This work reports on the physicochemical characterization of 21 exopolysaccharides (EPS) produced by Lactobacillus and Bifidobacterium strains isolated from human intestinal microbiota, as well as the growth and metabolic activity of the EPS-producing strains in milk. The strains belong to the species Lactobacillus casei, Lactobacillus rhamnosus, Lactobacillus plantarum, Lactobacillus vaginalis, Bifidobacterium animalis, Bifidobacterium longum, and Bifidobacterium pseudocatenulatum. The molar mass distribution of EPS fractions showed 2 peaks of different sizes, which is a feature shared with some EPS from bacteria of food origin. In general, we detected an association between the EPS size distribution and the EPS-producing species, although because of the low numbers of human bacterial EPS tested, we could not conclusively establish a correlation. The main monosaccharide components of the EPS under study were glucose, galactose, and rhamnose, which are the same as those found in food polymers; however, the rhamnose and glucose ratios was generally higher than the galactose ratio in our human bacterial EPS. All EPS-producing strains were able to grow and acidify milk; most lactobacilli produced lactic acid as the main metabolite. The lactic acid-to-acetic acid ratio in bifidobacteria was 0.7, close to the theoretical ratio, indicating that the EPS-producing strains did not produce an excessive amount of acetic acid, which could adversely affect the sensory properties of fermented milks. With respect to their viscosity-intensifying ability, L. plantarum H2 and L. rhamnosus E41 and E43R were able to increase the viscosity of stirred, fermented milks to a similar extent as the EPS-producing Streptococcus thermophilus strain used as a positive control. Therefore, these human EPS-producing bacteria could be used as adjuncts in mixed cultures for the formulation of functional foods if probiotic characteristics could be demonstrated. This is the first article reporting the physicochemical characteristics of EPS isolated from human intestinal microbiota.

Carbon dioxide treatment of refrigerated raw milk was evaluated as a method for extending storage life by inhibiting growth of psychrotrophic bacteria and other bacterial groups in raw milk. The effect of CO2 acidification followed by degasification and pasteurization on biochemical and microbiological properties of cold stored milk was studied on a pilot scale, Two CO2 treatments (acidification to pH 6.2 and to pH 6.0) were compared with a control (untreated) milk during 4 days of storage at 4°C. Total bacterial counts in the categories of milk established in this study were mainly determined by the proteolytic psychrotroph levels. The inhibitory capability of CO2 was greater in the low-quality than in the high-quality milk category. Acidification at pH 6.0 was more inhibitory than that at pH 6.2, especially against proteolytic psychrotrophs. Neither caseins nor whey proteins were affected by CO2 treatment and pasteurization. Organic acid (orotic, citric, uric, formic, acetic, propionic, and hippuric) concentrations did not change after CO2 treatment, cold storage, or the pasteurization process; the lactic acid content of CO2-treated milks remained constant during the refrigeration time but increased slightly in the control. In general, lower amounts of volatile compounds were produced in CO2-treated milks during refrigeration than in control milk. Ethanol and 2-propanol levels were most affected by degasification and pasteurization. Sensory evaluation revealed no significant differences between CO2-treated milk after degasification and pasteurization and the untreated milk used as control. It was concluded that degasification and pasteurization on a pilot scale eliminated CO2 from milk with minimum detrimental effects on its biochemical and sensory properties, making this process acceptable for milk preservation.

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The effect of the application of CO2 to extend the cold storage of raw and pasteurized milk on the content of fat-soluble vitamins of milk was investigated. CO2-treated milk (pH 6.2) was compared with a control (unacidified) milk. CO2-treated and control raw milk samples were stored at 4 °C for 4 days. CO2-treated milk was then vacuum degasified, and both control and treated samples were pasteurized and stored at 4 °C for 7 days. CO2 addition inhibited the growth of microorganisms in raw milk without affecting the stability of vitamin A (retinol and β-carotene) and vitamin E (α-tocopherol). Acidity and pH data indicated that subsequent vacuum degasification and pasteurization on a pilot scale partially removed CO2, making milk acceptable for liquid consumption. However, the residual CO2 present extended the cold-storage period of pasteurized milk by inhibiting bacterial survivors without detrimental effects on retinol, β-carotene, and α-tocopherol. Slightly higher (not statistically significant, p > 0.05) concentrations of retinol, β-carotene, and α-tocopherol were detected during cold storage in raw and pasteurized CO2-treated milk with respect to the control milk, which could be related to a certain protective effect of the CO2. Keywords: Milk; carbon dioxide; psychrotrophs; cold storage; fat-soluble vitamin

On-farm refrigeration of milk reduces the growth rate of mesophilic bacteria so allowing longer storage time of raw milk before processing. However, refrigeration creates a selective pressure favouring the multiplication of psychrotrophic bacteria present as normal contaminants in raw milk. Psychrotrophs are killed by most of the currently employed pasteurization and sterilization treatments of milk but they can produce extracellular heat-stable proteinases and lipases, which are capable of degrading various milk components, affecting the storage-life of heat-processed milk and the quality of dairy products (Champagne et al. 1994; Shah, 1994).

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