COMPARISON OF METHODS FOR THE DETERMINATION OF CELL VIABILITY IN STORED BAKER'S YEAST

The classical plate method for discriminating between viable and nonviable yeast cells in stored baker's yeast was compared with dead cell staining techniques using methylene blue and three fluorochrome stains. The increase of dead yeast cells during storage of baker's yeast for up to 16 days at 5°C, 20°C and 35°C was determined. During prolonged storage, especially at 35°C, the death rate increased rapidly and the yeast cake began to soften because of autolysis. In these conditions the choice of method for determining the proportion of dead cells proved to be of great importance. Useful results for yeast stored for some time at 35°C could be obtained only by the fluorochrome technique using primuline, acridine orange or acriflavine as fluorochromes.     

EFFECTS OF SHRINK FILM WRAP ON INTERNAL GAS CONCENTRATIONS, CHILLING INJURY, AND RIPENING OF HONEYDEW MELONS

Honeydew muskmelons (Cucumis melo L.) were individually wrapped with polyvinyl chloride (PVC) shrink film and stored at 2.5° or 7.5 °C for 21 days and examined, then held an additional 2 or 3 days at 20°C and examined again. Nonwrapped melons were the control. The concentration of CO₂ in the cavity of wrapped melons stored 21 days was 5.6% at 2.5°C, 9.1% at 7.5°C, but only 1.1% and 1.5% in the nonwrapped held at 2.5°C or 7.5°C, respectively. Wrapped fruit ripened slower than nonwrapped fruit during storage and subsequent holding at 20°C, after which time 70% of the wrapped melons were rated eating ripe, but 62% of the controls were overripe. Wrapped melons exhibited 30% less chilling injury (CI) symptoms than nonwrapped fruit stored at 2.5°C. The CI symptoms ranged from reddish-brown to dark-brown surface discolorations and sometimes included dry sunken areas of skin. Fresh weight loss was about 1 % in wrapped melons, but 5% in nonwrapped fruit, regardless of storage temperature. Decay incidence was about equal in wrapped and nonwrapped melons after storage at 2.5°C, but was greater for wrapped than nonwrapped melons after storage at 7.5°C. Soluble solids content was about 12.5% in wrapped and nonwrapped melons stored at either temperature.     

STABILITY OF RESINS IN PELLETISED HOPS

The α-acids content and Lead Conductance Value of hop pellets, manufactured from seedless Pride of Ringwood hops grown in Australia and stored at 5°C, 20°C or 30°C, were monitored over a twelve month storage period. No decrease in α-acids content occurred during storage at 5°C, whereas at 20°C the decrease (ca. 7%) was approximately one-third that experienced for baled hops of the same variety. At both 5°C and 20°C the Lead Conductance Value decreased at approximately one-third the rate of that in baled hops. During twelve months storage at 30°C the α-acids content of the hop pellets decreased by approximately 40% and the Lead Conductance Value by 30%. The hop pellets stored at 20°C and 30°C developed rancid odours during the twelve months storage period.     

The effects of ethanol treatment on the metabolism,shelf life and quality of stored tomatoes at different maturities and temperatures

Two maturity stages of commercially grown tomatoes (breaker and mature green) were exposed to ethanol vapour (2 ml ethanol kg⁻¹ fruit) for 6 h at 20 °C prior to storage at 5 °C and 20 °C. During storage the colour, firmness and composition changes were examined every 3 and 7 days. The results showed that ethanol vapour treatment could significantly slow down the colour changes and softening of both mature green and breaker tomatoes with greater effects when stored at 5 °C. There was no difference between the two maturity stages in retardation of softening during storage; in contrast the maturity stage had a highly significant effect in the colour development of stored tomatoes. When the fruit stored at 5 °C was then held at 20 °C for 7 days the ripening process was accelerated but the fruit did not reach the same level of colour development as the fruit stored at 20 °C continuously. The results suggest that ethanol vapour pretreatment could be used as a cheap and easy method to extend the storage life of tomatoes. © 1999 Society of Chemical Industry     

Production and Regeneration of Principal Volatiles in Apples Stored in Modified Atmospheres and Air

In a series of exploratory experiments, storage of McIntosh apples (Malus domestica Borkh.) in modified atmospheres (MA) (5% CO₂+ 3% O₂ at 2.8°C) suppressed the development of headspace ethanol and acetaldehyde from that in apples stored in air at 0°C (RA). Acetaldehyde, ethanol, ethyl butyrate and hexanal production from intact fruit was further suppressed when the apples were stored in 1.5% CO₂+ 1.5% O₂ or 1.5% CO₂+ 1.0% O₂ at 2.8°C. Placement of fruit in RA following MA storage initially regenerated ethyl butyrate and hexanal in preference to ethanol and acetaldehyde. However storage of fruit in 1.5% CO₂+ 1.0% O₂ for 320 days completely suppressed the principal headspace volatiles and blocked their subsequent regeneration in RA.     

