Effect of carbon dioxide on improving the keeping quality of raw milk

Impact of Milk Preacidification with CO2 on the Aging and Proteolysis of Cheddar Cheese

To determine the influence of milk preacidification with CO(2) on Cheddar cheese aging and proteolysis, cheese was manufactured from milk with and without added CO(2). The experiment was replicated 3 times. Carbon dioxide (approximately 1600 ppm) was added to the cold milk, resulting in a milk pH of 5.9 at 31 degrees C in the cheese vat. The starter and coagulant usage rates were equal for the control and CO(2) treatment cheeses. The calcium content of the CO(2) treatment cheese was lower, but no difference in moisture content was detected. The higher CO(2) content of the treatment cheeses (337 vs. 124 ppm) was maintained throughout 6 mo of aging. In spite of having almost one and a half times the salt-in-moisture, proteolysis as measured by pH 4.6 and 12% trichloroacetic acid soluble nitrogen expressed as percentages of total nitrogen, was higher in the CO(2) treatment cheeses throughout aging. The ratio of alpha(s)-casein (CN) to para-kappa-CN decreased faster in the CO(2) treatment cheeses than in the control cheeses, especially before refrigerated storage. No difference was detected in the ratio of beta-CN to para-kappa-CN between the control and CO(2) treatment cheeses. Intact alpha(s)- and beta-CN were found in the expressible serum (ES) from the CO(2) treatment cheese as well as alpha(s1)-I-CN, but they were not detected in the ES from the control cheese. No CN was detected in the ES from the curd before the salting of either the control or CO(2) treatment cheese. Higher proteolysis in the cheese made from milk preacidified with CO(2) may have been due to increased substrate availability in the water phase or increased chymosin activity or retention in the cheese.

Impact of Milk Preacidification with CO2 on Cheddar Cheese Composition and Yield

Preacidification of milk for cheese making may have a beneficial impact on increasing proteolysis during cheese aging. Unlike other acids, CO(2) can easily be removed from whey. The objectives of this work were to determine the effect of milk preacidification on Cheddar cheese composition, the recovery of individual milk components, and yield. Carbon dioxide was injected inline after the cooling section of the pasteurizer. Cheeses with and without added CO(2) were made simultaneously from the same batch of milk. This procedure was replicated 3 times. Carbon dioxide in the cheese milk was about 1600 ppm, which resulted in a milk pH of about 5.9 at 31 degrees C. The starter culture and coagulant addition rates were the same for both the CO(2) treatment and the control. The whey pH at draining of the CO(2) treatment was lower than the control. Total make time was shorter for the CO(2) treatment compared with the control. Cheese manufactured from milk acidified with CO(2) retained less of the total calcium and fat than the control cheese. The higher fat loss was primarily in the whey at draining. Preacidification with CO(2) did not alter the crude protein recovery in the cheese. The CO(2) treatment resulted in a higher added salt recovery in the cheese and produced a cheese that contained too much salt. Considering the higher added salt retention, the salt application rate could be lowered to achieve a typical cheese salt content. Cheese yield efficiency of the CO(2) treated milk was 4.4% lower than the control due to fat loss. Future work will focus on modifying the make procedure to achieve a normal fat loss into the whey when CO(2) is added to milk.

Effect of CO2 addition to raw milk on proteolysis and lipolysis at 4 degrees C

Fresh raw milks, with low (3.1 x 10(4) cell/ml) and high (1.1 x 10(6) cells/ml) somatic cell count (SCC), were standardized to 3.25% fat, and from each a preserved (with 0.02% potassium dichromate) and an unpreserved portion were prepared. Subsamples of each portion were carbonated to contain 0 (control, pH 6.9) and 1500 (pH 6.2) ppm added CO2, and HCl acidified to pH 6.2 Milk pH was measured at 4 degrees C. For the preserved low- and high-SCC milks, two additional carbonation levels, 500 (pH 6.5) and 1000 (pH 6.3) ppm, were prepared. Milks were stored at 4 degrees C and analyzed on d 0, 7, 14, and 21 for microbial count, proteolysis, and lipolysis. The addition of 1500 ppm CO2, but not HCl, effectively delayed microbial growth at 4 degrees C. In general, in both the low- and high-SCC unpreserved milks, there was more proteolysis and lipolysis in control and HCl acidified milks than in milk with 1500 ppm added CO2. Higher levels of proteolysis and lipolysis in the unpreserved milks without added CO2 were related to higher bacteria counts in those milks. In preserved low- and high-SCC milks, microbial growth was inhibited, and proteolysis and lipolysis were caused by endogenous milk enzymes (e.g., plasmin and lipoprotein lipase). Compared with control, both milk with 1500 ppm added CO2 and milk with HCl acidification had less proteolysis. The effect of carbonation or acidification with HCl on proteolysis in preserved milks was more pronounced in the high SCC milk, probably due to its high endogenous protease activity. Plasmin is an alkaline protease and the reduction in milk pH by added CO2 or HCl explained the reduction in proteolysis. No effect of carbonation or acidification of milk on lipolysis was observed in the preserved low- and high-SCC milks. The CO2 addition to raw milk decreased proteolysis via at least two mechanisms: the reduction of microbial proteases due to a reduced microbial growth and the possible reduction of endogenous protease activity due to a lower milk pH. The effect of CO2 on lipolysis was mostly due to a reduced microbial growth. High-quality raw milk (i.e., low initial bacteria count and low SCC) with 1500 ppm added CO2 can be stored at 4 degrees C for 14 d with minimal proteolysis and lipolysis and with standard plate count < 3 x 10(5) cfu/ml.

