Effect of heat treatment on the antimicrobial properties of tef dough,injera, kocho and aradisame and the fate of selected pathogens

A known population from each of a 24h culture of Bacillus cereus, Pseudomonas aeruginosa, Salmonella spp., Shigella spp., Klebsiella spp. and Staphylococcus aureus was inoculated into tef flour–water/kocho–water mixtures in screw-capped flasks and allowed to ferment for 30h at room temperature (18–21°C). The flasks were then heat-treated. Cultures of the test bacteria were inoculated into tubes containing graded volumes of 30-h-fermented tef dough/kocho extracts which had been heat-treated at 45, 61 and 80°C in assay broth containing aqueous extracts from injera and aradisame. They were incubated for 24h at 32°C and optical densities determined. Populations of the major indigenous bacteria, yeasts and moulds in fermented tef dough (30h), kocho samples, injera and aradisame were determined from other control portions of the same samples. Higher temperature (80°C) heat-treatment promoted the inhibitory potential of extracts from doughs of both foods as compared with lower temperature heat-treatments (45 and 61°C). Asporogenous test bacteria were affected more than the spore-formers. Better efficacy of extracts from injera and aradisame suggested improved antimicrobial properties of the baked products than in doughs. Heat of baking inactivated all vegetative cells although spores of B.cereus, the yeasts and moulds survived the heat (100°C) applied for 5min. The c.f.u./g of food for B. cereus was below the disease-causing level (0.5×101 and 1.5×103, in injera and aradisame, respectively). Actual baking temperatures in homes are higher than the ones used here; if post-baking contamination is minimized or prevented, the products would be microbiologically safe with respect to the asporogenous pathogens when served fresh. Further studies on aflatoxins and improved storage conditions for kocho are recommended.     

The effect of fermentation on the growth and survival ofSalmonella typhimurium,Staphylococcus aureus,Bacillus cereus andPseudomonas aeruginosa in fermenting tef (Eragrostis tef)

Summary Growth ofSalmonella typhimurium, Staphylococcus aureus, Bacillus cereus andPseudomonas aeruginosa was inhibited when the pH of fermenting tef approached 5.0, 5.0, 5.5 and 5.0, respectively. However the test organisms grew in far more acidic conditions in broth than in fermenting tef and this is due to antimicrobial substance(s) being produced by some of the lactic acid bacteria. Except forBacillus cereus spores, all the test organisms were heat-inactivated during the baking process of the final tef injera.

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Effet de la fermentation sur la croissance et la survie de Salmonella typhimurium, Staphylococcus aureus, Bacillus cereus et Pseudomonas aeruginosa dans le tef (Eragrostis tef) en fermentation

Résumé La croissance deSalmonella typhimurium, deStaphylococcus aureus, deBacillus cereus et dePseudomonas aeruginosa est inhibée lorsque les pH du tef en fermentation approchent respectivement 5:0, 5.0, 5.5 et 5.0. Toutefois, les organismes tests croissent dans des conditiones bien plus acides que dans le tef en fermentation. Ceci est dû à des substances antimicrobiennes produites par certaines bactéries lactiques. A l'exception des spores deBacillus cereus, tous les organismes tests sont inactivés par la chaleur durant le processus de cuisson.

     

A polyphasic study on the taxonomic position of industrial sour dough yeasts

The sour dough bread making process is extensively used to produce wholesome palatable rye bread. The process is traditionally done using a back-slopping procedure. Traditional sour doughs in Finland comprise of lactic acid bacteria and yeasts. The yeasts present in these doughs have been enriched in the doughs due to their metabolic activities, e.g. acid tolerance. We characterized the yeasts in five major sour bread bakeries in Finland. We found that most of the commercial sour doughs contained yeasts which were similar to Candida milleri on the basis of 18S rDNA and EF-3 PCR-RFLP patterns and metabolic activities. Some of the bakery yeasts exhibited extensive karyotype polymorphism. The minimum growth temperature was 8 degrees C for C. milleri and also for most of sour dough yeasts.     

