Paper- Int. Dairy J.

Piraino P., T. Zotta, A. Ricciardi, P.L.H. McSweeney and E. Parente (2008). Acid production, proteolysis, autolytic and inhibitory properties of lactic acid bacteria isolated from pasta filata cheeses: a multivariate screening study. International Dairy Journal 18, 81-92.

To degrade or not to degrade...

Named after Adolph Strecker (1822-1871), the Strecker degradation is a reaction between an alpha-amino acid and a dicarbonyl (usually produced through the Maillard reaction) and has been well studied in food chemistry as it produces volatile flavour compounds in many products that undergo Maillard browning. An example of the Strecker degradation between diacetyl and valine is shown below. As can be seen, a product of the Strecker degradation is an aldehyde corresponding to the amino acid (isobutyraldehyde in this case).Early studies on volatile flavour compounds in cheese also found aldehydes with structures that were obviously related to amino acids and the assumption was made that these compounds also originated from the Strecker degradation since that reaction had been well characterised in the context of other foods. However, later work has shown that amino acids in cheese are catabolised by other pathways, some of which produce aldehydes.

The crucial step in the catabolism of most amino acids in cheese during ripening is catalysed by enzymes known as aminotransferases, principally from starter organisms.
The co-substrate usually used by aminotransferases is alpha-ketoglutarate and thus the products of the reaction are glutamic acid and an alpha-keto acid corresponding to the original amino acid. These alpha-keto acids then degrade by various reactions to yield a range of volatile flavour compounds. Amino acid catabolism is perhaps the most important series of reactions for the production of volatile flavour compounds in many ripened cheeses.

Further reading:

Ardo, Y. (2006). Flavour formation by amino acid catabolism. Biotechnol. Adv. 24, 238-242.

Curtin, Á.C. and P.L.H. McSweeney (2004). Catabolism of amino acids in cheese during ripening. In Cheese: Chemistry, Physics and Microbiology, Volume 1, General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier Applied Science, Amsterdam. pp. 436-454.

Yvon, M. and Rijnen, L. (2001). Cheese flavour formation by amino acid catabolism. Int. Dairy J. 11, 185-201.

Did you know...? Eyes in Swiss-type cheese

Mechanical openings (resulting from the incomplete fusion of curd pieces) are common in many cheese varieties where they may or may not be considered desirable. However, some internal bacterially-ripened varieties are characterized by the development of eye holes caused by gas, produced by bacterial metabolism, being trapped in the curd. In many cheeses, gas production is undesirable but there are two main families of cheeses where eyes are desired. Dutch types (Edam, Gouda and related varieties) have occasional small eyes while cheeses related to Swiss Emmental are characterized by large eyes. While eye development in Dutch-type cheeses is due to citrate metabolism, in Emmental-type varieties, CO2 is produced by Propionibacterium freudenreichii from lactate during the "hot-room" step of ripening (see below for overall chemical equations). CO2 migrates through the cheese curd until it finds a point of weakness where it accumulates to form an eye. Interestingly, only a small proportion of the CO2 remains in the eyes; most remains dissolved in the aqueous phase of the cheese or is lost from the surface. Eye development in Swiss cheese depends on the rate and quantity of CO2 production, the number and size of loci for eye development, CO2 partial pressure and diffusion rate and of course cheese texture and temperature. P. freudenreichii is very sensitive to salt and consequently Emmental-type cheeses have amongst the lowest NaCl levels of all varieties.

For more on eyes in cheese see:

McSweeney, P.L.H. (2003). Cheeses with “eyes”. In Encyclopedia of Food Science and Nutrition, 2nd edition, B. Caballero, L. Trugo and P.M. Finglas (eds), Academic Press, London, pp. 1087-1094.

McSweeney, P.L.H. and P.F. Fox (2004). Metabolism of residual lactose and of lactate and citrate. In Cheese: Chemistry, Physics and Microbiology, Volume 1, General Aspects, 3rd edition, P.F. Fox, P.L.H. McSweeney, T.M. Cogan and T.P. Guinee (eds), Elsevier Applied Science, Amsterdam. pp. 361-371.

5th IDF Symposium on Cheese Ripening

I'm just home from the 5th International Dairy Federation Symposium on Cheese Ripening held this week in Bern. Needless to say, the event ran like Swiss clockwork for the 400+ delegates and it was very nice to catch up with so many cheese friends. A lovely feature of the world of cheese research is that it is really like a large family; it is a small area and so one can so easily get to know well many scientists from around the world.

In between cobbling together a grant proposal with some Irish colleagues, I managed to attend nearly all sessions quietly sitting up in the top corner soaking up the latest thoughts on cheese.

The symposium programme reflected the current themes in cheese research; we learned much about PDO varieties but perhaps there were fewer talks than before on the big industrial products, Cheddar, Mozzarella, Gouda and processed cheese. The poster sessions were very well attended and contained many interesting ideas for future work. One that stood out for me was Wim Engel's method for making micro-scale cheese in a microtitre plate developed at the NIZO; Wim easily trumped our mini-scale cheesemaking technique (20 g) by about two orders of magnitude!

The symposium dinner was held in a marquee on top of the Gurten in the midst of the tail-end of the terrible storm that hit Ireland earlier in the week. Science was largely left behind that evening which was a symposium in the more classical sense of the term(!) but with Swiss folk music. Ylva Ardo of Copenhagen received a much deserved award for all her dedication and hard work through the years. I had my first introduction to Raclette, a cheese I had read about but never before tasted. It was absolutely gorgeous when melted and the organisers arranged a few hundred grammes for each conference delegate as a very welcome going-away present.

All-in-all, congratulations and thanks to the local organising committee and particularly to Ueli Butikofer, Hans-Peter Bachmann and of course Johann, the quirky cartoon character who kept us all entertained with his yodelling!

The world of cheese

It is nice to see that www.cheesescience.net is being accessed; here are the locations of hits received in the last two weeks or so. Thanks to everyone for looking...!