THE EFFECT OF STORAGE TEMPERATURE ON THE STABILITY OF THE ALPHA-ACIDS CONTENT OF BALED HOPS

The α-acids content of baled hops (Pride of Ringwood) stored at ?20°C, ?5°C, +5°C and +22°C was monitored over a twelve month storage period. The α-acids content of the hops was found to decrease at a linear rate, i.e., by zero order kinetics, at each storage temperature. The relationship between reaction rate and storage temperature was found to accord with the predictions of classical reaction kinetics, enabling a prediction of the stability of hops stored at various temperatures to be made.     

Atmospheric Composition and Quality of Fresh Mushrooms in Modified Atmosphere Packages As Affected by Storage Temperature Abuse

Mushrooms (Agaricus bisporus) were packaged in 4-liter modified atmosphere (MA) containers, and a steady-state atmosphere of 5% and 10% was maintained at 4 °C. Temperature was fluctuated from 4 °C to 20 °C during 12-d storage period in cycles: 2 d at 4 °C followed by 2 d at 20 °C. Temperature increase during fluctuations caused anoxic atmospheres both in O₂ (1.5%) and CO₂ (22% to 10%). The quality of mushrooms stored under temperature fluctuating regime was severely affected as indicated by extensive browning, loss of firmness, and the level of ethanol in the tissue compared to mushrooms stored at constant temperature. It was clear that temperature fluctuation, even if it should occur once, can seriously compromise the benefits of MA packaging and safety of the packaged produce.     

USE OF FLEXIBLE POLYMER MATERIAL FOR PACKAGING FRESH PEACHES

Fresh peaches (Prunus persica) were overwrapped in trays with 1 of 3 formulations of flexible polyvinyl chloride (PVC) film that differed in gas transmission rate or they were held in nonwrapped trays (controls). The CO₂ transmission rate at 0°C for PVC type III film was 280 mL/m^2. h (1 atmos); that of type II was 4 times greater and that of type I, about 5 times greater. The peaches were stored either 14 days at 0° or 7.5°C, or 7 days each at 0° and 7.5°C plus 2 days at 20°C to simulate retail display. The mean CO₂ levels were 10, 7.2 and 4.7% in packages that were wrapped with PVC III film and held at 7.5°, 0°/7.5° and 0°C, respectively. CO₂ in packages wrapped with PVC I or II was below 3% at each storage temperature. O₂ concentration remained about 4% in all packages. Weight loss was less and fruit was firmer among those packaged in PVC III than among nonwrapped controls at each of the 3 storage temperatures. Storage temperature had no effect on weight loss or of fruit held in PVC III film. External appearance of fruit packaged with the 3 types of film was significantly better than that of the controls. Internal appearance of the peaches was unaffected by any of the treatments. A microatmosphere favorable for fresh peaches can be maintained within packages overwrapped with polymer films that are selectively permeable to respiratory gases.     

1-MCP Treatment Prolongs Postharvest Life of 'Santa Rosa'Plums

'Santa Rosa’plums were treated with concentrations of 0‐ (control), 0.5‐, or 0.75‐μL/L 1‐methylcyclopropene (1‐MCP) for 24 h at 1°C and stored at 1 °C for up to 40 d. Ethylene and CO₂ production, firmness, weight loss, Brix, acidity, color evolution, and ethanol and acetaldehyde concentrations were determined periodically after cold storage and after subsequent shelf life of 5 and 8 d at 20 °C. 1‐MCP strongly inhibited ethylene and CO₂ production. Higher values of firmness were observed in 1‐MCP‐treated plums compared with control. 1‐MCP treatment delayed color evolution, reduced acidity loss, and inhibited ethanol and acetaldehyde production. 1‐MCP did not affect weight loss or sugar content.     

