and behaviour and; the cheese could be melted by the consumer to make mac and cheese, cheese toasties etc.

The regulatory names are Pasteurised Process Cheese (sometimes American Cheese), with or without ‘Food’ or ‘Spread’ at the end. Products must meet specific FDA requirements to use these designations. Kraft tried reducing the quantity of cheese and substituted milk protein concentrate and anhydrous milk fat in the formulation. They got a ‘please explain’ letter from the FDA. Rather than conforming to FDA definitions, Kraft rebranded their products as a ’Pasteurised Prepared Cheese Product’, freeing them from regulatory requirements. Click the picture to see this designation in a easily-overlooked shade of blue on blue (note too that the product has been branded Kraft Singles, not Kraft Cheese). You might also see ‘Pasteurised Cheese Snack’ used on some other products. Branding might vary with country.

But never mind because now, with sodium citrate, you can make your own – and with a real cheese of your choice!


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Further reading:

Modernist cuisine recipe and explanations: [link]
Chef steps recipes [link] and science [link]
Harold McGee’s invaluable resource for everything: [link]
Further science on melty/stretchy cheese: [link]
Scientific report on sodium citrate: [link]
Cellular respiration and the citric acid cycle: [link]