HIGH HYDROSTATIC PRESSURE EFFECT ON SACCHAROMYCES CEREVISIAE,ESCHERICHIA COLI AND LISTERIA INNOCUA IN PEAR NECTAR

ABSTRACT

An isostatic pressure system was used for processing fresh‐prepared pear nectar, inoculated with Saccharomyces cerevisiae, Escherichia coli and Listeria innocua, at selected pressures (0 to 241 MPa), and times (2 s, and 0 to 15 min). The come up time (time to reach the working pressure) had an important microbial inactivation effect in view of the fact that the initial counts of 6.0 × 10⁵, 1.02 × 10⁷ and 2.4 × 10⁷ cfu/mL were reduced to 2.4 × 10⁵, 6.3 × 10⁵ and 2.2 × 10⁷ cfu/mL, for S. cerevisiae, E. coli and L. innocua, respectively, in nectar. Decimal reduction time values, in the range of 2.0–35.3, 0.6–20.6 and 9.2–588.2 min, were calculated from the first order kinetics modeling of S. cerevisiae, E. coli, and L. innocua survivors, respectively. z_p values of 120.5, 92.6 and 75.2 MPa were obtained for the three types of microorganisms, respectively.

PRACTICAL APPLICATIONS

The application of high hydrostatic pressure may deliver a “cold pasteurized” pear nectar. This “cold pasteurization” may inactivate microorganisms and enzymes as thermal pasteurization does; however, high hydrostatic pressure does not generate unwanted chemical compounds that may change the nectar fresh sensory characteristics. Thus, a fresh‐like fruit product, free from chemicals and with high quality and sensory characteristics, could be offered to consumers.

     

SUCROSE AND ULTRA HIGH PRESSURE INACTIVATION OF SACCHAROMYCES CEREVISIAE AND LISTERIA INNOCUA

D‐values for ultra high pressure inactivation of Saccharomyces cerevisiae and Listeria innocua were evaluated in commercial applesauce with selected concentrations of soluble solids ranging from 13 to 60%. Applesauce with S. cerevisiae was compressed to 300 MPa and immediately decompressed (identified as 0 s) or held at 300 MPa for 30‐s intervals for 150 s at room temperature. Applesauce with L. innocua was treated at 375 MPa for 0 to 300 s at room temperature. D‐values for both microorganisms at soluble solids concentrations of 13, 20 and 30% were calculated. D‐values for inactivation of S. cerevisiae were 26.3, 29.9 and 46.5 s in applesauce adjusted with sucrose to 13, 20 and 30% soluble solids, respectively. D‐values for inactivation of L. innocua were 40.2, 55.2 and 120.6 s in applesauce also adjusted with sucrose to 13, 20 and 30% of soluble solids. Ultra high pressure treatment of commercial applesauce inoculated with S. cerevisiae or L. innocua adjusted to 40, 50 or 60% soluble solids reduced an initial inocula of 10⁷to 10⁶by less than 1 log. Soluble solids concentrations greater than 30% provide a baroprotective effect on inactivation by high pressure treatment. The use of ultra high pressure technology by the food industry to inactivate microorganisms may be limited in food models with high soluble solids concentrations. Soluble solids adjusted with sucrose concentrations greater than 30% protect against the destruction of microorganisms. An effective pressure treatment that ensures the microbial stability of foods depends on an understanding of the relationship between microorganisms and food components.     

SURVIVAL OF LISTERIA INNOCUA IN APPLE JUICE AS AFFECTED BY VANILLIN OR POTASSIUM SORBATE

Survival of stationary phase Listeria innocua ( as surrogate microorganism for L. monocytogenes) inoculated in apple juice (pH 3.3 or 3.8) supplemented with vanillin (1,500 ppm or 3,000 ppm) or potassium sorbate (500 ppm or 1,000 ppm) and stored at room temperature was studied. L. innocua survived in apple juice without the preservatives at pH 3.3 or 3.8, with minimal population reductions. In the juices with the incorporation of potassium sorbate or vanillin , L. innocua behavior depended on the pH value, the type of antimicrobial and its concentration. At pH 3.3, the presence of vanillin (3,000 ppm) or potassium sorbate (1,000 ppm or 500 ppm) decreased L. innocua counts, with population reductions ranging from 4 to 5 log cycles after a 4 h – 8 h exposure at 30C. However, at pH 3.8 , L. innocua showed sensitivity only to 3,000 ppm vanillin. Survival curves were successfully fitted using a Weibull type distribution of resistances.
The results suggest that the use of potassium sorbate or vanillin could prevent the survival of L. innocua in contaminated unpasteurized and pasteurized apple juice. Vanillin, a natural antimicrobial, would be particularly suitable as an antilisterial additive for less acidic apple juice.     

