This question was posed to the twittersphere by Eat The Rind. Curious, I decided to take on the challenge of answering this question.
There are two main types of mechanism by which browning of food occurs, depending on whether or not the process is mediated by enzymes. If enzymes have been inactivated during heat processing, only nonenzymic browning can occur.
The Maillard reaction is one of three non-enzymatic browning reactions. This reaction is a heat-induced reaction that occurs between a carbonyl compound, which is usually a reducing sugar, and an amine, usually an amino acid, a peptide, or a protein. If cheese contains a high level of residual galactose (or lactose or glucose) due to the use of a starter unable to utilize galactose, it is susceptible to Maillard browning. The intensity of browning depends on the sugar content of the cheese and the ability of the cheese proteins to remain hydrated during cooking.
reducing sugar (galactose > mannose > glucose) + amine –> brown pigments + flavors
The Maillard reaction is actually incredibly complex. In order to understand it, a simplified and subdivided scheme of the reaction has been done most successfully by Hodge. (Nursten)
I. Initial stage: products colourless, without absorption in the ultraviolet (about 280 nm).
Reaction A: Sugar-amine condensation
Reaction B: Amadori rearrangement
II. Intermediate stage: products colourless or yellow, with strong absorption in the ultraviolet
Reaction C: sugar dehydration
Reaction D: Sugar fragmentation
Reaction E: Amino acid degradation (strecker degradation)
III. Final Stage: Products highly coloured.
Reaction F: Aldol condensation
Reaction G: Aldehyde-amine condensation and formation of heterocyclic nitrogen compounds.
Since it has been noted that a positive correlation between galactose content and brown color intensity in heated Mozzarella cheese exists, starter cultures containing lactic acid bacteria strains able to equally convert glucose and galactose to lactic acid are needed if browning is to be avoided. According to A.H. Jana, the only starter used for fresh Italian Mozzarella is Streptococcus thermophilus. Streptococcus spp. and Lactobacillus helveticus are used in Mozzarella because they release inappreciable amounts of galactose into the cheese curd by fermenting accumulated galactose through metabolism. Manufacture of high-moisture Mozzarella entails use of starters like S. lactis, S. durans or S. faecalis, wheras for low-moisture Mozzarella the starters recommended are S. thermophilus, L. bulgaricus and/or L. helveticus. It has been reported that use of culture composed of one or more of the species of P. cerevisiae, L. plantarum, L. casei, S. faecalisand S. durans in addition to the standard starter culture yielded cheese containing less than 0.3% lactose, also helping in avoiding burning or blistering during baking on pizzas. Mozzarella cheese having reduced lactose content is a key factor in order to avoid burning or blistering of cheese during baking.
THE CASEIN MATRIX AND FAT CONTENT
The melting properties of Mozzarella are based on the number and strength of casein-casein interactions (bonds between casein proteins). When cheese is first placed into an oven, the cheese temperature quickly rises, but the shape does not change. As the cheese reaches a critical temperature called the softening point, it begins to flow. At this point, the cheese not only continues to rise in temperature, it also changes shape. The cheese’s casein protein matrix begins to collapse, fusing into a semi-solid mass. Technically, caseins do not melt, but interactions with each other result in an outcome referred to as melt. As cheese is heated, there is a more rapid relaxation of protein-protein bonds and a greater thermal motion of casein strands, which results in cheese becoming more liquid-like. Next, the melt profile reaches a third critical point called the complete melt point. After this point, cheese height changes are minimal, and cheese temperatures slowly approach the oven temperature.
A relatively low number of fat globules in reduced-fat cheese results in a denser structural matrix leading to a firm and dry cheese that melts poorly. As fat content decreases, changes in physical properties and flavor lower the cheese quality. The change in physical and functional properties are presumably due to loss of plasticizing action of the fat and increased cross-linking within the curd and hence in the cheese.
LOW-FAT CHEESE WILL MELT WITH THE ADDITION OF OIL
Covering the skim or low-fat cheese surface of a pizza with a hydrophobic barrier (such as olive oil) will affect the melting and browning behavior of Mozzarella cheese. The barrier will prevent moisture loss that will limit melting. Therefore, fat within the interior microstructure of the cheese was not necessary to achieve the proper functionality of fat-free and lower fat Mozzarella cheeses during pizza baking.
Buffalo Mozzarella uses a starter that metabolizes sugars that cause browning and even though buffalo milk has a high percentage of proteins, it also has a high percentage of fat causing a less dense protein matrix that is easy to melt.
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A.H. Jana and P.K. Mandal, 2011. Manufacturing and Quality of Mozzarella Cheese: A Review. International Journal of Dairy Science, 6: 199-226. (http://scialert.net/abstract/?doi=ijds.2011.199.226)
Sundaram Gunasekaran, M. Mehmet Ak. Cheese Rheology and Texture. CRC Press LLC. 2003
Y. H. Hui, ed. et al. Handbook of Food Science, Technology, and Engineering, Volume Four. CRC Press LLC. 2006
Matzdorf, Beatriz; Cuppett, Susan L.; Keeler, Laurie J.; and Hutkins, Robert W., Browning of Mozzarella Cheese During High Temperature Pizza Baking. (1994).Faculty Publications in Food Science and Technology.Paper 29. (http://digitalcommons.unl.edu/foodsciefacpub/29)
Nursten,Harry E. The Maillard Reaction: Chemistry, Biology And Implications. The Royal Society of Chemistry. 2005
Fox, Patrick F., McSweeney, Paul L. H. Dairy Chemistry and Biochemistry. Kluwer Academic/Plenum Publisher. New York, New York. 1998
Fox, Patrick F. Fundamentals of Cheese Science. Aspen Publishers, Inc. 2000
F. Addeo, V. Alloisio, L. Chianese, V. Alloisio. Tradition and Innovation in the Water Buffalo Dairy Products. Department of Food Science University of Naples “Federico II” 80055 Portici (Napoli),
Figure 1: Calandrelli, Matilde. Manual on the Production of Traditional Buffalo Mozzarella Cheese. Food and Agriculture Organization of the United Nations. http://www.fao.org/ag/againfo/themes/documents/milk/mozzarella.pdf
Figure 2: Tommaso R.I. Cataldi, Massimiliano Angelotti, Giuliana Bianco. Determination of mono- and disaccharides in milk and milk products by high-performance anion-exchange chromatography with pulsed amperometric detection. Dipartimento di Chimica, Università degli Studi della Basilicata, Via N. Sauro 85, 85100 Potenza, Italy
Figure 3: Kindstedt, Paul. American Farmstead Cheese: The Complete Guide to Making and Selling Artisan Cheeses. Chelsea Green Publishing Co. White River Jct., VT. 2005
Image 1: Twitter screen capture
Image 2: Taken from “The Girl Who Ate Everything” Blog http://www.roboppy.net/food/2009/07/lasso-veloce-artichoke-pizzerias-pizzas-east-village-nyc.html Accessed: March 7, 2013