Experiences with the use of a starter culture in the fermentation of maize for ‘kenkey’ production in Ghana

Controlled fermentation of maize was carried out using six strains of Lactobacillus fermentum and one strain of yeast, Saccharomyces cerevisiae, isolated from traditionally fermented maize dough as starter cultures for inoculum enrichement. The fermentations were monitored by pH, acidity, microbiological analysis and taste panel evaluation of two products, kenkey and koko, prepared from the fermented doughs. The strains of L. fermentum used as starter culture dominated the microflora during fermentation and in most inoculated doughs the required pH was attained by 24 h instead of 48 h of dough fermentation. Higher contents of lactic acid bacteria and yeasts were observed in inoculated doughs at the initial stages of fermentation but the spontaneously fermented doughs attained similar lactic acid bacteria and yeasts counts by 24 h of dough fermentation. The organoleptic quality of kenkey and koko prepared from doughs fermented with starter culture for 48 h was not significantly different from the traditional products. Kenkey prepared from doughs fermented for 24 h with starter culture were found to be unacceptable by the taste panel although similarly produced koko was acceptable.The authors are with the Food Research Institute, Council for Scientific and Industrial Research, P.O Box M 20. Accra, Ghana.     

Survival and growth of Bacillus cereus in bread

Bread doughs were artificially inoculated with spores of six Bacillus cereus strains at different inoculum levels and counts of survivors in bread determined during storage at 27·5°C. No B. cereus were isolated from the centre crumb of 400 g loaves when the dough contained less than 10⁴ spores/g whereas with 800 g loaves survival occurred with doughs containing 5·0 times 10³ spores/g. With all strains there was a period of at least 24 h before multiplication took place in the bread. The inclusion in dough of 0·2% of calcium propionate, based on flour, effectively delayed germination and subsequent multiplication of B. cereus spores. It is concluded that the risk of food poisoning due to the presence of B. cereus in bread is minimal.     

Survival and growth of Bacillus cereus in bread

Bread doughs were artificially inoculated with spores of six Bacillus cereus strains at different inoculum levels and counts of survivors in bread determined during storage at 27.5 degrees C. No B. cereus were isolated from the centre crumb of 400 g loaves when the dough contained less than 10(4) spores/g whereas with 800 g loaves survival occurred with doughs containing 5.0 X 10(3) spores/g. With all strains there was a period of at least 24 h before multiplication took place in the bread. The inclusion in dough of 0.2% of calcium propionate, based on flour, effectively delayed germination and subsequent multiplication of B. cereus spores. It is concluded that the risk of food poisoning due to the presence of B. cereus in bread is minimal.     

The contribution of moulds and yeasts to the fermentation of 'agbelima' cassava dough

Agbelima, a fermented cassava meal widely consumed in Ghana, Togo and Benin, is produced by fermenting grated cassava with one of several types of traditional cassava dough inoculum. During fermentation a smooth textured sour dough is produced, the toxicity of cassava is reduced and there is a build up of volatile aroma compounds. Four types of inocula were included in the present investigation. In one type moulds were found to form a dominant part of the microbiota, the species present being Penicillium sclerotiorum, P. citrinum, P. nodulum, Geotrichum candidum and a basidiomycete. All these moulds were found to possess cellulase activity which was responsible for the hydrolysis of cassava tuber cellulose during fermentation leading to a breakdown of the coarse texture of cassava dough. The yeasts Candida krusei, C. tropicalis and Zygosaccharomyces spp. were present in high numbers in the four types of inocula including themouldy inoculum. The yeasts C. tropicalis and some strains of Zygosaccharomyces, all ofwhich possessed cellulase activity, were also found to contribute to the modification of cassavatexture during fermentation. All yeasts and moulds exhibited linamarase activity and were therefore capable of breaking down the cyanogenic glucosides present in cassava.     

A note on the leavening activity of yeasts isolated from Nigerian palm wine

The role of the yeast flora of Nigerian palm wine in the leavening activity of the beverage was investigated by subjecting organisms from the wine to dough-raising tests. Those with appreciable leavening activity were identified as Saccharomyces cerevisiae and Candida spp. They produced maximum dough volumes in 3–4 h at 37°C. The study has provided experimental evidence that yeasts contribute to the leavening activity of palm wine and has identified strains which have potential utility in commercial bread baking.     