FACTORS AFFECTING THE ULTRAVIOLET INACTIVATION OF ESCHERICHIA COLI K12 IN APPLE JUICE AND A MODEL SYSTEM*

Ultraviolet (UV) light has been used successfully for years to sterilize water and was recently approved as an acceptable irradiation treatment for the processing of juice. Although there is considerable information on the efficacy of UV processing in the treatment of water, limited data are available on its efficacy in fluid food systems. The objectives of this work were to determine the effects of apple‐juice properties on the UV inactivation of Escherichia coli K12 and the interdependence of intensity and time on the efficacy of UV light. Results showed that absorbance (A) and suspended solids (SS) affected UV inactivation, while pH and dissolved solids did not. Concerning the interdependence of intensity and time, intensity levels can only be changed without sacrificing effectiveness at a limited range of intensity and dose levels. This means that the range of the intensity level of the actual UV reactor must be considered in process‐parameter determination.     

INACTIVATION of SACCHAROMYCES CEREVISIAE IN APPLE JUICE BY SQUARE-WAVE and EXPONENTIAL-DECAY PULSED ELECTRIC FIELDS

With equivalent electrical energy input, inactivation of microorganisms by pulsed electric fields depends on pulse waveform. Exponential-decay and square-wave pulsed electric fields were selected to treat Saccharomyces cerevisiae suspended in apple juice. A parallel-plate static treatment chamber with 25 ml volume and 0.95 cm electrode gap was used. Peak electric field and pulse electric energy input were 12 kV/cm and 260 Joules per pulse for both waveforms. Both waveforms were found effective in the microbial inactivation. However, inactivation of S. cerevisiae treated with square-wave pulses was greater than yeast treated with exponential-decay pulses. For the purpose of food pasteurization, square-wave pulsed electric fields may result in significant energy savings compared to exponential-decay pulses.     

POTENTIAL USE OF 18 TESLA STATIC AND PULSED MAGNETIC FIELDS ON ESCHERICHIA COLI AND SACCHAROMYCES CEREVISIAE

Escherichia coli and Saccharomyces cerevisiae were subjected to 18T static and pulsed magnetic fields at different temperatures in either phosphate buffer, McIlvaine buffer, or peptonated water. Colony forming units were recorded after incubation in nutrient and Violet Red Bile agar for E. coli and potato dextrose agar for S. cerevisiae. No inactivation or cell injury was detected due to the influence of the magnetic field whether static or pulsed. As expected, at higher temperatures both inactivation and cell injury were increased. Although further research is needed, magnetic field technology at 18T or lower does not appear as a feasible option for processing foods.     

紫外线对梨清汁中大肠杆菌的杀灭作用研究

研究了果汁厚度、辐射距离、辐射时间和杀菌温度4个因素对紫外线杀灭接种在梨清汁中的大肠杆菌的影响.并通过正交实验得出各因素的最佳组合.结果表明:辐射时间和果汁厚度对紫外线的杀菌效果影响最为显著.当果汁厚度为0.5mm,辐射时间为5 min,辐射距离为10 cm,杀菌温度为30℃时,杀菌率可达99.95%.     

KINETICS OF INACTIVATION OF ESCHERICHIA COLI IN CARROT JUICE BY PULSED ELECTRIC FIELD

The inactivation kinetics of Escherichia coli inoculated into carrot juice by pulsed electric field (PEF) was investigated, and the experimental data were fitted to Hülsheger and Peleg models. The electric field strength ranged from 5 to 20 kV/cm, and the number of pulses was from 207 to 1449. The level of E. coli inactivation increased with the increment of the electric field strength and the number of pulses. As the number of pulses increased, the kinetic constants b_E and E_c^a (Hülsheger model) varied from 0.2429 to 0.5778 cm/kV and from 7.1301 to 5.7842 kV/cm, respectively. The k and E_c^b obtained using the Peleg model varied from 2.3277 to 1.4725 kV/cm and from 12.2523 to 7.4755 kV/cm, respectively. The fitting performance of the two models was evaluated by using a series of indices including accuracy factor, bias factor, sum of the squares of the differences of the natural logarithm of the observed and predicted data, correlation coefficient and the root mean square error between the observed and the predicted data. A comparison among these corresponding parameters indicates that the Peleg model better describes the inactivation kinetics of E. coli by PEF than the Hülsheger model.     

