George R. Stephens

Effect of mulches on bacterial populations and enzyme activity in soil and vegetable yields

SummaryMulches in five combinations of clear plastic, black plastic, and undecomposed leaves covered by or on top of black or clear plastic film to modify soil temperature, were used to study the enzyme activity of the soil and numbers of bacteria able to produce degradative enzymes. Yield of pepper and broccoli planted through the mulches was measured.The numbers of bacteria in the soil able to excrete protease, phosphatase, cellulase, and lipase were correlated with one another suggesting that the same group of organisms excreted the different enzymes. The activity of enzymes in the soil was similarly correlated. Bacterial urease-producers and the urease activity in the soil were not correlated with the number of other enzyme producers or activity of other enzymes in soil suggesting an independent population of urease producers degraded the urea fertilizer used. Temperature differences of 11–14°C provided by the various mulches appeared insufficient to create important differences in populations and biochemical activities of soil bacteria.Yield of broccoli was greatest early in the season in soil warmed under black polyethylene plastic and later in the season in the soil cooled with leaf mulch over black plastic. Yield of pepper was greatest in mid-to-late summer on soil warmed with black plastic, but the highest yield was obtained in early fall in soil with clear and black plastic over leaf mulch.

Influence of crown class and shade tolerance on individual tree development during deciduous forest succession in Connecticut, USA

Abstract Sixty years of individual tree crown class records were used to elucidate the influence of crown class (dominant, codominant, intermediate, or suppressed), shade tolerance (intolerant, midtolerant, or tolerant), and their interactions on the probability of individual tree movement among crown classes. Trees were measured at 10 year intervals between 1927 and 1987, excluding 1947, on 364 nominal 0.01 ha plots. A total of 14 154 individual tree records were used in this analysis. The transition rates among crown classes for 30 year intervals (1927–1957 and 1957–1987) were examined for all combinations of antecedent crown classes and tolerance rankings. The distribution of ingrowth among tolerance rankings was also examined. Mortality rates increased with decreasing crown class for all tolerance rankings, and mortality rates increased with decreasing tolerance. Compared with tolerant trees, midtolerant and intolerant trees had higher rates of ascension into dominant and codominant crown classes and exhibited higher persistence rates in the dominant crown class. These factors suggest that midtolerant and intolerant trees have an advantage over tolerant trees in the higher crown classes, In contrast, tolerant trees had the advantage in suppressed and intermediate crown classes, with lower mortality rates, higher persistence, and higher rates of crown class ascension than for midtolerant or intolerant trees. Crown class stratification was driven by the change in relative advantage of each tolerance ranking among the crown class. Results of this study suggest that the canopy stratum (upper canopy, lower canopy, or mixed) affected by disturbance is as important as disturbance scale, intensity, and frequency in influencing the composition of the suppressed crown class. Because the suppressed crown class is in a constant state of high flux, with fewer than 30% of the suppressed trees remaining in the suppressed crown class for any 30 year period, a small change in the relative persistence or ascension rates among tolerance rankings, whether by a different disturbance or climate regime, could alter the proportion of tolerance rankings in the suppressed crown class and ultimately result in an alternative succession.

Keeping Quality of Pasteurized Milk for Retail Sale Related to Code Date, Storage Temperature, and Microbial Counts

Keeping quality of milk samples collected in original containers from fillers and stored at 1.7, 5.6, and 10.0 C remained organoleptically acceptable, on the average, 17.5, 12.1, and 6.9 days, respectively. Samples were tested for specific groups of bacteria at collection and when the milk became unacceptable (flavor score < 36). In addition to a total aerobic count the specific groups included pseudomonads, lipolytic, proteolytic, acid-producing, and coliform bacteria, and lipolytic and proteolytic pseudomonads. Keeping quality at any storage temperature was unrelated to the manufacturers code date (last day product is to be sold). There was a significant correlation between keeping quality at 10.0-C storage and the other two storage temperatures, suggesting a practical test to measure keeping quality at the lower temperatures. Microbial counts, made at bottling and when the sample became unacceptable, were not consistently related to the time required for milk to become unacceptable at any storage temperature. When samples were stratified by flavor defect, certain microbial tests were significantly related to keeping quality.

WHAT TESTS USEFULLY PREDICT KEEPING QUALITY OF PERISHABLE FOODS

The keeping quality of milk and salad samples is shown to be generally unrelated to standard microbiological and chemical tests. Relatively little of the observed variation in flavor score, assigned to milk samples upon receipt from retail outlets, could be accounted for by microbiological and chemical tests made at the same time. The flavor score given to milks immediately after pasteurization seems to be most useful to predict keeping quality, i.e., the number of days required to attain an unsatisfactory flavor score.

Comparison of Code Date Reliability for Freshly Bottled Whole, Lowfat and Nonfat Fluid Milk

Whole, lowfat, and nonfat fluid milk samples were collected from dairy plants on the day of bottling and each set of the three types of milk was processed from the same raw milk supply. Bacterial analyses for total aerobic count and for specific degradative types were made immediately after collection. Samples were also stored at 1.7, 5.6 and 10.0 C to test for flavor deterioration. Total count of aerobic bacteria and of specific degradative types did not differ significantly among the three types of milk. Organoleptic analysis was made periodically to determine milk quality (flavor score) and defect (flavor). At any of the storage temperatures the keeping quality (days required from bottling to reach a flavor score of < 36) was unrelated to the length of time between bottling and last day of sale (code date) assigned by the processor. At 5.6- and 10.0-C storage, more whole milk samples were criticized for more serious flavor defects (e.g. putrid) than for the less serious ones (e.g. lacking freshness) found in the lower fat milks. Keeping quality of all three types of milks at 1.7- and 5.6-C storage could be predicted from keeping quality determined at 10.0 C with equations previously developed for whole milk.

Antibiotic-resistant Bacteria in Raw Milk and Ability of Some to Transfer Antibiotic Resistance to Escherichia coli

Raw milk samples were examined for number and percentage of bacteria resistant to seven antibiotics: penicillin, ampicillin, chloramphenicol, neomycin sulfate, polymyxin B sulfate, tetracycline and streptomycin sulfate. A significant negative correlation was found between the total aerobic count of the milk sample and the concentration (above 5 or 10% of the total count) of bacteria in each milk resistant to each of the antibiotics tested. Three of 42 gram-negative isolates were capable of transferring their antibiotic resistance to Escherichia coli . Substantial numbers of antibiotic-resistant bacteria in raw milk were found and some survived pasteurization. Inspection of farms failed to indicate a relationship between farm practices or use of antibiotics in feed or as pharmaceuticals and number of antibiotic-resistant bacteria in the raw milk.

Relation of Code Dates to Quality of Milk Sold in Retail Markets

Milk collected at retail markets and in schools in Connecticut in 1970-71, 1974, and 1975 was examined for flavor quality, temperature at collection, code date (last day product is intended to be offered for sale), and age of sample (days from bottling). The relationships between quality and age of sample and the processors code date were tested. Regression analysis of 1975 samples showed that, on the average, processors overestimated shelf life by about 2 days, but this overestimate varied from 0 to 7 days for individual dairies. Although the temperature of samples collected in 1974 and 1975 was lower than in the 1970-71 sampling, there was no diminution in the percentage of samples of unsatisfactory quality.