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 ofPseudomonas growing in sterilised milk. Changes of sensory properties of CO₂-treated samples after heat treatment were also analysed. Inhibition of psychrotrophic growth at 7 °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 °C for 20 s or 110 °C for 5 min. No sensory differences were found between non-acidified and acidified milks degassed before heat treatment.

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Konservierung von Rohmilch mit Kohlendioxid. Sensorische Untersuchung wärmebehandelter Milch

Zusammenfassung Es wird der Einfluß von CO₂ auf das Wachstum von psychrotrophen Milchfäulnisorganismen in frischer Rohmilch bzw. in reinen Kulturen von drei Spezies von in sterilisierter Milch wachsendenPseudomonas untersucht. Auch die Veränderungen in den sensorischen Eigenschaften nach der Wärmebehandlung der mit CO₂ behandelten Proben werden untersucht. Die Hemmwirkung auf das von Psychrotrophen bei 7 °C in durch CO₂ auf pH 6.2 bzw. 6.0 eingestellte Milch schwindet durch die allmähliche Abnahme an CO₂ während der Lagerung. Der hemmende Einfluß steigt erheblich durch tägliche Einstellung des pH-Werts. Die sensorischen Analysen zeigten signifikante Unterschiede zwischen nicht angesäuerten und angesäuerten Proben nach einer Erhitzung auf 75 °C für 20 s bzw. 110 °C für 5 min. Es konnten jedoch keine signifikanten sensorischen Unterschied zwischen nicht angesäuerter und angesäuerter Milch, die vor der Hitzebehandlung entgast worden waren, beobachtet werden.

     

Manufacture of cheese made from CO2-treated milk

Cheeses were manufactured from pasteurised milk (control), pasteurised milk acidified to pH 6.0 with CO₂, and milk acidified to pH 6.0 with CO₂ prior to pasteurisation. Production of cheese from CO₂-treated milk at pH 6.0 reduced the amount of rennet necessary for coagulation by about 75%. Although acidification reduces the amount of lactic acid produced by starter during incubation of milk, no significant differences in lactic acid content were detected between cheeses manufactured from non-acidified or CO₂-acidified milks. Cheeses produced from CO₂-treated milk showed less proteolysis than control cheeses, but no significant differences in sensory characteristics between cheeses were detected.     

Characteristics of carbonated fermented milk and survival of probiotic bacteria

The carbonation of pasteurised milk was evaluated as a method for improving bacterial viability in fermented milk added with probiotic bacteria (Lactobacillus acidophilus and/or Bifidobacterium bifidum). The behaviour of microorganisms during fermentation and cold storage, and the biochemical and sensory properties of the products were assessed. In AT (Streptococcus thermophilus/L. acidophilus) and ABT (S. thermophilus/L. acidophilus/B. bifidum) products, the fermentation times to decrease the pH to 5 were significantly lowered when CO₂ or lactic acid was added to milk. The higher acidity levels of carbonated (as a result of production of carbonic acid) and lactic acidified samples enhanced growth and metabolic activity of the starter during fermentation and was the reason for this reduction in incubation time. Cell counts of S. thermophilus, L. acidophilus and B. bifidum gradually decreased through the cold storage of carbonated and non-acidified fermented milk, although the counts were always higher than 10⁶ viable cells g⁻¹. The CO₂ did not exert any influence on the viability of S. thermophilus and L. acidophilus in AT fermented milk stored at 4°C but the presence of B. bifidum and CO₂ in ABT-type products was associated with lower viability of L. acidophilus during the refrigerated storage. The higher acetate concentrations of ABT products made with non-acidified milk as compared with the carbonated products could have contributed to major survival of L. acidophilus in the former. The use of milk acidified with CO₂ had no detrimental effects on the sensory properties of ABT fermented milk. Therefore, we concluded that the carbonation of pasteurised milk prior to the starter addition could be satisfactorily used to reduce the manufacture time of fermented milk.     

Inhibition of Psychrotrophic Bacterial Growth in Refrigerated Milk by Addition of Carbon Dioxide

Treatment of refrigerated milk with 20 to 30 mM CO₂ was evaluated as a method for extending storage-life by inhibiting growth of psychrotrophic bacteria. Generation times for each of five psychrotrophic pseudomonads were significantly longer when grown at 7°C in sterile milk treated with CO₂ than when the same bacteria were grown in ungassed sterile milk.When raw milks were stored at 7°C and treated with CO₂, the time required for aerobic plate counts to increase 10-fold was at least 24 h longer than in the same milks left untreated. Numbers of coliforms, psychrotrophs, and anaerobes (facultative and obligate) were significantly lower in raw milks treated with CO₂ than in untreated milks incubated at 7°C for 6 d.     