HIGH PRESSURE INACTIVATION OF SACCHAROMYCES CEREVISIAE,ENDOGENOUS MICROFLORA AND PECTINMETHYLESTERASE IN ORANGE JUICE1

Decimal reduction times (D-values) for Saccharomyces cerevisiae ascospores inoculated into pasteurized orang juice ranged from 4 to 76 s at pressures between 500 and 350 MPa. At the same pressures, D-values of S. cerevisiae vegetative cells ranged from 1 to 38 s while those for the native microflora in nonpasteurized Hamlin orange juice were between 3 and 74 s. Corresponding z-values were 123, 106 and 103 MPa for ascospores, vegetative cells and native microflora, respectively. Native microorganisms that survived high pressure treatments included yeasts, gram-positive and gram-negative bacilli. Pectinmethylesterase activity in nonpasteurized Hamlin orange juice was reduced to 5% of initial activity after 30 s at 900 MPa.     

REDUCTION OF PATULIN IN APPLE JUICE CONCENTRATES DURING STORAGE

In this study, the reduction of patulin content in apple juice concentrates during 6 months of storage at 22 and 30C was investigated. Results demonstrated that reduction in patulin content was dependent on the storage temperature and time. Patulin reductions after 1 month of storage at 22 and 30C were in the ranges of 45–64% and 66–86%, respectively. Levels of patulin were below detectable limits after 4 months of storage at 22 and 30C.     

INACTIVATION OF ESCHERICHIA COLI O157:H7 DURING THE STORAGE UNDER REFRIGERATION OF APPLE JUICE TREATED BY PULSED ELECTRIC FIELDS

The sensitivity of pulsed electric fields (PEF)‐treated E. coli O157:H7 cells to subsequent holding in apple juice has been evaluated. Escherichia coli O157:H7 cells in apple juice were resistant to PEF. A PEF treatment of 400 µs at any electrical field strength was not sufficient to inactivate one log₁₀cycle of cells. However, PEF injured a large proportion of E. coli O157:H7 cells that became sensitive to a subsequent storage under refrigeration in apple juice. The total lethal effect of the combined process depended on the electrical field strength and storage time. The combination of a PEF treatment at 25 kV/cm for 400 µs and a subsequent storage of the apple juice under refrigeration for 48 h allowed five log₁₀cycles of inactivation to be achieved. The combination of PEF and maintenance under refrigeration has been demonstrated to be an effective pasteurization method, by sufficiently reducing the presence of E. coli O157:H7 in apple juice in order to meet U.S. FDA recommendations.     

FLOCCULATION IN SACCHAROMYCES CEREVISIAE AS INFLUENCED BY WORT COMPOSITION AND BY ACTIDIONE

The stage during the fermentation of wort at which cells flocculate is shown to vary with the extent of exponential growth, so that the use of worts with a high content of assimilable nitrogen leads to delayed flocculation. Additionally, the presence of a high concentration of lysine, which inhibits post-exponential growth, retards flocculation still further and may prevent its occurrence. Addition of actidlone to a growing culture inhibits further increase in cell mass and prevents flocculation occurring, although the use of sugar continues; cell wall synthesis is restricted and mannan production is particularly repressed by actidione treatment.     

STEROL BIOSYNTHESIS BY STRAINS OF SACCHAROMYCES CEREVISIAE IN THE PRESENCE AND ABSENCE OF DISSOLVED OXYGEN

The content of sterols in Saccharomyces cerevisiae which has been harvested after anaerobic growth and then added to a complex nutrient medium, rises rapidly from ca. 1 mg/g dry yeast to ca. 10 mg in the presence of dissolved oxygen. A range of sterols, present principally as sterol esters, is formed during this period. The concentration of free sterols does not rise above 3 mg/g and esters are thought to form a reserve sterol pool. Cyclization of squalene to lanosterol in the presence of oxygen seems not to be markedly affected by oxygen concentration in contrast to demethylation and desaturation reactions on the pathway to ergosterol. When oxygen concentration falls to zero, further metabolism of preformed sterols continues, with the accumulation of episterol and ergosterol and reduction in the concentration of zymosterol and 24(28)-dehydroergosterol. During anaerobic growth a marked hydrolysis of sterol esters occurs and free sterols eventually predominate.     