THE FATE OF LIVE BREWERS' YEAST SLURRY IN BOVINE RUMEN FLUID

Live brewers' yeast slurry was incubated under carbon dioxide at 27°C and 39°C in 0·1% peptone solution and in bovine rumen fluid which had been clarified by removal of the population of bacteria and protozoa normally present. Numbers of viable yeast in both media remained constant for 12 h at 27°C; at 39°C loss in viability was 81 % in peptone and 94% in rumen fluid during the same period. When glucose was added to clarified or unclarified rumen fluid containing yeast slurry and incubated for 6 h at 39°C, ethanol was produced. Ethanol production was prevented if the slurry was treated with heat or chemical preservatives before addition to the rumen fluid. Unclarified rumen fluid from a steer fed a brome-alfalfa hay-grain ration contained 10²–10³ yeasts and moulds per ml. The results suggested that the feeding of live brewers' yeast slurry to ruminants could result in ethanol toxicity if fermentable carbohydrate were also present, though many of the yeast cells would succumb to heat inactivation at normal rumen temperatures. This risk could be eliminated by prior treatment of the slurry with heat or chemical preservatives.     

Effect of aminoethoxyvinylglycine and low-pressure storage on the post-storage production of aroma volatiles by Golden Delicious apples

Golden Delicious apple fruits untreated (control) or treated pre-harvest with the ethylene biosynthesis inhibitor aminoethoxyvinylglycine (AVG) were stored at 2°C under either ambient pressure or low pressure (6.7 kPa; LPS). The production of aroma volatiles during post-storage periods at 20°C was measured on fruits stored for 3, 5, 7 and 9 months. Aroma production of AVG- or LPS-treated fruits immediately after storage was considerably lower. However, after 14 days at 20°C, aroma production almost reached values for control fruits (AVG fruits had to be treated with 50 μl C₂H₄ litre^?1). After the longer storage this effect of a post-storage period at 20°C declined gradually and finally was almost absent. Only a post-storage period at 2°C in a normal atmosphere followed by a period at 20°C was now able to partially revive long-stored fruits from this ‘residual’ effect of AVG or LPS. It is suggested that changes in the sensitivity of fruits to C₂H₄ are responsible for the observed decline in aroma production.     

Efficacy of different chemicals on shelf life extension of parsley stored at two temperatures

Four different chemical treatments, GA₃, 1‐MCP, essential oils and nano‐Cu, were applied immediately after harvest to Petroselinum crispum (Mill) plants. The efficacy of the above chemicals on shelf life extension of parsley stored at 5 °C and 20 °C was determined by analysing physiological and biochemical factors that determine quality standards of storage fresh parsley. Nonsprayed parsley revealed the highest loss of weight, ascorbic acid, pigments and an enhancement of CO₂ production and lipid peroxidation at 5 °C and 20 °C of storage. Nano‐Cu was more effective for delaying weight loss and revealed a better storage capacity. GA₃, 1‐MCP and essential oils sprays were more effective in ascorbic acid retention at 20 °C than at 5 °C, whereas all substances protect samples from lipid peroxidation. Essential oils were more clearly inhibitory towards both total viable counts and yeast infection. Our results suggest that GA₃, 1‐MCP, essential oils and Nano‐Cu exert their function through different mechanisms during ripening and could provide an effective and complementary means for maintaining high‐quality parsley leaves after harvest.     

THE INFLUENCE OF TEMPERATURE ON SOME PROPERTIES OF YEAST

The effects of temperature on the growth of yeast and on its metabolic activity in distiller's malt wort have been studied. In un-aerated fermentations, maximum yeast production takes place at about 30° C. whereas the growth rate in aerated cultures is highest at 35° C. The lag phase of the yeast studied fell from 6 hr. at 20° C. to 2·8 hr. at 25° C. and was not thereafter greatly affected by increases of temperature until 42° C. was reached, at which point growth ceased. Maltase activity was maximal at 25° C. when considered in terms of unit quantities of either yeast or fermenting wort, but the optimum temperature for initial fermentation velocity varied according to the time over which the measurement was made, being maximal at 40° C. for 0·5 hr., and at 35° C. for 2 hr. Alcohol production was highest at 25° C. whereas glycerol and higher alcohol formation took place optimally at 30° C.     

CHANGES IN CYTOCHROME LEVELS DURING THE STORAGE OF BAKER'S YEAST AT DIFFERENT TEMPERATURES

The content of cytochromes determined per g fresh yeast was found to stay constant during nearly 3 months' storage of baker's yeast at 5°C. When stored at 24°C and 35°C the content of cytochromes apparently increased during the first 14 and 4 days of storage, respectively, and then decreased continuously.     