Growth and metabolic activity of a cheese starter in CO2-acidified and non-acidified refrigerated milk

 The growth and activity of two Lactococcus strains and one Leuconostoc strain in CO₂-acidified and non-acidified refrigerated milk were evaluated separately and as a mixed culture to determine their suitability for use as a starter in the manufacture of Afuega’l Pitu, an acid-coagulated Spanish cheese, from refrigerated CO₂-acidified milk. The growth of the strains studied and their production of organic acids were similar in CO₂-acidified and non-acidified refrigerated or fresh milk, indicating that CO₂ treatment does not affect the metabolic activity of the strains. However, refrigeration enhanced the production of acetaldehyde, ethanol and diacetyl in CO₂-acidified and non-acidified milk. The level of diacetyl was also greater in refrigerated CO₂-acidified milk than in refrigerated non-acidified milk. It was concluded that refrigerated milk acidified with CO₂ can be satisfactorily used in the manufacture of Afuega’l Pitu cheese, and that this technique can be also used in the production of other acid-coagulated cheeses. Received: 1 July 1997 / Revised version: 1 September 1997     

Growth and metabolic activity of a cheese starter in CO2-acidified and non-acidified refrigerated milk

 The growth and activity of two Lactococcus strains and one Leuconostoc strain in CO₂-acidified and non-acidified refrigerated milk were evaluated separately and as a mixed culture to determine their suitability for use as a starter in the manufacture of Afuega’l Pitu, an acid-coagulated Spanish cheese, from refrigerated CO₂-acidified milk. The growth of the strains studied and their production of organic acids were similar in CO₂-acidified and non-acidified refrigerated or fresh milk, indicating that CO₂ treatment does not affect the metabolic activity of the strains. However, refrigeration enhanced the production of acetaldehyde, ethanol and diacetyl in CO₂-acidified and non-acidified milk. The level of diacetyl was also greater in refrigerated CO₂-acidified milk than in refrigerated non-acidified milk. It was concluded that refrigerated milk acidified with CO₂ can be satisfactorily used in the manufacture of Afuega’l Pitu cheese, and that this technique can be also used in the production of other acid-coagulated cheeses. Received: 1 July 1997 / Revised version: 1 September 1997     

Lactic acid production and rheological properties of yogurt made from milk acidified with carbon dioxide

The influence of milk acidification up to pH 6.0 with CO₂ on D ‐ and L ‐lactic acid production and lactose consumption by yogurt starter, changes in the pH, and rheological and sensory properties of yogurt were studied. A slight influence of CO₂ on lactic acid production during yogurt manufacture was detected. No significant changes in lactic acid concentration were observed during storage, although the final concentration was significantly lower in control than in pH 6.2 and 6.0 acidified samples. A great influence of CO₂ on D ‐lactic acid production was not observed. Yogurt manufactured from milk with lower pH values showed lower final pH values after 7 days of storage. The viscosity was similar in all analysed samples. No significant differences in sensory characteristics between unacidified and acidified yogurts were detected. © 1999 Society of Chemical Industry     

Manufacture of cheese made from CO2-treated milk

Cheeses were manufactured from pasteurised milk (control), pasteurised milk acidified to pH 6.0 with CO₂, and milk acidified to pH 6.0 with CO₂ prior to pasteurisation. Production of cheese from CO₂-treated milk at pH 6.0 reduced the amount of rennet necessary for coagulation by about 75%. Although acidification reduces the amount of lactic acid produced by starter during incubation of milk, no significant differences in lactic acid content were detected between cheeses manufactured from non-acidified or CO₂-acidified milks. Cheeses produced from CO₂-treated milk showed less proteolysis than control cheeses, but no significant differences in sensory characteristics between cheeses were detected.

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Herstellung von Käse aus CO₂-behandelter Milch

Zusammenfassung Käse wurde in drei verschiedenen Varianten hergestellt: aus pasteurisierter Milch (Kontrollkäse) sowie aus mit CO₂ auf pH 6.0 angesäuerter Milch, die nach der Säuerung pasteurisiert wurde bzw. angesäuerter Milch, die im voraus pasteurisiert worden war. Es wurde beobachtet, daß die für die Koagulation erforderliche Labmenge bei der Herstellung von Käse aus durch CO₂ auf pH 6.0 angesäuerter Milch, im Vergleich mit Kontrollkäse, auf 75% abnimmt. Der Gehalt der Milch an Starter-Kulturen nahm durch die CO₂-Behandlung ab. Allerdings konnten keine signifikanten Unterschiede im Milchsäuregehalt zwischen den aus nicht angesäuerter bzw. angesäuerter Milch hergestellten Käsen festgestellt werden. Aus CO₂-behandelter Milch hergestellter Käse zeigte geringere Proteolyse als der Kontrollkäse, jedoch konnten keine signifikanten sensorischen Unterschiede zwischen den untersuchten Varianten festgestellt werden.