Serum protein and casein concentration: effect on pH and freezing point of milk with added CO2

The objective of this study was to determine the effect of protein concentration and protein type [i.e., casein (CN) and serum protein (SP)] on pH (0 degree C) and freezing point (FP) of skim milk upon CO2 injection at 0 degree C. CN-free skim milks with increasing SP content (0, 3, and 6%) and skim milks with the same SP content (0.6%) but increasing CN content (2.4, 4.8, and 7.2%) were prepared using a combination of microfiltration and ultrafiltration processes. CO2 was injected into milks at 0 degree C using a continuous flow carbonation unit (230 ml/min). Increasing SP or CN increased milk buffering capacity and protein-bound mineral content. At the same CO2 concentration at 0 degree C, a milk with a higher SP or a higher CN concentration had more resistance to pH change and a greater extent of FP decrease. The buffering capacity provided by an increase of CN was contributed by both the CN itself and the colloidal salts solublized into the serum phase from CN upon carbonation. Skim milks with the same true protein content (3%), one with 2.4% CN plus 0.6% SP and one with 3% SP, were compared. At the same true protein content (3%), increasing the proportion of CN increased milk buffering capacity and protein-bound mineral content. Milk with a higher proportion of CN had more resistance to pH change and a greater extent of FP decrease at the same carbonation level at 0 degree C. Once CO2 was dissolved in the skim portion of a milk, the extent of pH reduction and FP depression depended on protein concentration and protein type (i.e., CN and SP).

Impact of CO2 addition to milk on selected analytical testing methods

The addition of CO2 to raw milk and dairy products controls the growth of psychrotrophic bacteria at refrigeration temperatures. The objective of this study was to determine the effects of dissolved CO2 in milk on the performance of four important routine testing methods: antibiotic residue test, freezing point test, infrared milk component analysis, and alkaline phosphatase test. Raw or pasteurized whole milk was carbonated at <4 degrees C to contain approximately 0 (control), 200, 400, 600, and 1000 ppm of CO2. The addition of CO2 to raw milk up to 1000 ppm had no effect on the performance of the three antibiotic (beta-lactams) residue tests: IDEXX SNAP, Charm II Sequential Tablet, and Delvo-P Ampule. Milk freezing point decreased linearly with increasing concentration of dissolved CO2, from -0.543 degrees H (control) to -0.595 degrees H (1000 ppm). Carbonation to 1000 ppm decreased milk pH (measured at 38 degrees C) from 6.61 (control) to 6.15 (1000 ppm). The effects of CO2 on milk freezing point and pH were reversible upon removal of dissolved CO2. Increased CO2 levels in milk changed the infrared absorption spectrum of milk and caused the corrected lactose readings to decrease and the corrected fat B readings to increase. For the alkaline phosphatase tests, 0 (none), 0.05, 0.1, and 0.2% raw milk were deliberately added to pasteurized milks of six levels of carbonation (0 to 1000 ppm). The addition of CO2 did not influence the ability of Fluorophos, Charm PasLite, and Scharer Modified Rapid tests to differentiate between a pasteurized milk and a pasteurized milk with raw milk contamination.

Preservation of raw milk with CO2. Sensory evaluation of heat-processed milks

The effect of CO2 on the growth of psychrotrophic milk spoilage organisms was studied, both in raw fresh milk and in pure cultures of three species of Pseudomonas growing in sterilised milk. Changes of sensory properties of CO2-treated samples after heat treatment were also analysed. Inhibition of psychrotrophic growth at 7 degrees C in milk treated with CO2 to a pH 6.2 or 6.0 was impaired by a gradual reduction of the CO2 content during storage. Growth inhibition was considerably improved by pH adjustment at 24-h intervals. Sensory analysis showed significant differences between non-acidified and acidified samples after heat treatment at 75 degrees C for 20 s or 110 degrees C for 5 min. No sensory differences were found between non-acidified and acidified milks degassed before heat treatment.

Effect of carbon dioxide on growth and extracellular enzyme production by Pseudomonas fluorescens B52

The effect of carbon dioxide (30 mM.l-1) on growth and extracellular protease and lipase production by Pseudomonas fluorescens B52 growing in a simulated milk medium at 7 degrees C in fermenter was investigated. The addition of carbon dioxide was shown to have a differential effect with extracellular enzyme production, particularly lipase, being inhibited to a greater extent than bacterial growth and final cell numbers.