Fathy E. El-Gazzar

Ultrafiltration and reverse osmosis in dairy technology : a review

Ultrafiltration and reverse osmosis processes can be useful in the dairy foods industry. When milk is processed, milk fat and casein are rejected fully (e.g., are in retentate) and thus are concentrated by ultrafiltration and reverse osmosis membranes. Lactic cultures are slow to reduce the pH of retentate because of its increased buffering capacity since concentrated calcium phosphate and proteins are present. Conditions for growth of pathogenic microorganisms and inhibition of such bacteria in ultrafiltered milk differ from those of unfiltered milk. The principal advantage of using ultrafiltered milk for conversion into such cheeses as Cheddar, cottage, Havarti, Feta, brick, Colby, and Domiati is an increase in yield of product. Additional benefits claimed for use of ultrafiltered milk in cheese making include reduction in costs of energy, equipment, and labor; improved consistency of cheese flavor; and possible production of new byproducts.

Growth and Aflatoxin Production by Aspergillus parasiticus NRRL 2999 in the Presence of Lactic Acid and at Different Initial pH values

Twenty-five milliliters of glucose-yeast-salts medium containing 0, 0.5, 0.75, 1.0, 1.5 and 2.0% lactic acid with an initial pH of 3.5 or 4.5 were inoculated with 1 ml of a spore suspension containing 106 conidia of Aspergillus parasiticus NRRL 2999 and incubated at 28°C for 10 d. The pH of the medium, weight of mycelium and aflatoxin production were determined after 3, 7, and 10 d of incubation. Amounts of aflatoxin produced were determined using reversed-phase high-performance liquid chromatography. Cultures grown in the presence of 0.5 and 0.75% lactic acid at an initial pH of 4.5 produced more aflatoxin B1 than did the other cultures at the end of 3 d of incubation. This was not true for aflatoxin G1; with increasing concentrations of lactic acid, cultures produced decreasing amounts of aflatoxin G1. Also, cultures growing in the medium with an initial pH of 3.5 produced more aflatoxin B1 in the presence of lactic acid at the end of 3 d of incubation than did control cultures. Cultures growing in the presence of 0.5 and 0.75% lactic acid produced the most aflatoxin. Maximum amounts of aflatoxin G1 were produced after 7 d of incubation, with cultures growing in the presence of 0.5 and 0.75% lactic acid producing the most. Lactic acid did not inhibit growth (mycelium weight) of cultures in the medium with initial pH values of 3.5 or 4.5 except there was a slight decrease in mycelial weight when the medium contained 0.5% lactic acid and had an initial pH value of 3.5.

Growth and Aflatoxin Production by Aspergillus parasiticus NRRL 2999 in the Presence of Acetic or Propionic Acid and at Different Initial pH Values

Experiments were done to determine effects of different concentrations of acetic or propionic acid in a glucose-yeast extract-salts medium with an initial pH value of 4,5 or 5.5 on growth and aflatoxin production by Aspergillus parasiticus NRRL 2999. Amounts of aflatoxin were measured with reversed-phase high-performance liquid chromatography. The maximum concentration of acetic or propionic acid that permitted growth at an initial pH of 5.5 was 1% after 7 d of incubation and 0.25% after 3 d of incubation, respectively. When the initial pH of the medium was 4.5, the maximum concentration of acetic or propionic acid that permitted growth was 0.25 or 0.1%, respectively. There was no significant difference (p>0.05) in amount of mycelial (dry weight) produced by cultures in the presence of 0.0, 0.25, 0.50 or 0.75% acetic acid. Amounts of aflatoxin B1 and G1 produced decreased with an increasing concentration of acetic acid. Increasing concentrations of propionic acid caused a decrease in the amount of mycelial dry weight and aflatoxin produced by cultures growing in the medium with an initial pH of 5.5. At an initial pH of 4.5 mycelial growth was slow and at 3 d of incubation amounts of aflatoxin B1 and G1 produced were reduced as concentrations of acetic acid increased. This also was true for propionic acid in the medium with an initial pH of 4.5. Cultures with an extended lag phase in the presence of acetic or propionic acid overcame this and then produced large amounts of aflatoxin B1 and G1 at 7 and 10 d of incubation.

