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Thursday, August 28, 2008

Blue Brie


Blue Brie is an interesting cheese in that it has the white mould, Penicillium camemberti, growing on the outside (as does Camembert and Brie) while the blue mould, Penicillium roqueforti, grows inside the cheese. The cheese has a creamy consistency, not unlike that of Camembert, but with a distinct methyl ketone flavour note from fatty acid metabolism by P. roqueforti. However, its flavour is much less pungent in this respect than Blue cheese.

One method of making Blue Brie is to add spores of P. roqueforti to the milk; spores thus become well distributed throughout the cheese mass. After manufacture, the cheese can then be dipped into hot water to inactivate the spores of P. roqueforti on the surface of the cheese which is then sprayed with spores of P. camemberti. Unlike Blue cheese, which has a crumbly texture characterised by many mechanical openings, Blue Brie has a close texture that allows no air into the cheese mass. Penicillium spp. require oxygen for growth and so the cheese is pierced with skewers to allow some air into the centre of the cheese mass. The spores of P. roqueforti which lie along the channels formed on piercing then germinate in the centre of the cheese. Unlike the random veins in Blue cheese, the "veins" in Blue Brie are noticeably straight. Blue Brie is thus a mycological tour de force which helps explain its relatively high price.

Perhaps surprisingly, I am not a major eater of cheese; however, when I do buy cheese, Blue Brie is amongst my favourite varieties. A major brand on the market is Cambozola (the name is apparently a portmanteau of "Camembert" and "Gorgonzola") produced by the German company Champignon. The picture above does not do Cambozola justice; usually the surface is much whiter and has a denser covering of P. camemberti. Even if you feel the pungent flavour of Blue cheese is too much, you should try a Blue Brie like Cambozola; the flavour is much more subtle than for example a Danish Blue and should be to everyone's taste.

Wednesday, August 20, 2008

Paper- J Appl Microbiol

Milesi, M.M., P.L.H. McSweeney and E.R. Hynes (2008). Viability and contribution to proteolysis of an adjunct culture of Lactobacillus plantarum in two model cheese systems. Journal of Applied Microbiology 105, 884-892.

Friday, August 15, 2008

Peptidase specificity


Lactic acid bacteria (LAB) are auxotrophic for a range of amino acids and hence need to obtain them pre-formed from their environment. The LAB thus possess a wide range of proteolytic enzymes which allow them to degrade the caseins to provide the amino acids necessary to support their growth in milk (a growth substrate relatively poor in free amino acids but rich in protein). The principal proteinase in LAB is a cell wall-associated enzyme which degrades the caseins to short peptides which are further degraded by a wide range of intracellular peptidases to free amino acids. These peptidases are essential for LAB growth in milk but also contribute greatly to proteolysis in cheese during ripening. Lysis after cell death allows the peptidases to enter the cheese matrix where they act principally upon short peptides produced by proteinases during ripening.


The caseins are relatively rich in the amino acid proline. Proline has a cyclic structure that is quite different to that of all other amino acids. Hence, many "normal" peptidases are unable to degrade proline-containing peptides. Thus, LAB also have a range of proline-specific peptidases (PepX, I, Q, R, P) to allow them to utilise the caseins fully.


Although it never occurs in cheese in practice, the LAB have enzymes capable of hydrolysing every peptide bond in the casein system.


Further reading:


Upadhyay, V.K., P.L.H. McSweeney, A.A.A. Magboul and P.F. Fox (2004). Proteolysis 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. 392-433.

Paper- Int. Dairy J.


Sheehan, J.J., M.G. Wilkinson and P.L.H. McSweeney (2008). Influence of processing and ripening parameters on starter, non-starter and propionic acid bacteria and on the ripening characteristics of semi-hard cheeses. International Dairy Journal 18, 905-917.

Tuesday, August 12, 2008

Paper- Food Chem.


Kongo, J.M., F.X. Malcata and P.L.H. McSweeney (2009). Microbiological, biochemical and compositional changes during ripening of São Jorge - a raw milk cheese from the Azores (Portugal). Food Chemistry 112, 131-138.

Friday, August 8, 2008

Proteolysis of alpha-s1-casein by chymosin


Proteolysis of alpha-s1-casein by chymosin in cheese follows the following pattern in most ripened cheeses. Chymosin initially cleaves at Phe23-Phe24 (its primary cleavage site on alpha-s1-casein) forming a short peptide, f1-23 and a larger polypeptide, f24-199. The former peptide does not accumulate in cheese but is rapidly degrated by the cell envelope-associated proteinase (CEP) of the starter to a range of shorter peptides, depending on the specificity of the CEP. Fragments such as f1-9, f1-13 and f1-16 are not uncommon and resolve as major peaks in reverse-phase HPLC chromatograms of extacts from cheeses such as Cheddar.


Meanwhile, the polypeptide f24-199, which resolves well on urea-polyacrylamide gel eletrophoresis (urea-PAGE), is next cleaved by chymosin towards its centre at Leu101-Lys102 yielding a polypeptide, f102-199, which is also easily resolved by urea-PAGE.


(Please note that "alpha-s1-casein" is usually written as shown in the accompanying figure but I cannot seem to use symbols on this web site!)

Tuesday, August 5, 2008

Newspaper clipping...


During my recent trip to Argentina, I had a chance to visit briefly the most active dairy research group in Universidad Nacional del Litoral, Santa Fe; mention of the visit was made on the local paper, El Litoral! Now, if only I could understand Spanish...