Sources of microorganisms in pozol,a traditional Mexican fermented maize dough

Freshly prepared pozol, a traditional Mexican fermented maize dough, contained (c.f.u./g wet wt): lactic acid bacteria, 10⁴ to 10⁶; aerobic mesophiles, 10⁴ to 10⁵; Enterobacteriaceae, 10² to 10³; yeasts, 10² to 10⁴; and mould propagules, <10³. After 30 h at 28°C the numbers were, respectively: 10⁹, 7×10⁶, 5×10⁵, 10⁶ and 10⁴. Soaking alkali-treated grains overnight allowed lactic acid bacteria, aerobic mesophiles and Enterobacteriaceae to grow and these then constituted the primary microbial flora of the pozol dough. Grinding in a commercial mill inoculated the dough with lactic acid bacteria, aerobic mesophiles, Enterobacteriaceae and yeasts. Other processing stages, including the nature of the surface upon which the balls were made, handling of the dough, and air, contributed only minor numbers of microbes compared with the two major sources, soaking and grinding. The pH of pozol fell from an initial value of 7.3 to 4.6 after 30 h incubation at 28°C. The numbers of Enterobacteriaceae and other aerobic mesophilic bacteria remained constant between 11 and 30 h incubation and there was no evidence of the acidic conditions having any lethal effects on these organisms.     

Randomly amplified polymorphic DNA (RAPD) for discrimination of Pediococcus pentosaceus and Ped. acidilactici and rapid grouping of Pediococcus isolates

In a taxonomic study of the known Pediococcus species, together with 116 isolates from fermenting tef dough and fermented kocho using a PCR-based RAPD procedure, all the different species developed well differentiated agarose gel electrophoresis profiles. Analyses of the images with the Pearson product moment correlation coefficient (r) and upgma clustering algorithm in the GelCompar version 4·0 software gave a distinct identification scheme within and between type strains and food isolates of Pediococcus species. The procedure is simple, rapid for grouping of isolates, applicable to all species of pediococci and particularly useful for differentiating between strains of Ped. pentosaceus and Ped. acidilactici .     

Amylase production by a Gram-positive bacterium isolated from fermenting tef (Eragrostis tef)

Bacillus sp. A-001, which produced large amounts of amylase, was isolated from fermenting tef ( Eragrostis tef ) on tryptone soya agar supplemented with 1% starch. The organism grew between pH 4.5 and 10.5 with an optimum at 7–7.5. Growth occurred between 20 and 55°C but the optimum was about 35–40°C. At optimum medium pH (7.5) and a temperature of 35°C the organism entered the stationary phase after about 72 h and amylase production was at its highest (9.6 units ml^-1) at this time. Enzyme activity was optimal at pH 5.5 and 40°C and showed good thermal stability; it required 110 min to lose 50% of its activity at 70°C. The enzyme hydrolysed native starch (flour from tef, corn and kocho) to various oligosaccharides, including maltotriose, maltose and glucose.     

Improvement in the biocontrol of postharvest diseases of apples with the use of yeast mixtures

Mixtures of yeasts were tested for theirability to control Penicillium expansum andBotrytis cinerea on Red Delicious apple fruits. The occurrence of synergistic or antagonisticinteractions between yeast strains in differentmixtures was also evaluated. Two strains ofRhodotorula (R. glutinis SL 1 and R. glutinisSL 30) and two strains of Cryptococcus (C. albidus SL 43 and C. laurentii SL 62) were selected fordeveloping yeasts mixtures.The R. glutinis SL 1–R. glutinis SL 30 mixtureexhibited a lower effectiveness than eachstrain alone, against both moulds. Othermixtures (R. glutinis SL 1–C. albidus SL 43 and R. glutinis SL 30–C. albidus SL 43) showedsynergism against P. expansum but not against B. cinerea. The R. glutinis SL 1–C. laurentii SL62 mixture was the only mixture which showedsynergism against gray mould. There was not anymixture, which showed high effectivenessagainst both moulds at the same time. Differentresults could be explained by the dynamics ofthe population of the yeasts.By using yeast mixtures, it was possible toimprove biocontrol without increasing theamount of antagonists applied. The synergismobserved could be useful in enhancingbiological control.     

Cold hardiness and winter survival of the grape berry moth, Endopiza viteana, in New York State