     

Quality of plain yoghurt made from refrigerated and CO2-treated milk

The suitability of milk preserved by refrigeration and CO₂ addition for the manufacture of plain yoghurt was evaluated using two commercial strains of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus. Yoghurts manufactured, after milk pasteurisation, from refrigerated CO₂-treated samples (pH 6.15) were compared with two different controls made from pasteurised milk, either fresh or refrigerated. The multiplication and acidification capacity of the starter as well as the evolution of organic acids were not affected by the previous refrigeration and CO₂-treatment of raw milk nor by the residual CO₂ present in pasteurised milk. However, refrigeration enhanced the production of ethanol and diacetyl. No differences on sensory properties were detected through the cold storage between yoghurts made from CO₂-treated milk and those made from refrigerated control milk. It was concluded that refrigerated milk acidified with CO₂ could be satisfactorily used in the manufacture of yoghurt.     

Continuous dense‐phase CO2 processing of a coconut water beverage

Effects of dense‐phase CO₂ (DPCD) on microbial, physical, chemical and sensorial quality of coconut water (CW) beverage were evaluated. Pressure during DPCD treatment was not significant in microbial reduction whereas temperature and % CO₂ levels were significant. DPCD‐treated (34.5 MPa, 25 °C, 13% CO₂, 6 min), heat‐pasteurised (74 °C, 15 s) and untreated CW beverages were evaluated during 9 weeks of refrigerated storage (4 °C). Total aerobic bacteria of DPCD and heat‐treated samples decreased whereas that of untreated samples increased to >10⁵ CFU mL^?1 after 9 weeks. DPCD increased titratable acidity but did not change pH (4.20) and °Brix (6.0). Likeability of DPCD‐treated CW was similar to untreated. Heat‐treated samples were less liked (α = 0.05) at the beginning of storage. Off flavour and taste‐difference‐from‐control scores of heated samples were higher than DPCD during the first two weeks. DPCD extended shelf life of acidified, sweetened and carbonated CW over 9 weeks at 4 °C.     

Properties of Orange Juice with Supercritical Carbon Dioxide Treatment

The effects of supercritical CO₂ (SC-CO₂) on the microbiological, sensory (taste, odour, and colour), nutritional (vitamin C content), and physical (cloud, total acidity, pH, and °Brix) qualities of orange juice were studied. The CO₂ treatment was performed in a 1 litre capacity double-walled reactor equipped with a magnetic stirring system. Freshly extracted orange juice was treated with supercritical CO₂, pasteurised at 90°C, or left untreated. There were no significant differences in the sensory attributes and physical qualities between the CO₂ treated juice and freshly extracted juice. The CO₂ treated juice retained 88% of its vitamin C, while the pasteurised juice was notably different from the fresh juice and preserved only 57% of its vitamin C content. After 8 weeks of storage at 4°C, there was no microbial growth in the CO₂ treated juice.     

Lipid breakdown and sensory analysis of milk inoculated with Chryseobacterium joostei or Pseudomonas fluorescens

The lipolytic activity, lipid oxidation, sensory effect and production of volatile compounds by Chryseobacterium joostei and Pseudomonas fluorescens in full cream milk, incubated at 4 and 25 °C, were investigated. Lipolytic activity, measured by the amount of free fatty acids released, showed high levels for both organisms. Although no significant differences between C. joostei and Ps. fluorescens were observed at 4 °C, C. joostei was significantly (P < 0.001) more lipolytic than Ps. fluorescens at 25 °C. Lipid oxidation was measured by thiobarbituric acid analysis. No significant differences between any of the inoculated milk samples were noted. During sensory analysis, C. joostei produced stronger odours than Ps. fluorescens. Different odours were also described for the two species. Volatile compound analysis determined that C. joostei has the ability to produce the same amount of off-odours during psychrotrophic and mesophilic growth conditions as Ps. fluorescens, but with different odour profiles.     

