Merry Christmas

The figure above shows the locations of the last couple of dozen hits to www.cheesescience.net. As can be seen, the world of cheese is very diverse, but hereabouts it is shortly going to be Christmas, a time for goodwill and thanks. Hence, I would like to thank the people behind the 4,129 hits to www.cheesescience.net since about last March. It is very gratifying to see that this little site receives so many hits and I hope the miscellany of snippets about cheese science and technology have been of some interest and use to you.

It was a busy year for me travel-wise: conferences in Berne, Buenos Aires and Mexico City, teaching courses in Ankara and a university in Bolu (a city half-way between Ankara and Istanbul) and visiting students in parts of north Cork as far-flung as Kanturk! Holidays this year were tied mainly onto work trips. I had a couple of fascinating days revelling in museums and the Byzantine remains of Istanbul and finally got to explore a bit of Argentina properly with a few days in Buenos Aires (and yes, I did see the tango in its native habitat!) with a short trip right up north to Salta in the foothills of the Andes (wonderful scenery). As for publishing, with great relief to me and Prof Fox, the manuscript for the third and final volume of the third edition of Advanced Dairy Chemistry (covering lactose, water, vitamins and minor constituents of milk) left my desk for the publishers during the summer with publication due for early 2009. Our main publishing project now is the revised edition of the four volume Encyclopedia of Dairy Sciences which will be published by Elsevier in 2011.

I wish a most Merry Christmas and a Prosperous New Year to those who celebrate these festivals at this time of the year and, to those who do not, please accept my good wishes for your special days. There are not many Christmas traditions related to cheese, but I do suggest you try the combination of Port wine and Stilton (or the Blue variety of your choice). The full bodied intensity of Port perfectly complements the strong methyl ketone flavour of Blue cheese.

I leave you with my best wishes and a photo of the Christmas tree in chez McSweeney.

Cheese... but not as we know it

A present (photo) from a visiting student from Norway reacquainted me with a distinctive group of Norwegian unripened "cheese" varieties which are made by thermal evaporation, concentration and crystallisation of whey to which may be added skim milk or cream. These cheeses (collectively referred to as Brunost, meaning brown cheese, and including Geitost, Mesost/Myseost [popular in Sweden and Denmark, respectively], and Gudbrandsdalsost) are characterised by a smooth, creamy body and a sweet, caramel-like taste. Unusually for dairy products, the Maillard reaction is encouraged (the only other example that immediately springs to mind where extensive non-enzymatic browning is desirable in dairy products is the Argentinean dessert confection, Dulche de Leche). It is arguable whether Norwegian whey cheeses should be called "cheese" as they are really fat/protein-enriched by-products of concentrated heated whey. The vast majority of cheeses involve dehydration of milk by controlled syneresis of rennet- or acid-induced casein gels. In the case of these products, dehydration is achieved quite differently (by thermal evaporation) and their manufacture involves extensive lactose crystallisation. Still, to paraphrase the Bard, in this case, a cheese by any other name would taste as sweet!

Further reading:

Fox, PF, TP Guinee, TM Cogan and PLH McSweeney (2000). Fundamentals of Cheese Science. Aspen Publ., Gaithersburg, MD.

Kosikowski, FV and VV Mistry (1997). Cheese and Fermented Milk Foods (3rd edn, 2 vols), FV Kosikowski LLC, Westport, CT.

Production of succinate from citrate by Lb. plantarum

Succinic acid has an acidic, salty/bitter, taste and contributes to savoury flavour of cheese; Emmental cheese contains approximately 0.8-1.4 g succinate/kg. Succinate may be produced from citrate by Lactobacillus plantarum via a reductive tricarboxylic acid pathway (see below). Lb. plantarum is a component of the non-starter lactic acid bacterial (NSLAB) flora of many cheeses.

However, succinate is not usually found in Cheddar at concentrations above its taste threshold (~37 ppm). This may be because (i) strains of Lb. plantarum in NSLAB may not be able to use pathway, (ii) not enough organisms and/or (iii) other citrate-positive microorganisms outcompete Lb. plantarum.

