Carrot slicing method impacts E.coli contamination depth
Slicing with a blunt machine blade enhanced penetration of E.coli O157:H7 and its survival during storage at 8 °C, according to a study.
Carrots sliced with a blunt machine blade may have increased surface areas and greater nutrient availability compared to conditions at the surfaces of carrots sliced with a razor blade.
The effects of slicing and peeling, of storage atmosphere and temperature on the survival of E.coli O157:H7 at and below the cut surfaces of fresh-cut carrot discs were determined.
“Optimum cutting during harvesting and processing might also increase the efficiency of washing and anti-microbial dipping treatments in reducing pathogen count,” said O’Beirne et al.
E.coli attachment
Initially, there were no significant differences between slicing with a blunt machine blade or a razor blade on attachment levels of E.coli O157:H7 at the surface (0–25 μm) of the carrot slice. Both surfaces had similar counts for the first two sections sampled (0–25 and 225–250 μm).
However, higher numbers of cells (0.5 log CFU g−1) penetrated deeper (475–500, 725–750 and 975–1000 μm) into carrot tissue sliced with a blunt machine blade.
During storage, E.coli O157:H7 survived better at the surface and deeper into carrot tissue sliced with a blunt machine blade. It is likely that cracks, provided bacterial access and more nutrients due to damaged carrot tissue located deeper into the carrot slice.
Samples were taken at depths of 0–25, 225–250, 475–500, 725–750 and 975–1000 μm from the carrot slice.
Greater survival of E.coli O157:H7 was observed for carrot slices stored in 20% CO2/1% O2 than in 5% CO2/3% O2.
At higher CO2 levels, naturally occurring background microflora may be inhibited, decreasing competition for nutrients and space, allowing better survival of E.coli O157:H7, found the study.
Peeling and slicing
There were no significant effects of peeling method on penetration and survival.
E.coli O157:H7 did not survive as well on peeled surfaces as it did on sliced surfaces.
Slicing horizontally causes damage to the vascular tissue and nutrients and inorganic salts may be released, contributing to better survival of E.coli O157:H7 on sliced surfaces, said the researchers.
To investigate temperature effects, carrots were sliced with a sharp machine blade and stored at 4 °C or 10 °C.
To determine temperature abuse on survival of E.coli, at different tissue depths inoculated carrot slices, packaged within oriented polypropylene film (OPP film) were stored at 4 or 10 °C for three and five days.
During storage at 4 degrees, numbers decreased at all sample depths, and populations survived better at the surface compared to cells that had penetrated into the tissue. Storage of carrot slices at 10 degrees resulted in growth of E.coli O157:H7 populations at all sample depths.
Source: Food Control, Volume 40, June 2014, Pages 71–77
Online ahead of print, DOI: 10.1016/j.foodcont.2013.11.026
“Effects of processing and storage variables on penetration and survival of Escherichia coli O157:H7 in fresh-cut packaged carrots”
Authors: David O'Beirne, E. Gleeson, M. Auty, K. Jordan