Stability of casein micelle subjected to reversible CO2 acidification: Impact of holding time and chilled storage

Casein micelle stability and reactivity were assessed on milk subjected to reversible acidification by carbonation. Pressurised CO₂ was injected at 4 °C, leading to controlled acidification from 6.63±0.02 to a target pH (5.5 or 5.2). After holding the pressurised milk under these conditions for 15 or 60 min, the pressure was released and the milk pH returned to its initial value under stirring and vacuum degassing. Upon CO₂ treatment, calcium and protein partition, zeta potential and size of casein micelles were restored directly after neutralisation. The rheological properties of the gel obtained by acid coagulation of CO₂-treated milk did not change as a result of carbonation. Micelle hydration increased after neutralisation and during storage. Milk buffering capacity in the pH range of 4.5–5.5 decreased after neutralisation of milk acidified by carbonation, but increased during chilled storage of this milk. Holding time of carbonated milk at low pH was found to have no impact on the physicochemical characteristics of casein micelles and the rheological properties of the gel obtained by acid coagulation of this milk.     

Effect of Carbon Dioxide Atmosphere on Microbial Growth and Quality of Salmon Slices

Salmon slices ( Salmo salar ) were packed under CO₂ and stored in a chill room at 2±1°C for 3 weeks. A study was carried out on the effect of CO₂ atmosphere on bacterial growth as well as on chemical deterioration (pH, total volatile bases, (TVB), trimethylamine (TMA), lipid oxidation) and sensory features (exudation, raw fish odour, cooked fish flavour). Results were compared with a control stored in air. Salmon slices stored under CO₂ had a shelf-life of nearly twice as long as those in air, having lower bacterial counts, pH, TMA and TVB, as well as improved sensory features after 18 days at 2±1°C. Exudation was not significantly different between control and CO₂-stored samples after 10 days ice storage. Exudate values extended from 1 to 2% for CO₂-stored samples after 10 and 20 days storage, respectively. Salmon slices can be stored under CO₂ at 2±1°C for about 18 days, with no substantial loss of quality. Shelf-life can vary depending upon the state of fish, handling practices, initial bacterial load and storage temperature.     

The diversity and proteolytic properties of psychrotrophic bacteria in raw cows' milk from North China

Most psychrotrophic bacteria have the ability to produce thermoresistant proteases that can destroy the quality of milk and dairy products. To investigate the population dynamics of psychrotrophic bacteria during refrigeration, three raw cows' milk samples (sample A comprising milk from 10 farms in Beijing, sample B comprising milk from 5 farms in Heihe, and sample C comprising milk from 7 farms in Harbin) were refrigerated at 0–5 °C and 5–10 °C. PCR-DGGE (Polymerase chain reaction-denaturing gradient gel electrophoresis) analysis revealed that the bacterial community profiles varied from geographical site to site, and with refrigeration temperature. The dominant psychrotrophic bacteria among the samples after storage were affiliated with the order Pseudomonadales. Following isolation and identification, 8 psychrotrophic isolates were selected as stronger protease producers and their growth and proteolytic activities were assessed. The results indicate that the composition of psychrotrophic bacteria play an important role in the determination of the quality of milk and dairy products.     

Effect of Nitrogen Flushing on the Production of Proteinase by Psychrotrophic Bacteria in Raw Milk

Raw milk was flushed with 100 ml N₂ min ^-1 during storage at 4°C. Microflora (total psychrotrophs, proteolytic psychrotrophs, lactic acid bacteria) in nitrogen-flushed milk exhibited a longer lag phase and slower growth rates than those in milk stored aerobically at 4°C. Although proteolytic psychrotrophs grew in nitrogen-flushed milk, proteinases could not be detected in these milk samples. Proteinase assays and electrophoresis showed extensive proteolytic activity and hydrolysis of β-casein in control milk but no detectable casein degradation in nitrogen-flushed milk, even after storage for 18 days at 4°C. This study shows the potential of controlled atmosphere storage of raw milk for inhibition of the accumulation of proteolytic enzymes from psychrotrophic bacteria.     

Synthesis of extracellular lipase by a strain ofPseudomonas fluorescensisolated from raw camel milk

Ten strains of psychrotrophic bacteria were isolated from raw camel milk and were arranged according to their lipase production. Lipolytic activities ranged between 0.26 to 3.43 meq of palmetic acid 100 g^-;1of bovine milk fat h^-;1.Pseudomonas fluorescensRM₄was the most active strain. This bacterium could grow and secrete lipase over a wide range of temperatures. The optimum temperature for growth was 37°C, and the maximum lipase production was at 25°C. Growth was maximal after 96 h of incubation at 25°C, and lipase activity was maximal at 72 h post-inoculation at 25°C (during the late logarithmic phase). Shaking of the cultures (100 rpm) led to an increase in both growth and enzyme activities. Pseudomonas fluorescensRM₄was able to grow and secrete lipase over a pH range of 5.5-;8.50. Synthesis of the enzyme appeared to be inducible because no enzyme was detected in the absence of organic nitrogen. Supplementation of the basal medium with milk proteins enhanced lipase production. Tryptophan and lysine induced enzyme synthesis most effectively. Addition of 4 g l^-;1of glucose to broth stimulated both growth and production of lipase. Beyond this level of supplementation, lipase activity was considerably depressed.     