Further reading:

Dudley, EG and Steele, JL (2005). Succinate production and citrate catabolism by Cheddar cheese nonstarter lactobacilli. J. Appl. Microbiol. 98, 14-23.

Christmas is a coming...!

Christmas must be around the corner since the cheese researchers of Lab 232 had their annual lunch last Friday! From left to right, Diletta, Shirinda, Veronica, Paul, Anna, Kim and Himanshu. This year we had four different nationalities around the table (Ireland, Italy, Norway and India). As usual, we went to Cafe Paradiso, a prize-winning vegetarian restaurant on Lancaster Quay near UCC. I strongly recommend Cafe P should anyone be visiting Cork; the food is superb.

Last week also saw the winter conferrings at which our 2007-08 MSc (Applied Science) in Food Science graduated.

Lactose metabolism in lactic acid bacteria

Since cheese is a fermented dairy product, lactose metabolism by lactic acid bacteria has been thoroughly studied and is now well understood. Lactococcus lactis transports lactose into the cell using the phosphoenolpyruvate phosphotransferase system which phosphorylates lactose to lactose-6-phosphate. Lactococci possess a phospho-beta-galactosidase which hydrolyses lactose-6-phosphate to glucose and galactose-6-phosphate. The latter is converted, via the tagatose pathway, to dihydroxyacetone-phosphate which is isomerised to glyceraldehyde-3-phosphate and thus into the glycolytic pathway. The glucose moiety of lactose is metabolised to lactic acid by glycolysis. Lactococci produce 4 mol L-lactate per mol lactose.

Most other lactic acid bacteria transport lactose into the cell as lactose using a permease. These organisms hydrolyse lactose to glucose and galactose using a beta-galactosidase; galactose is converted to glucose-6-phosphate by the Leloir pathway and thus into the glycolytic pathway. Streptococcus thermophilus metabolises 1 mol lactose to 2 mol L-lactate since they metabolise only the glucose moiety of the disaccharide. Certain species of lactobacilli produce D- or DL-lactate, depending on the type of lactate dehydrogenase they possess. Leuconostoc sp. use a different sequence of biochemical events for lactose metabolism; the end products of phosphoketolase pathway are quite different (1 mol lactose is converted to 2 mol D-lactate, 1 mol ethanol and 2 mol CO2) to those of glycolysis.

Further reading:

Fox, P.F., T.P. Guinee, T.M. Cogan and P.L.H. McSweeney (2000). Fundamentals of Cheese Science. Aspen Publishers, Gaithersburg, MD. 587 pp

Predicting cheese yield

The manufacture of many cheeses involves an approximate 10-fold concentration of the solids in milk. Thus, a very rough rule-of-thumb suggests that one should obtain about 1 kg cheese from 10 L milk. Various formulae have been developed down through the years to estimate more accurately the yield of cheese of specific varieties. A widely used formula is the Van Slyke yield formula:

Where Ya is actual cheese yield (kg per 100 kg milk), F and C are the fat and casein contents of the cheesemilk (with added starter culture), respectively, %FR is fat recovery (%FR/100 is often 0.93 for Cheddar cheese), a is a coefficient to account for casein loss (often 0.1 for Cheddar) and b is a coefficient to account for cheese solids non-fat, non-protein (often 1.09 for Cheddar). This formula often underestimates yield of high moisture cheeses. Also, 93% recovery of fat may not be achieved. Factors must be varied for other cheese varieties and can also vary between factories and application of a generic yield formula may not accurately predict yield in all plants. If pronounced seasonal variation, theoretical formulae give less accuracy, mainly due to errors in estimating casein concentration.

Plant specific formulae are developed by analysing historical data on cheese yield in relation to milk composition, milk quality, fat and protein recovery and cheese salt and moisture contents. These must be updated regularly but give very accurate predictions for a particular plant. However, they are not applicable to other plants and are dependent on accuracy of historical data. Also, it is important to remember that accurate prediction does not mean that a plant is operating at maximum efficiency!

Further reading:

Banks, J.M. (2007). How can cheese yield be predicted? In Cheese Problems Solved, P.L.H. McSweeney (ed.), Woodhead, Cambridge, pp. 105-6.