Chemical and microbial stability of high moisture dried apricots during storage

BACKGROUND: This study was conducted to determine the chemical and microbial stability of high moisture (HM) dried apricots during storage at 5, 20 and 30 °C for a period of 8 months. HM dried apricots were obtained by rehydrating dried apricots in ‘water’ and ‘water + H₂O₂’. RESULTS: Analysis of kinetic data suggested first‐order models for loss of SO₂ and non‐enzymatic browning reactions. Higher storage temperatures increased the rate of SO₂ loss and formation of brown colour in HM dried apricots. Results from extensive colour measurements (non‐enzymatic browning, reflectance colour and β‐carotene) revealed that the colour of HM dried apricots stored at 5 °C was almost unchanged during 8 months of storage. The colour of samples stored at 30 °C was unacceptable starting from 2 months of storage. Total mesophilic aerobic bacteria counts decreased 0.7, 1.1 and 1.5 log cycles after 8 months of storage at 5, 20 and 30 °C, respectively. For the same storage period, the decrease in mesophilic bacteria was 0.62 log cycle in samples rehydrated in ‘water + H₂O₂’ and stored at 20 °C. CONCLUSION: These results suggest that HM dried apricots should be stored at temperatures lower than 20 °C to preserve the characteristic golden yellow colour. A relatively low level of SO₂ (1458 mg kg^?1 at 200 g kg^?1 moisture level) was sufficient to prevent the growth of spoilage organisms in HM dried apricots at all three storage temperatures. Copyright © 2008 Society of Chemical Industry     

EFFECT OF PHORONE ON ACCUMULATION AND FATTY ACID COMPOSITION OF SURFACE LIPID OF APPLE DURING STORAGE

Phorone added to apples after harvest stimulated the rate of accumulation of surface lipid and decreased the rate of water loss when fruit were stored at 0°C but had the opposite effect in fruit stored at 5, 10 or 20°C where lipid accumulation was inhibited and water loss enhanced. The fatty acid composition of surface lipid from phorone-treated fruit showed a higher proportion of linoleic acid at all temperatures. For oleic and linolenic acids, a differential effect of storage temperature occurred with phorone-treated fruit at 0°C developing a lower proportion of oleic acid and a higher proportion of linolenic acid compared to control fruit whereas at 5° and 20°C the reverse occurred.     

Changes in appearance and natural microflora on iceberg lettuce treated in warm,chlorinated water and then stored at refrigeration temperature

The objective of this study was to determine the effect of warm, chlorinated water on the survival and subsequent growth of naturally occurring microorganisms and visual quality of fresh-cut iceberg lettuce. After dipping cut lettuce leaves in water containing 20 mg l⁻¹free chlorine for 90 s at 50°C, samples were stored at 5 or 15°C for up to 18 or 7 days, respectively. Populations of aerobic mesophiles, psychrotrophs, Enterobacteriaceae, lactic acid bacteria, and yeasts and molds were determined. The visual appearance and development of brown discoloration were monitored. Treatment of lettuce in warm (50°C) chlorinated water delayed browning of lettuce. Shelf life of lettuce stored at 5°C, as determined by subjective evaluation of color and general appearance, was about 5 days longer than that of lettuce stored at 15°C. Treatment in warm (50°) water, with or without 20 mg l⁻¹chlorine, and in chlorinated water at 20°C significantly (α= 0·05) reduced the initial population of mesophilic aerobic microflora by 1·73–1·96 log₁₀cfu g⁻¹. Populations increased, regardless of treatment, as storage time at 5°C and 15°C increased. The same trends were observed in populations of psychrotrophs and Enterobacteriaceae. Yeast populations increased slightly in lettuce stored at 5°C but were consistently about 3 logs lower than mesophilic aerobes. Populations of molds and lactic acid bacteria were less than 2 log₁₀cfu g⁻¹throughout storage at 5 or 15°C. Results suggest that heat (50°C) treatment may have delayed browning and reduced initial populations of some groups of micro-organisms naturally occurring on iceberg lettuce, but enhanced microbial growth during subsequent storage.     

EFFECT OF STORAGE ON THE NUCLEIC ACID COMPOSITION OF BAKER'S YEAST*

General changes are described in the nucleic acid content of yeast stored for 20 days under extreme conditions (35 °C). The total amount of nucleic acid increased in the first phase of 6–8 days storage. In the second phase, during which the yeast started to autolyse, the nucleic acid content diminished, first slowly and then more quickly. The quantity of acid-soluble nucleotides increased with storage. The amount of dead yeast cells is low during the first period, but starts to increase quickly during the second period. Changes in the rRNA, 5S RNA, tRNA, mRNA and DNA, fractionated on MAK columns, were investigated over a 20-day storage period. The contents of the RNA fractions increased initially during storage. The amount of rRNA attained its maximum value after 5 days, followed by DNA (7 days), then tRNA (10–12 days) and lastly 5S RNA; thereafter they all decreased. The mRNA fraction diminished rapidly: after only two days' storage the incorporation of labelled uracil into mRNA had declined by about 90%, showing that the ability to synthesize mRNA was soon lost. The short half-life of mRNA ensures its absence in yeast stored for longer periods.     

