• Enterococci in Food Fermentations: Functional and Safety Aspects

      Cogan, Tim; Rea, Mary; Drinan, Finbarr; Gelsomino, R. (Teagasc, 2001-04-01)
      Enterococci are natural residents of the human and animal gastrointestinal tracts; many species are also found in soil, plants and food. These organisms also form an important part of the microflora of many cheeses, especially those made in Southern Europe, where they can reach levels of 107 - 108 cfu/g. There is contradictory information on their role in flavour development in cheese with some studies showing that they have a positive effect and others a negative one. Enterococcus faecalis, Ec. faecium and Ec. durans are the important species found in cheese, though recent results from our laboratory show that Ec. casseliflavus may also be important (see below). Many of these species withstand pasteurisation. Their presence in food has been questioned because they are responsible for many nosocomial infections in hospitals. They are also promiscuous and easily transfer antibiotic resistance to other organisms and acquire resistance to vancomycin themselves. Cheddar cheese has a complex microflora and is conducive to growth of many bacteria, especially lactic acid bacteria. Enterococci are facultative anaerobes, which ferment lactose and can grow in high salt concentrations. Therefore, they should grow in cheese if they are present in the raw milk. Phenotypically they can be confused with starter lactococci. Traditionally, they are separated from lactococci by their ability to grow at 45°C and in 6.5% salt. However, these tests have serious drawbacks since some species of enterococci cannot grow at 45°C and some lactococci can grow at 45°C and in 6.5% salt. The effect of enterococci on flavour development in Cheddar cheese has not been studied to any great extent. The overall objectives of this collaborative project were to investigate the taxonomic relationships between food, veterinary and clinical isolates of enterococci, their virulence, their ability to produce toxins, their antibiotic resistance and their technological performance in cheesemaking. The specific objectives of the Moorepark team were to study the co-metabolism of citrate and sugar by enterococci, develop a DNA probe to distinguish between Enterococcus and Lactococcus and evaluate the contribution of enterococci to flavour development in Cheddar cheese.
    • Identification of the key compounds responsible for Cheddar cheese flavour

      Beresford, Tom; Wallace, J.; Aherne, Seamus; Drinan, Finbarr; Eason, D.; Corcoran, M.O.; Mulholland, E.; Hannon, John A. (Teagasc, 2000-09-01)
      There is a poor understanding of the relationship between organoleptic assessment of cheese and quantitative analysis of flavour compounds. Further, the contribution of particular cheese-making parameters such as ripening temperature and starter culture has not been fully elucidated. During the ripening of most cheese varieties complex chemical conversions occur within the cheese matrix. In most cheese varieties breakdown of protein is the most important flavour development pathway. The primary cheese protein, casein, is degraded enzymatically to short peptides and free amino acids. The agents primarily responsible for these conversions are the residual rennet that is retained in the cheese curd at the end of the manufacturing phase and the proteinases and peptidases that are associated with the starter bacteria. While the rate and degree of proteolysis are of vital significance for desired flavour development, the direct products of proteolysis do not fully define cheese flavour. Much research is now demonstrating that the further biochemical and chemical conversions of the products of proteolysis, in particular the amino acids, are necessary for full flavour development. The products produced by these pathways are volatile at low boiling points and are thus released during mastication of the cheese in the mouth. Many of these volatile compounds contribute to the flavour sensation experienced by the consumer. A very wide spectrum of such compounds have been isolated from cheese, in excess of two hundred in some cheese varieties. It is now generally accepted that there is no individual compound which defines cheese flavour completely and that the flavour sensation is the result of numerous compounds present in the correct proportions. This has become known as the Component Balance Theory . The application of modern analytical techniques as proposed in this project would provide a greater understanding of the significant flavour compounds in Cheddar cheese and help to identify the impact of specific cheese-making parameters such as starter flora and ripening temperature on the production of volatile flavour compounds. This data would assist the general programme on flavour improvement of cheese which should ultimately benefit the cheese manufacturer. Hence this project set out to develop methods to identify the key flavour compounds in Cheddar cheese. These techniques would then be applied to experimental and commercial cheeses during ripening in an effort to identify key compounds and the influence of starter cultures and ripening temperature on their production.
    • Role of Lactobacilli in Flavour Development of Cheddar Cheese.