Growth of Listeria monocytogenes at 4, 32, and 40°C in Skim Milk and in Retentate and Permeate from Ultrafiltered Skim Milk

Pasteurized skim milk and retentate (concentrated fivefold or twofold by volume) and permeate from ultrafiltered skim milk were inoculated with Listeria monocytogenes strains California or V7 and incubated at 4, 32, or 40°C. Changes in populations of the pathogen were determined, growth curves were derived, and generation times and maximum populations calculated for each combination of strain, product, and temperature. Both strains grew faster and achieved higher (ca. 1 to 2 orders of magnitude) populations at 4°C in retentate of either concentration than in skim milk. The pathogen grew in permeate at 4°C and attained maximum populations of ca. 106 to 107/ml. Tyndallized samples of skim milk and retentate and permeate from ultrafiltered skim milk were inoculated with the same strains of L. monocytogenes and incubated at 32 or 40°C. Populations achieved by the pathogen at these temperatures, ca. 107 to 108/ml, were similar in skim milk, retentate, and permeate.

Growth and Aflatoxin Production by Aspergillus parasiticus in the Presence of Sodium Chloride

Twenty-five milliliters of glucose-yeast-salt medium containing 0, 2, 4, 6, 8, or 10% NaCl was inoculated to contain, approximately 105 or 107 conidia of Aspergillus parasiticus NRRL 2999 and then incubated at 13 or 28°C. Amounts of aflatoxin produced were determined using Reversed-Phase High Performance Liquid Chromatography (HPLC). Increasing the concentration of NaCl reduced accumulation of aflatoxin and also induced a lag in growth of the culture. At 13°C, the mold produced small amounts of aflatoxin after an extended lag phase, and NaCl was markedly more inhibitory at 13 than at 28°C.

Thermal Destruction of Listeria monocytogenes in Ground Pork Prepared with and without Soy Hulls

D-values and z-values were determined for Listeria monocytogenes Scott A cells heated in raw ground pork prepared with and without soy hulls and in a soy hull/water mixture. Products inoculated with ca. 107 colony-forming units (CFU) per g were sealed in glass vials, immersed in a water bath, and held at 50, 55, 60, or 62°C for predetermined times. Survival was determined by testing heated samples with McBride listeria agar. The D-values for L. monocytogenes cells at 50, 55, and 60°C were 108.81, 9.80, and 1.14 min, respectively, when heating was in ground pork and 113.64, 10.19, and 1.70 min, respectively, when heating in ground pork with added soy hulls. At 62°C L. monocytogenes cells were inactivated too rapidly to permit determination of the D-value. The D-values for L. monocytogenes in the soy hull/water mixture at 50 and 55°C were 19.84 and 3.94 min, respectively. L. monocytogenes cells were inactivated too quickly to determine the D-value at 60°C. The z-values for L. monocytogenes in ground pork prepared with and without soy hulls were 5.45 and 5.05°C, respectively. If ground pork naturally contains 102 L. monocytogenes cells per g and if we want to assure safety with a 4-D Listeria cook (reducing the L. monocytogenes population by four orders of magniatude), then according to results of this study, ground pork must be heated to an internal temperature of 60°C for at least 4.6 min and ground pork with added soy hulls for at least 6.8 min.

