Pre-peeled potatoes, treated with heated ascorbic/citric acid solutions to control browning, undergo firming and separation of superficial tissues that affect texture after mashing and slicing following cooking. Examination of superficial parenchyma cells in cooked potatoes by scanning and transmission electron microscopy indicated greater cell wall rigidity and middle lamella retention in samples treated with browning inhibitors than in untreated controls. Lumps in mashed potatoes prepared from treated samples showed similar characteristics. Treated samples sometimes displayed a discontinuity beneath the superficial tissue, corresponding to site of tissue separation during slicing. These observations are consistent with treatment-induced changes in cell wall and/or middle lamella pectins as the cause of such textural defects.

Freshly harvested mushrooms were found to induce a near-anaerobic environment (<2% O2) in unventilated, PVC-overwrapped packages within 2 to 6 h when incubated at 20 to 30°C. Mushrooms were inoculated with an enterotoxigenic strain of Staphylococcus aureus and incubated in overwrapped trays at different temperatures. S. aureus grew and produced staphylococcal enterotoxin (SE) in unventilated PVC-overwrapped mushroom packages when inoculated at levels of 103, 104, and 105 CFU/g of mushroom after 4 days of incubation at 30°C. Growth of S. aureus was observed at all levels of inoculation at 25°C, but no SE was detected after 7 days of incubation. When mushroom packages were ventilated, S. aureus growth was suppressed and no SE was detected after 7 days at 25°C and 4 days at 30°C. However, S. aureus growth in ventilated packs exceeded growth in unventilated packages when the incubation temperature was increased to 35°C; SE was detected within 18 h of incubation at this temperature, even in mushrooms inoculated at a low level (102 CFU/g). These results show the extreme importance of proper sanitation and worker hygiene during mushroom harvesting and packaging, ventilation of fresh mushroom packages, and proper storage temperatures for fresh mushrooms at all points of the food chain.

Growth of Listeria monocytogenes on the surface of fresh peeled potatoes, treated with sulfite or a commercial browning inhibitor (CBI), packaged under vacuum and stored at 4, 15 and 28°C was determined. At 4°C, L. monocytogenes did not grow in all treated potatoes even after 21 days. At 15°C, L. monocytogenes grew to 7 log 10 CFU/g within 12 days in the potatoes treated with sulfite or CBI. At 28°C, L. monocytogenes population was greater than 3 log 10 CFU/g by 24 h in all samples regardless of treatment. Sulfites or a CBI appeared to provide a measure of safety in pre-peeled potatoes packaged under vacuum when kept at proper refrigeration temperatures.Key Words: Listeria monocytogenes, browning inhibitor, potatoes for inoculum preparation prior to all experiments. Preparation of inoculumInoculated BHI was incubated at 37°C for 18h. The cells were harvested by centrifugation at room temperature for 10 min at 7,700 ∞ g, the cell pellet washed twice, resuspended and diluted in sterile 0.1% peptone water (w/v) (Difco). Sample preparation and inoculationFresh, raw Russet potatoes ( 80-110g each) were obtained from a commercial potato distributor. Potatoes were abrasionpeeled for 2 min with a Vegetable Peeler (Model A1-15, Toledo Scale Co., Toledo, OH) and held under water up to 30 min prior to treatment. Potatoes received one of 4 different treatments: (1) immersion in 1.2% (w/ v) sodium bisulfite for 2 min, (2) immersion in a 2% (w/v) solution of a commercial browning inhibitor (CBI) consisting of citric acid, ascorbic acid, sodium acid pyrophosphate and L-cysteine HCl for 5 min, (3) immersion in a 2% solution of the CBI for 5 min, followed by a brief rinse with tap water (30 mL rinse/kg potatoes), or (4) untreated control. Residual sulfite levels were determined in uninoculated control samples by the Optimized Monier-Williams Method (Hillary et al., 1989).Duplicate sets of two potatoes ( 100g each) were weighed into filter stomacher bags (SFB-0410; Spiral Biotech., Bethesda, MD) and inoculated with 1 mL of an appropriate dilution of L. monocytogenes cell suspension to yield a final concentration of 1-2 log 10 CFU/g. Thereafter, the bags were manually massaged to ensure even distribution of organisms on the surface of the potatoes. The bags were placed in 17.8 ∞ 20.3 cm barrier bags (Koch Model 01 46 09, Kansas City, MO). The oxygen transmission rate (by manufacturer) of the nylon/polyethylene film was 54.2 cc/m 2 in 24h measured at 24°C and 75% relative humidity. The bags were evacuated to a negative pressure of 1000 millibars and heat sealed using a Multivac Model A300/ 16 gas packaging machine (Germany). Two replications were performed for each treatment. For each replicate experiment, two bags of each treatment were prepared for each sampling time and temperature. Controls consisted of inoculated, untreated potatoes stored under vacuum.