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Mikrobiologiske og kemiske forandringer i rødkitost - fremstillet af råmælk og pasteuriseret mælk

Højgaard, Kenneth S. (2004) Mikrobiologiske og kemiske forandringer i rødkitost - fremstillet af råmælk og pasteuriseret mælk. [Microbiological and chemical changes in red-smeared soft cheese - prepared from raw and pasteurized milk.] Thesis, KVL, Department of Food Science. .

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Summary

I Danmark bliver al mælk til fremstilling af bløde oste varmebehandlet, hvorved nogle mener, at ostene mister deres specielle smag. Et af hovedargumenterne mod pasteurisering er, at diversiteten i mælkesyrebakterierne forsvinder, hvilket underbygges med, hvad man ved om hårde oste; i cheddar og hårde schweizeroste er man inden for de sidste årtier begyndt at anerkende betydningen af medfølgefloraen som smagsdanner.
I dette speciale er forskellen i udviklingen af mikrobiologiske og enkelte kemiske parametre mellem oste fremstillet af rå og pasteuriseret mælk blevet undersøgt. Dette blev gjort ved at fremstille fire produktioner af en ost af münstertypen. I hver produktion blev der fremstillet en batch af rå mælk og en af pasteuriseret mælk.
Under produktionen og modningen af disse oste blev der på Den Kgl. Veterinær- og Landbohøjskole foretaget en række mikrobiologiske undersøgelser, mens der samtidigt blev foretaget en række fysiske og kemiske målinger på Danmarks Jordbrugsforskning.
De fire produktioner foregik på forskellige tidspunkter af året, og der var derfor forskel på, om køerne var på græs eller gik på dybstrøelse inde i stalden, og på om foderet var græs eller ensilage. Der viste sig meget stor variation mellem ostene fra de fire produktioner, forskelle, der dog ikke alene kan tilskrives årstidsvariationen, men må forklares med fejl under produktionerne; i anden produktion blev der fundet flere Enterobacteriaceae i den pasteuriserede mælk end i den rå mælk, hvilket kun kan skyldes en efterkontaminering af mælken efter varmebehandling; i tredje produktion var der ikke ret mange Enterobacteriaceae i mælken, men pga. en inaktiv starterkultur nåede indholdet i begge typer oste op på ca. 10.000.000 CFU pr. g. efter 10 dage. I første og fjerde produktion foregik ostefremstillingen uden problemer, men de fremstillede oste var forskellige i størrelse og kan derfor ikke sammenlignes. Der blev i alle fire produktioner fundet et signifikant højere indhold af bakterier fra starterkulturen i råmælksostene, selvom den tilsatte mængde var den samme som i den pasteuriserede mælk
Medfølgefloraen i ostene kom først til sidst i modningen op på et niveau, hvor den ville kunne have betydning for smagsdannelsen. Derimod var der flere oste, som havde et meget højt niveau af Enterobacteriaceae, deriblandt Hafnia alvei, som danner putrescin sammen med mælkesyrebakterier.
Plasminaktiviteten i de tre produktioner - hvori den blev målt - var varierende og ikke som forventet højere i ostene af den pasteuriserede mælk. Der var en tendens til hurtigere modning i de oste, der havde høj plasminaktivitet.
I flere undersøgelser er indholdet af vand i fedtfri ost (VFFO) højere i oste fremstillet af pasteuriseret mælk end af rå mælk. Dette svarer til vores resultater i to af produktionerne, hvorimod indholdet var på samme niveau i en tredje produktion og højere i råmælksostene i en fjerde. Sidstnævnte kan muligvis forklares med, at fedtindholdet ikke var det samme i den rå mælk og i den pasteuriserede mælk.

Summary translation

In Denmark, all milk used for production of soft cheese is pasteurized, a process believed by some to cause a loss of taste in the cheese. One of the main arguments against pasteurization is the loss of diversity in the lactic acid bacteria. This is supported by evidence from hard cheeses; during the past decades the importance of non-starter lactic acid bacteria as flavour producers in Cheddar and hard Swiss cheeses is increasingly being acknowledged.
This thesis examines the differences in the development of microbiological and some chemical indicator parameters in cheeses produced by raw and pasteurized milk, respectively. This was done by producing four productions of a cheese of the Münster type. In each production a batch was developed by both raw milk and by pasteurized milk.
During production and maturity of the cheeses a number of microbiological tests were carried out at the Royal Veterinary and Agricultural University. At the same time, a number of physical and chemical tests were carried out at the Danish Institute of Agricultural Sciences. The four productions took place at different times of the year. This caused different results depending on whether the cows were grazing or in the stable. The results furthermore differed depending on the feed being grass or silage. The cheeses from the four productions showed quite large differences. These differences cannot be fully explained by seasonal differences but could be related to accidents during production. During the second production more Enterobacteriaceae were identified in the pasteurized milk than in the raw milk. This can only be caused by a post-contamination of the milk following pasteurization.
The third production did not have significant numbers of Enterobacteriaceae in the milk. However, due to an inactive starter culture, the content of Enterobacteriaceae reached approximately 10,000,000 CFU per gram after 10 days. For the first and the fourth batches the production of cheeses did not create problems but the cheeses produced were of different sizes and were therefore not easily comparable. In all four batches a significantly higher level of bacteria from the starter culture was found in the cheeses from raw milk in spite of the quantity being identical to the quantity used in the pasteurized milk.
Only at a late stage of ripening did the Non-Starter Lactic Acid Bacteria (NSLAB) reach a level where it could have had an impact on the taste. On the other hand, several cheeses had a quite high level of Enterobacteriaceae, this includes Hafnia alvei which together with lactic acid bacteria produces putrescin.
The level of plasmin activity in the three batches varied and was unexpectedly not higher in the cheeses made from pasteurized milk. A tendency to mature faster was identified in the cheeses with a high level of plasmin activity.
In several other studies the content of water in fat free cheese was higher in cheeses developed from pasteurized milk than in cheeses developed from raw milk. This confirms our results in two productions. On the other hand, the content of water was at the same level (in the two types of cheeses) in a third production and even higher in the cheeses from raw milk in a fourth production. The last result may be explained by different levels of fat in the raw milk and in the pasteurized milk.

EPrint Type:Thesis
Thesis Type:Master thesis
Subjects: Food systems > Food security, food quality and human health
Research affiliation: Denmark > DARCOF II (2000-2005) > II.11 (RAWMICHEESE) Production of organic raw milk cheese and content of phyto-estrogens
Deposited By: Hansen, Dr. Tina Beck
ID Code:3915
Deposited On:25 Oct 2004
Last Modified:12 Apr 2010 07:30
Document Language:Danish - Dansk
Status:Published
Refereed:Not peer-reviewed

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