Microbial and Sensory Effects of Combined High Hydrostatic Pressure and Dense Phase Carbon Dioxide Process on Feijoa Puree

High hydrostatic pressure (HHP) is used for microbial inactivation in foods. Addition of carbon dioxide (CO₂) to HHP can improve microbial and enzyme inactivation. This study investigated microbial effects of combined HHP and CO₂ on Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae, and evaluated sensory attributes of treated feijoa fruit puree (pH 3.2). Microorganisms in their growth media and feijoa puree were treated with HHP alone (HHP), or saturated with CO₂ at 1 atm (HHPcarb), or 0.4%w/w of CO₂ was injected into the package (HHPcarb+CO₂). Microbial samples were processed at 200 to 400 MPa, 25 °C, 2 to 6 min. Feijoa samples were processed at 600 MPa, 20 °C, 5 min, then served with and without added sucrose (10%w/w). Treated samples were analyzed for microbial viability and sensory evaluation. Addition of CO₂ enhanced microbial inactivation of HHP from 1.7‐log to 4.3‐log reduction in E. coli at 400 MPa, 4 min, and reduction of >6.5 logs in B. subtilis (vegetative cells) starting at 200 MPa, 2 min. For yeast, HHPcarb+CO₂ increased the inactivation of HHP from 4.7‐log to 6.2‐log reduction at 250 MPa, 4 min. The synergistic effect of CO₂ with HHP increased with increasing time and pressure. HHPcarb+CO₂ treatment did not alter the appearance and color, while affecting the texture and flavor of unsweetened feijoa samples. There were no differences in sensory attributes and preferences between HHPcarb+CO₂ and fresh sweetened products. Addition of CO₂ in HHP treatment can reduce process pressure and time, and better preserve product quality.     

Effect of the use of carbon dioxide on Prato cheese making

The effects of the addition of carbon dioxide (CO₂) under pressure (1.6 × 10⁵ Pa at 8 °C) to pasteurised Prato cheese milk (pH 6.0) was investigated through 120 d of refrigerated storage. The addition of CO₂ decreased the curd formation time (30 min), the total manufacturing time (47 min), and the pH of Prato cheese, thus leading to reduced moisture content. The CO₂ treated cheese showed higher firmness and fracturability due to the greater whey loss. In contrast, the microorganism counts, cheese yield, protein loss, cohesiveness, springiness, and gumminess were not significantly affected by the treatment. For the lactose fermentation, no significant differences were observed. The addition of CO₂ did not change the proteolysis indexes, and no significant differences were observed in the sensory acceptance of the CO₂ treated cheese, which was well accepted by consumers.     

Evaluation of the microbiological safety and sensory quality of rainbow trout (Oncorhynchus mykiss) processed by the sous vide method

This work evaluates the microbiological quality and sensorial characteristics of rainbow trout processed by the sous vide method under different time/temperature conditions and stored at 2°C and 10°C.The equivalent heat treatments applied in core were: 3.3 min at 90°C (PV₇₀ in core 331.64), 1.04 min at 90°C (PV₇₀ in core 104.5) and 5.18 min at 70°C (PV₇₀ in core 5.18). The product stored at 2°C or 10°C was periodically evaluated to investigate its sensory quality, microbiological condition and proximate composition. The batches stored at 2°C had lower growth counts of mesophiles and psychrotrophs. Moreover, these counts decreased by increasing the heating temperature and time. Staphylococcus aureus, Bacillus cereus, Clostridium perfringens and Listeria monocytogenes were not found in any of the samples. Neither aerobic nor anaerobic spores were detected in the trout samples processed at 90°C for 3.3 min in core and stored at 2°C after 45 days. A heat treatment of 90°C for 3.3 min in core was the most effective to extend the shelf-life of trout (>45 days).Therefore, it can be concluded that the treatment at 90°C for 3.3 min in core is the most effective to ensure the safety and extend the shelf-life of sous vide trout preserving its sensory characteristics.This study establishes the microbiological quality of sous vide trout and emphasizes the relevance of the microbiological quality of the raw materials, heat treatment and storage temperature to ensure the safety of the product, since this fatty fish is not eviscerated and high counts can be found in the raw fish.