KEEPING QUALITY Research Articles

CHARACTERIZATION OF 'MARSH' GRAPEFRUIT PEEL MATURATION AND ITS RELATIONSHIP TO POSTHARVEST HANDLING AND KEEPING QUALITY

This reports a preliminary study to understanding the role of peel maturation in citrus as it relates to postharvest handling and keeping quality of the fruit. Currently, citrus is harvested based on internal quality (edibility). Peel changes have not been related to best harvest time. Several physical parameters (weight loss, decay, chilling injury, color index, puncture resistance, and detachment force), and chemical parameters (TSS:acid ratio of the juice, and sugars, glycosidases, abscisic acid and volatile components of the peel) were measured for harvest dates to determine if peel maturation and senescence can be monitored by some combination of factors in order to minimize fruit disorders resulting from immature or senescent peel or unusual stress levels that the peel may have been subjected to. Changes in levels of sugars, glycosidases, abscisic acid and volatile components during maturation (harvest dates) coincided with changes in peel color, percent fruit drop and with fruit characteristics during storage (weight loss, decay and chilling injury) and may be useful for establishing harvest date guidelines to avoid harvesting too early (immature fruit) or too late (senescent fruit) so that less postharvest injuries occur while the fruit still has acceptable internal quality. Suggested harvest period was January-March. Days from bloom date, detachment force and peel reducing sugars were the best indicators for a peel senescence index.

Challenge Testing the Lactoperoxidase System Against a Range of Bacteria Using Different Activation Agents

Lactoperoxidase (LP) exerts antimicrobial effects in combination with H2O2 and either thiocyanate (SCN−) or a halide (e.g., I−). Garlic extract in the presence of ethanol has also been used to activate the LP system. This study aimed to determine the effects of 3 LP activation systems (LP+SCN−+H2O2; LP+I−+H2O2; LP + garlic extract + ethanol) on the growth and activity of 3 test organisms (Staphylococcus aureus, Pseudomonas aeru-ginosa, and Bacillus cereus). Sterilized milk was used as the reaction medium, and the growth pattern of the organisms and a range of keeping quality (KQ) indicators (pH, titratable acidity, ethanol stability, clot on boiling) were monitored during storage at the respective optimum growth temperature for each organism. The LP+I−+ H2O2 system reduced bacterial counts below the detection limit shortly after treatment for all 3 organisms, and no bacteria could be detected for the duration of the experiment (35 to 55h). The KQ data confirmed that the milk remained unspoiled at the end of the experiments. The LP + garlic extract + ethanol system, on the other hand, had no effect on the growth or KQ with P. aeruginosa, but showed a small retardation of growth of the other 2 organisms, accompanied by small increases (5 to 10h) in KQ. The effects of the LP+SCN−+H2O2 system were intermediate between those of the other 2 systems and differed between organisms. With P. aeruginosa, the system exerted total inhibition within 10h of incubation, but the bacteria regained viability after a further 5h, following a logarithmic growth curve. This was reflected in the KQ indicators, which implied an extension of 15h. With the other 2 bacterial species, LP+SCN−+H2O2 exerted an obvious inhibitory effect, giving a lag phase in the growth curve of 5 to 10h and KQ extension of 10 to 15h. When used in combination, I− and SCN− displayed negative synergy.

Alternative strategies for activation of the natural lactoperoxidase system in cows' milk: trials in Tanzania

Thiocyanate content and lactoperoxidase activity of individual cow's milk of different breeds were determined, and the effects of different lactoperoxidase system (LP-s) activation strategies were compared. Lactoperoxidase activity varied significantly between Friesian and both Ayrshire and Tanzania Short Horn Zebu (TSHZ), but differences between Ayrshire and TSHZ were not significant. There was no significant variation in SCN- content between breeds. The LP-s was activated using three strategies based on SCN-: namely; equal concentrations of SCN- and H2O2 (7:7, 10:10, 15:15 mg/l), excess SCN- concentrations (15:10, 20:10, 25:10 mg SCN-:H2O2/l), and excess H2O2 concentrations (10:15, 10:20, 10:25 mg SCN-:H2O2/l), plus a fourth strategy based on I- (15:15 mg I-:H2O2/l). The keeping quality (KQ) was assessed using pH, titratable acidity, clot on boiling and alcohol stability tests. All activation strategies enhanced the shelf life of milk (typically increasing KQ from around 10 to around 20 h), but it was clear that the effectiveness of the LP-s depends on the type and concentrations of the activators of the system. The LP-s activated using I- as an electron donor was more effective than the LP-s activated using SCN- as an electron donor, increasing the KQ by a further 6-8 h compared with SCN-.