Laboratory and field investigations were undertaken to evaluate the effect of winter mortality on subsequent infestation levels of the grape berry moth (GBM), Endopiza viteana (Clemens) (Lepidoptera: Tortricidae). Supercooling points (SCP) were used as a measure of the minimum survivable temperature for GBM pupae. The SCP of winter forms (diapausing) from a laboratory colony averaged –25.2 °C in December, –24.3 °C in March, and –24.0 °C in April. When diapausing pupae from a laboratory colony were placed in the field from December to March, the SCP averaged –23.3 °C. The SCP of summer forms from a laboratory colony averaged –21.6 °C in March. Significant levels of mortality (ca. 80%) of diapausing GBM pupae were observed after 24 h at –21 °C. Comparable mortality occurred after 1-week at –15 °C. Eighty-four percent of diapausing pupae survived for 8-months at 3 °C; non diapausing pupae suffered high levels of mortality (ca. 60%) after only 7-days at 3 °C. Field survival of diapausing pupae were significantly greater at soil level than 1-meter above the soil. Pupal survival was greater at 75% r.h. than at 0% or ambient r.h.. In 1986–1987 and 1987–1988, pupae covered with snow or leaf-litter had higher of survival than exposed pupae although these differences were only statistically significant in 1986–1987. The above findings are discussed within the context of potential implications to management strategies for GBM.

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Zusammenfassung Labor und Freilandversuche wurden unternommen um den potenziellen Einfluss von Wintersterben auf subsequenten Wurmbefall des nordamerikanischen Traubenwicklers Endopiza viteana (Clemens) (Lepidoptera: Tortricidae) auszuwerten. Die niedrigste Temperatur wobei Puppen ueberleben konnten ist durch Ueberkuehlungstemperaturen der Puppen gemessen worden. Fuer das ueberwinternde Stadium (in Diapause) einer Laborkolonie lag diese Temperatur durchschnittlich bei minus 25.2 °C in Dezember, minus 24.3 °C in Maerz, und minus 24.0 °C in April. Der Ueberkuehlungspunkt lag um minus 23.3 °C im Durchschnitt fuer Laborkolonie Puppen die waehrend der Diapause von Dezember bis Maerz im Freiland ausgesetzt wurden. Der Ueberkuehlungspunkt von Sommerstadien von der gleichen Kolonie lag im Vergleich um minus 21.6 °C im Maerz. Signifikante Sterblichkeit (um 80%) der Traubenwicklerpuppen wurde bemerkt nach 24 Stunden bei minus 21 °C. Vergleichbar war die Sterblichkeit nach einer Woche bei minus 15 °C. Nach 8 Monaten bei 3 °C ueberlebten 84% der Puppen in Diapause; 60% der Puppen nicht in Diapause starben nach 7 Tage bei 3 °C. Das Ueberlebensprozent diapausierender Puppen im Freien war signifikant hoeher auf der Erdoberflaeche als fuer die 1 m ueber der Oberflaeche. Mehr Puppen ueberlebten bei 75% relativer Luftfeuchtigkeit als bei 0% oder umgebender Luftfeuchtigkeit. In 1986–1987 und 1987–1988 Puppen die mit Schnee oder Laub bedeckt waren, ueberlebten prozentual mehr als die ohne jedes Bedeckungsmaterial. Die Ergebnisse werden im Kontext ihrer potenzieller Verwendung fuer Kontrolle des Traubenwicklers diskutiert.

     

Medium optimization for glucoamylase production by a yeast,Pichia subpelliculosa ABWF-64, in submerged cultivation

An amylolytic yeast strain Pichia subpelliculosawas shown to produce glucoamylase in submerged cultivation. The yeast strain produced the enzyme optimally at 30 °C and pH 5.6 in shake flasks agitated at 200 rev min^–1 in the optimized glucoamylase production medium containing 1% starch, 0.2% yeast extract, 0.4% K₂HPO₄, 0.035% NaCl and 0.1% MgCl₂. Maximum enzyme production was attained during early growth of 11 h in shake flasks, and 6 h in a laboratory fermenter. By optimizing media components and cultivation parameters, a 15-fold increase in glucoamylase secretion was achieved.     

Leavening ability and freeze tolerance of yeasts isolated from traditional corn and rye bread doughs.

Strains of Saccharomyces cerevisiae and Torulaspora delbrueckii isolated from traditional bread doughs displayed dough-raising capacities similar to the ones found in baker's yeasts. During storage of frozen doughs, strains of T. delbrueckii (IGC 5321, IGC 5323, and IGC 4478) presented approximately the same leavening ability for 30 days. Cell viability was not significantly affected by freezing, but when the dough was submitted to a bulk fermentation before being stored at -20 degrees C, there was a decrease in the survival ratio which depended on the yeast strain. Furthermore, the leavening ability after 4 days of storage decreased as the prefermentation period of the dough before freezing increased, except for strains IGC 5321 and IGC 5323. These two strains retained their fermentative activity after 15 days of storage and 2.5 h of prefermentation, despite showing a reduction of viable cells under the same conditions. The intracellular trehalose content was higher than 20% (wt/wt) in four of the yeasts tested: the two commercial strains of baker's yeast (S. cerevisiae IGC 5325 and IGC 5326) and the two mentioned strains of T. delbrueckii (IGC 5321 and IGC 5323). However, the strains of S. cerevisiae were clearly more susceptible to freezing damages, indicating that other factors may contribute to the freeze tolerance of these yeasts.     