Growth or Survival of Listeria monocytogenes in Ready-to-Eat Meat Products and Combination Deli Salads During Refrigerated Storage

Growth or survival of Listeria monocytogenes in cold‐smoked salmon; sliced, cooked ham; sliced, roasted turkey; shrimp salad; and coleslaw obtained at retail supermarkets stored at 5 °C, 7 °C, or 10 °C (41 °F, 45 °F, or 50 °F, respectively) for up to 14 d was evaluated. Cold‐smoked salmon, ham, and turkey were obtained in case‐ready, vacuum packages. All food products were stored aerobically to reflect additional handling within the retail supermarket. Cold‐smoked salmon, ham, and turkey supported the growth of L. monocytogenes at all 3 storage temperatures. Fitted growth curves of initial populations (about 3 log₁₀ colony‐forming units [CFU]/g) in cold‐smoked salmon, ham, and turkey stored at 5 °C achieved maximal growth rates of 0.29, 0.45, and 0.42 log₁₀ CFU/g growth per day, respectively. Storage at 10 °C increased the estimated maximal growth rate of the pathogen by 0.56 to 1.08 log₁₀ CFU/ g growth per day compared with storage at 5 °C. A decline in populations of L. monocytogenes was observed in shrimp salad and coleslaw, and the rate of decline was influenced by storage temperature. Retention of viability was higher in shrimp salad than in coleslaw, where populations fell 1.2, 1.8, and 2.5 log₁₀ CFU/g at 5 °C, 7 °C, and 10 °C, respectively, after 14 d of storage. Inability of shrimp salad and coleslaw to support the growth of L. monocytogenes may be attributed to the acidic pH (4.8 and 4.5, respectively) of the formulations used in this study. Results show that the behavior of L. monocytogenes in potentially hazardous ready‐to‐eat foods is dependent upon the composition of individual food products as well as storage temperature.     

DRYING AND STORAGE TREATMENTS FOR OVERCOMING DORMANCY IN MALTING BARLEY

A batch of deeply dormant Triumph barley was stored at ?18°C. The grain was so dormant that its viability, 97–98%, could not be determined using standard tests. Samples were dried to 12% moisture at various rates by varying the temperature and relative humidity of the drying air. Dried samples were stored at three temperatures (15°, 27° and 38°C). At intervals the germination characteristics of subsamples were determined. Germinability improved with storage time, improving faster at the higher temperatures. However some of the samples stored at 27°C and all the samples stored at 38°C suddenly showed a loss of viability, preceded by a loss in vigour. The rate of recovery from dormancy was independent of the drying regime used. Initially the germinability of the barley was 5–10% (1 ml agar test) and 0–4% (3 ml agar test). Recovery from dormancy was extremely slow at 15°C, so that after a year germination values were around 80% (1 ml agar test) and 45% (3 ml agar test). After 27 weeks the viability of grain stored at 27°C began to decline, germinability was 85–90% (1 ml agar test) and 50–60% (3 ml agar test). At 38°C the initial decline in dormancy was rapid, but germinability fell catastrophically at various times between 3 and 30 weeks storage. Other samples of the same lot of Triumph barley were dried to various moisture contents, 9.4%, 10.3%, 11.0%, 13.0% and 14.5%. These were stored at 38°C. The initial rate of recovery from dormancy was rapid, and was unrelated to the moisture content of the grain. The samples dried to 9.4% and 10.3% m.c. achieved germination values close to the viability value, around 95% (1 ml agar test) and 90% 3 ml (agar) tests in 15–18 weeks storage and showed no signs of deterioration in 30 weeks. Grain held at 11% moisture deteriorated after 12 weeks and that stored at 13% and 14.5% deteriorated after 3 weeks. The germinabilities of the samples dried to 9.4% and 10.3% and stored for 15–18 weeks at 38°C were so good, reaching maximal values at 2 days in the germination test, that it is concluded that they could probably not be matched by samples stored cool. The possibility of using higher storage temperature to overcome dormancy and water sensitivity more rapidly, is discussed. Experiments with other grain samples have confirmed that, in most respects, the original barley was typical of deeply dormant Triumph. Samples of Carmargue and Golden Promise matured much more rapidly than Triumph. However water sensitivity was extremely persistent in a sample of Doublet.