Challenge testing of the lactoperoxidase system in pasteurized milk.

To determine the role of lactoperoxidase (LP) in inhibiting the growth of micro-organisms in pasteurised milk. Four micro-organisms of importance in the spoilage of pasteurized milk were challenged in lactoperoxidase (LP)-enriched ultra-heat treated (UHT) milk after subsequent pasteurization. Milk samples were stored at the optimum temperatures for growth of the individual bacteria. Pasteurization was carried out at 72 degrees C/15 s and 80 degrees C/15 s to determine the effect of the LP system on the micro-organisms. An active LP system was found to greatly increase the keeping quality (KQ) of milks inoculated with Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus thermophilus and pasteurized at 72 degrees C, but had little or no effect in milks heated at 80 degrees C, presumably due to virtual inactivation of LP at 80 degrees C. However, pasteurization temperature had no effect on the KQ of milks challenged with Bacillus cereus spores. This study suggests that the LP system, rather than heat-shocking of spores, is responsible for the greater KQ of milk pasteurized at 72 degrees C/15 s compared with 80 degrees C/15 s. The study emphasizes the care required in selecting pasteurization temperatures in commercial practice and to avoid the temptation to compensate for inferior quality of raw milk by increasing pasteurization temperature.

The direct epifluorescent filter technique, cytochrome c oxidase test and plate count method for predicting the keeping quality of pasteurized cream

The ability of the direct epifluorescent filter technique (DEFT) to predict the keeping quality (KQ) of pasteurized cream, after an 18-h preincubation stage, was poor unless (particularly with samples that had a corresponding long KQ) undiluted cream was filtered. The DEFT then gave as good, if not better, prediction of the KQ as the plate count. The cytochrome c oxidase test (Kroll 1985, J. Appl. Bacteriol. 59 , 137–141) could also be used successfully to predict the KQ of pasteurized cream but its predictive value was not as good as the DEFT or plate count method. However, because of its simplicity, rapidity and low cost, the method could be of use to milk processors to monitor the potential KQ of their products.

Prediction of the keeping quality of pasteurized milk by the detection of cytochrome c oxidase.

The keeping quality (KQ) of pasteurized milk samples stored at 5 degrees C and 10 degrees C was satisfactorily predicted after 18 h pre-incubation with 0.05% benzalkonium chloride at 20 degrees C, by estimating the numbers of Gram-negative psychrotrophic bacteria using the simple, cheap and rapid (5 min) assay of cytochrome c oxidase. Correlation coefficients for the relationship between cytochrome c oxidase activity at 20 degrees C and KQ at 5 degrees C or 10 degrees C of -0.89 and -0.84 respectively were obtained. The method correctly predicted the KQ of more than 89% of the samples of pasteurized milk. The assay was not satisfactory for use on samples after pre-incubation at 30 degrees C.

Effect of pasteurization temperature on the keeping quality of whole milk

SummaryThe keeping quality (KQ) of whole milks pasteurized at 72 °C, 78 °C or 83 °C for 20 s and stored at 12 or 7 °C was examined by assessment of the naturally contaminating thermoduric microflora and of the numbers of Gram-negative rods, a single strain of the latter being introduced deliberately by post pasteurization inoculation. A pasteurization temperature of 83 °C reduced the KQ at 7 °C by about 15% compared with pasteurization at 72 °C or 78 °C, with a 2 d loss in the absence, and aid loss in the presence, of the post pasteurization contamination. The KQ of milks stored at 12 °C was not significantly affected by the pasteurization temperature. Milks pasteurized at 78 °C tended to have a slightly better KQ than those pasteurized at 72 °C, especially at 7 °C. However, pasteurization at both 78 °C and 83 °C caused a reduction in cream rising and considerably increased the risk of cream plugging.

Use of the Direct Epifluorescent Filter Technique for predicting the keeping quality of pasteurized milk within 24 hours

The keeping quality (KQ) of pasteurized milk stored at 5 degrees C and 11 degrees C was predicted within 24 h by pre-incubating samples and counting bacteria by the Direct Epifluorescent Filter Technique (DEFT). For samples from 5 degrees C storage, 0.03% (w/v) benzalkonium chloride and 0.002% (w/v) crystal violet (final concentration) were added to inhibit the growth of Gram positive bacteria during pre-incubation. The samples from milk stored at 11 degrees C were pre-incubated without the addition of inhibitors. After pre-incubation there was a satisfactory relationship between the DEFT count and the KQ of milks at both 5 degrees C and 11 degrees C. The DEFT count following pre-incubation correctly classified greater than 80% of pasteurized milks on the basis of KQ.