Fate of Listeria monocytogenes in Sweetened Condensed and Evaporated Milk During Storage at 7 or 21 °C

Sweetened condensed and evaporated milks were inoculated to contain ca. 103 to 107 cells of Listeria monocytogenes (strains Scott A, California, or V7)/ml. Both inoculated products were cooled from 25°C to 21°C in ca. 2 h or to 7°C in ca. 4 h. When inoculated sweetened condensed milk was held at 7°C for 42 d, there was no appreciable decrease in numbers of L. monocytogenes strains Scott A and V7, whereas the population of L. monocytogenes strain California decreased by ca. 1.2 orders of magnitude. Inoculum level had no effect on the magnitude of the decrease. At 21°C, 42 d of storage resulted in a more pronounced decrease in numbers of L. monocytogenes than it did during storage at 7°C, with numbers of the pathogen decreasing by 1.7, 1.6, and 3.4 orders of magnitude for strains Scott A, V7, and California, respectively. All strains of L. monocytogenes not only survived but grew in evaporated milk stored at 7 or 21 °C for 56 or 28 d, respectively.

Sodium Benzoate in the Control of Growth and Aflatoxin Production by Aspergillus parasiticus

Sodium benzoate, 0.0, 0.1, 0.2, 0.3 or 0.4%, was added to a glucose-yeast-salts medium which was inoculated with 1 ml of a spore suspension containing 108 conidia of Aspergillus parasiticus NRRL 2999 and then was incubated at 28°C. Cultures were analyzed after 3, 7 and 10 d for mycelial dry weight, pH and accumulation of aflatoxin B1 and G1. Amounts of aflatoxin produced were determined using reversed-phase high performance liquid chromatography (HPLC). The percentage of inhibition or stimulation by the additive was used to make comparisons between treatments and control. Generally, increasing the concentration of sodium benzoate increased the percentage of inhibition at the end of incubation (10 d). However, the average accumulation of mycelial dry weight was greater in the presence of benzoate than in its absence, with the greatest increase occurring when the medium contained 0.3% sodium benzoate.

Role of Hydrogen Peroxide in the Prevention of Growth and Aflatoxin Production by Aspergillus parasiticus

Hydrogen peroxide, 0.0, 0.03, 0.05, 0.3 and 0.5% was added to 25 ml of a glucose-yeast-salts medium which was inoculated with 1 ml of a spore suspension containing 106 conidia of Aspergillus parasiticus NRRL 2999 and then was incubated at 14 or 28°C. Cultures held at 28°C were analyzed after 3, 7 and 10 d for mycelial dry weight, pH and accumulation of aflatoxin B1 and G1. Incubation of some cultures at 28°C was continued for 90 d. Cultures held at 14°C were analyzed after 1, 2 and 3 months for mycelial dry weight, pH and aflatoxin production. Amounts of aflatoxin produced were determined using reversed-phase high-performance liquid chromatography (HPLC). The percentage of inhibition or stimulation by the additive was used to make comparisons between treatments and control. Overall, increasing the concentration of hydrogen peroxide to 0.3 or 0.5% completely prevented growth and aflatoxin production for up to 90 d of incubation at 14 or 28°C.

Growth of and Aflatoxin Production by Aspergillus parasiticus in a Medium Containing Potassium Chloride or a Mixture of Potassium Chloride and Sodium Chloride

Twenty-five milliliters of glucose-yeast-salts medium containing 0, 2, 4, 6, 8 and 10% KCl or a mixture of NaCl (%) and KCl (%) (0:0, 1.5:0.5, 3.25:0.75, 4.75:1.25, 6.5:1.5, and 8:2) was inoculated with 1 ml of a spore suspension containing 106 conidia of Aspergillus parasiticus NRRL 2999 and incubated at 28°C for 10 d. The pH, dry weight of mycelium and aflatoxin production were determined after 3, 7 and 10 d of incubation. Amounts of aflatoxin produced were determined using reverse-phase high-performance liquid chromatography (HPLC). The mold growing in the presence of 0, 2 and 4% KCl produced maximum amounts of aflatoxin after 3 d, whereas in the presence of 6, 8 and 10% KCl it did so after 7 d. This trend was also true when the mold grew in the presence of mixtures of NaCl and KC1. Amounts of aflatoxin produced decreased with increasing concentrations of KCl or of the mixture of NaCl and KCl. The mycelial dry weight increased with increasing concentrations of KCl or the mixture of NaCl and KCl, although there was an extended lag phase at higher concentrations of both treatments.