GROWTH OF SACCHAROMYCES CEREVISIAE AND SACCHAROMYCES UVARUM IN A TEMPERATURE GRADIENT INCUBATOR

A temperature gradient incubator has been used to determine the effect of temperature on the growth of strains of Saccharomyces cerevisiae and Saccharomyces uvarum (including lager brewing yeasts formerly classified as Saccharomyces carlsbergensis). The maximum temperatures for growth (T_max) for all strains of S. cerevisiae examined were in the range 37.5°C-39.8°C and the optimum temperatures for the most rapid initial growth (T_opt) were in the range 30.0°C-35.0°C. Strains of S. uvarum, however, formed two distinct groups: Group A (including all brewing strains of S. uvarum tested) had T_max values 31.6°C-34.0°C and T_opt values 26.8°C-30.4°C; Group B had T_max values 38.2°C-40.0°C and T_opt values 30.0°C-34.6°C. It is proposed, therefore, that the species name S. carlsbergensis should be re-introduced and applied to those strains of S. uvarum (Group A) which have the lower T_max values. Minimum temperatures for growth (T_min) of the yeasts were not investigated as initial studies had shown that they could not be measured satisfactorily. Measurements of the generation times for one brewing strain of S. cerevisiae and one brewing strain of S. uvarum (Group A) over the temperature range 6.0°C-22.0°C have shown that there are significant differences between the yeasts at the lower end of the temperature range and that the relationship between generation time (GT) and temperature (T) for both yeasts closely follows the mathematical expression:      

ADSORPTION OF SILICONE ANTIFOAM BY SACCHAROMYCES CEREVISIAE

Polydimethylsiloxane (PDS) dispersed in an aqueous emulsion containing stearate emulsifiers and cellulose-based thickeners is adsorbed to the walls of Saccharomyces cerevisiae NCYC 366 during growth in a defined medium. Including PDS at concentrations up to 30 μg per ml of medium has no effect on the duration of the lag phase or the rate of exponential growth. Cells suspended in buffer containing PDS take at least 2–4 h to become saturated with PDS. Cells saturated with PDS do not differ from PDS-free organisms in respiratory activity, but have a slightly lower electrophoretic mobility at pH values between 4.0 and 7.0. About half of the PDS on saturated cells appears in the supernatant when they are converted into sphaeroplasts, but proportionately less PDS is released into the supernatant from cells containing less PDS. The equilibrium constant for binding of PDS to isolated walls is about five times as great as for binding to cells. PDS is taken up by isolated walls faster than by intact organisms. Incubation of walls saturated with PDS in buffered EDTA leads to release of PDS from the walls, but incubation in the presence of 8-m urea does not.     

BEHAVIOR OF LISTERIA MONOCYTOGENES AND ESCHERICHIA COLI O157:H7 IN FRESH-SLICED CACTUS-PEAR FRUIT

Storage experiments were conducted to follow the behavior of Listeria monocytogenes and Escherichia coli O157:H7, deliberately inoculated on fresh‐cut cactus‐pear fruits before packaging under modified and control atmosphere and stored at four different temperatures (4, 8, 12 and 20C). L. monocytogenes was able to proliferate during storage at different temperature both in control and modified atmosphere. By comparing the sanitary‐risk values with those of shelf life, it is possible to conclude that the storage of cactus‐pear samples at temperatures greater than 4C, both in control and in modified atmospheres, could lead to a significant health‐time risk, and that this is strictly affected by temperature. E. coli O157:H7 was able to proliferate only in the sample stored at 4 and 8C in both package atmospheres. On the contrary, this species was completely suppressed at the higher temperatures. In our study, E. coli O157:H7 appeared to be much less suited for survival on the surface of the fruit than L. monocytogenes.     

ROMATIC COMPOUNDS AND SUGARS IN FLOCCULATION OF SACCHAROMYCES CEREVISIAE

Phenol, aniline, benzyl alcohol and pyridine in the range 0.1 m to 0.3 m appreciably lowered the floc dissociation temperature, T_f , of two strains of Saccharomyces cerevisiae, without affecting flocculation intensity, F. Cell surface inactivation and Sr salt addition lowered F but not T_f . Mannose lowered F specifically, over fructose, glucose and maltose. Results are interpreted as a specific complementary site binding in yeast flocculence.