      Beresford, Tom; Cogan, Tim; Rea, Mary; Drinan, Finbarr; Fitzsimons, Nora; Brennan, N.; Kenny, Owen; Fox, P.F. (Teagasc, 2001-05-01)
      Cheddar cheese is a complex microbial ecosystem. The internal cheese environment, in particular of hard and semi-hard cheeses, is not conducive to the growth of many microorganisms. At the beginning of ripening the dominant microorganisms are the starter bacteria which are present at high levels (~109/g). However, during ripening, non-starter lactic acid bacteria (NSLAB) grow from relatively low levels (<103/g) at the beginning of ripening, to 108/g within 6 - 8 weeks. Other bacteria, e.g. enterococci and staphylococci, may also be present but in much lower numbers. In a previous study of mature and extra mature Cheddar cheeses from different manufacturers (see End of Project Report No. 1), it was found that the NSLAB population was dominated by strains of Lb. paracasei. However, their contribution to cheese flavour and their source(s) are still unclear, nor is it known if the NSLAB flora is unique to each plant. Hence, understanding the growth of this group of organisms in cheese is a key to defining their role in flavour development. The biochemistry of flavour development in cheese is poorly understood. For most cheese varieties, including Cheddar, proteolysis, which results in the accumulation of free amino acids, is of vital importance for flavour development. Increasing evidence suggests that the main contribution of amino acids is as substrates for the development of more complex flavour and aroma compounds. The manner by which such compounds are generated in cheese is currently the focus of much research. Starter bacteria have been shown to contain a range of enzymes capable of facilitating the conversion of amino acids to potential flavour compounds. However, the potential of lactobacilli (NSLAB) to produce similar enzymes has only recently been investigated. Hence, although, it is generally accepted that the cheese starter flora is the primary defining influence on flavour development, the contribution of NSLAB is also considered significant. The objectives of these studies were: - to develop a greater understanding of the behaviour of NSLAB in cheese, and - to identify suitable strains, and other cheese bacteria, to be used as starter adjuncts for flavour improvement.
    • Significance of Lactobacilli in Cheddar Cheese

      Cogan, Tim; Beresford, Tom; Drinan, Finbarr; Palles, Tony; Fitzsimons, Nora (Teagasc, 1998-09-01)
      The objectives of this project were to isolate and identify the non-starter lactobacilli in mature Cheddar cheese, identify strains which impart mature flavours to cheese and determine their role in developing cheese flavour. The main conclusions were as follows: Based on an analysis of 18 mature Cheddar cheeses, selected from 7 commercial manufacturers, non-starter lactic acid bacteria typically numbered, as expected, 106-108 per gram and were dominated (97 percent) by Lactobacillus paracasei. Although a small number of strains (typically 1 to 4) was found in each cheese there was considerable strain diversity in cheeses within as well as between manufacturing plants. When selected strains were investigated for survival and flavour enhancement when added (as starter adjuncts) with the normal starter cultures in Cheddar cheese manufacture, it was found that they remained dominant for up to 3 months of ripening. Commercial grading of these cheeses at 3 and 6 months confirmed that the added strains did modify flavour development and one (DPC 4103), in particular, had a beneficial effect. It was confirmed that two selected strains of non-starter lactobacilli were capable of metabolising citrate under the conditions of Cheddar cheese ripening and, consequently, if present in sufficient numbers, would influence flavour development. The work was greatly facilitated by the successful and novel adaptation of a modern rapid molecular technique (RAPD) for species and strain classification. In summary these studies found that one species of lactobacilli (Lb. paracasei) was the dominant non-starter lactic acid bacteria in mature Cheddar cheese. Although a wide variety of strains were identified, only a few were found in any particular cheese, suggesting their likely role in cheese flavour diversity even within the same manufacturing plant. This suggests the potential for flavour control or enhancement through the selective and controlled use of non-starter lactic acid bacteria. Preliminary investigations of the metabolism of those organisms supports this view and a follow-up study now in progress should provide greater clarity on this matter.