Temperature and seedling age affect susceptibility of perennial ryegrass seedlings to pathogenic fungi

Summary Effects of temperature and seedling age on survival of perennial ryegrass (Lolium perenne L.) seedlings grown on sand-wheat wholemeal cultures of different isolates ofFusarium spp. (9 isolates),Pythium spp. (9 isolates), andChaetomium spp. (1 isolate) are reported. Some isolates were virulent over the whole range of temperatures tested (7.5–27.5°C). The virulence of others depended on temperature. Most isolates were less virulent at intermediate temperatures (12.5–22.5°C) than at higher or lower temperatures. At 25°C ryegrass seedlings were susceptible to fungal attack for only a limited period after germination commenced. This period differed for different fungi, but for most isolates tested, seedlings were resistant after 2–3 days.     

Bacterial species in raw and cured compost from a large-scale urban composter

Summary Raw and cured compost samples from a large-scale urban composter were studied over a period of eight months to gain information on bacterial species present. Total viable, aerobic heterotrophic bacteria, lactose-positive bacteria, antibiotic and metal-resistant bacteria and thermophilic bacteria were enumerated. Both raw and cured compost samples contained metal and antibiotic-tolerant bacteria (–1 compost) as well as high numbers (as high as Log 7.4 CFU g^–1 dry weight compost) of thermophilic bacteria isolated by growth at 55 °C. Selected colonies were also identified using the Biolog 95 substrate identification system.Escherichia coli andSalmonella spp. were not detected in compost samples.     

Effects of temperature and medium composition on the ethanol tolerance ofBacillus stearothermophilus LLD-15

The effects of temperature (60°–70°C) and medium composition (complex and defined) on ethanol tolerance ofBacillus stearothermophillus LLD-15, an L-lactate dehydrogenase mutant have been determined in shake flasks under aerobic conditions. In all cases, there was complete inhibition of growth in the presence of 6%v/v ethanol.B. stearothermophillus LLD-15 was found to be less tolerant to ethanol at 70°C than at 60°C and also less tolerant to ethanol in a defined medium than in a complex medium.     

Protein enrichment of sweet potato residue by solid-state cultivation with mono- and co-cultures of amylolytic fungi

The degree of protein enrichment of sweet potato residue by different amylolytic moulds in solid-state cultivation was much greater than that obtained using amylolytic yeasts. The optimum initial moisture content for protein enrichment was about 65% (w/w). Adding nitrogen sources to the culture twice (at the start of the incubation and after 24 h) considerably improved the final protein content. A co-culture of amylolytic mycelial fungi yielded a product with 32% (w/w) crude protein after 4 days' incubation at 30°C. In a column reactor, the highest temperatures reached were 42°C and 40°C and the minimum O₂ concentrations were 1.5% and 2.5% of full saturation in the central and bottom layers, respectively.     

The effects of acclimation to alternating environmental temperature on metabolic and endocrine responses in guinea pigs,during acute heat and cold exposure

Male Guinea pigs (n=80) were divided into four groups and maintained in a climatic chamber for three weeks in one of the following environmental conditions: (1) T_a20°C and 55% RH; (2) T_a35°C and 30–35% RH from 08:00 to 20:00 h and 5°C; 60–65% RH, from 20:00 h to 08:00 h; (3) T_a5°C and 60–65% RH; (4) T_a35°C and 30–35% RH. At the end of this period the animals were exposed to either –5°C, 60–65% RH or 45°C, 30–35% RH, for a period of 20 min, following which T_re, plasma 11-OHCS, thyroxin, glucose, and FFA, and body and organ weights were determined. The cold-warm adapted animals seemed to develop a more efficient adaptability to acute heat and cold exposure. It is suggested that on acute exposure to severe environmental conditions the endocrine and the nervous system play a dominant role in maintaining optimal body temperature, while on chronic exposure the metabolic rate of the various organs becomes relatively more important.