By J. Atilio De Frias, Yaguang Luo, Bin Zhou, Patricia Millner And Xiangwu Nou, Usda Agricultural Research Service
Time-temperature control of fresh-cut produce at 41°F (5oC) or less significantly reduces the growth of human pathogens. Since 2009, the US Food Code has required that packaged ready-to-eat leafy greens be kept at 41°F (5oC) or lower to minimize the potential of pathogen proliferation in the supply chain.
Under commercial settings, scientific studies reported large temperature variations and temperature abuse for produce displayed in open multi-deck refrigerated cases, often exceeding 41°F (5oC). Lower product temperatures in open cases are not achieved by simply lowering thermostat settings because freezing temperatures and loss of product quality can occur in the rear sections of the cases. To address these issues, we determined in our research supermarket facility at the USDA Agricultural Research Service (ARS) that installing clear glass doors was the most cost-effective solution for achieving compliance to US Food Code, improved quality and safety of fresh-cut leafy greens, and reduced energy consumption.
Open versus Doors
At USDA-ARS, two commercial-scale 12-ft multi-deck display cases were installed in a research supermarket facility. One of the cases was retrofitted with clear glass doors for comparative studies involving packaged fresh-cut leafy greens (e.g. baby spinach, chopped romaine lettuce and lettuce trio). Ambient conditions during the tests were 70°F (21oC), 60-70 percent relative humidity, and thermostats were set at 33°F (0.6oC).
Bagged leafy green products were displayed in both cases in three- to four-day trials, allowing for the collection of tens of thousands of temperature data points from the produce stored in both cases. The differences in temperature uniformity were striking. In the open case, product temperatures in the front often exceeded the US Food Code temperature requirement of 41°F (5oC). In the display case with doors, the non-compliance with Food Code was greatly reduced at the front of the case, and product temperature differentials decreased significantly.
Quality of Produce
In the open case, the quality of baby spinach samples in the front was substandard after four days of storage. Trained sensory panelists gave these samples the lowest ratings, often described as “wet,” and showing significant decay, particularly from samples by the front of the case. On the other hand, the visual quality of baby spinach in the case with doors was higher and with lower decay. Data showed a good statistical correlation between visual quality scores and storage temperatures.
To validate the effects of the Food Code compliance in the case with doors, packaged fresh-cut leafy greens were artificially contaminated with three major human pathogens, including E. coli, Salmonella enterica, and Listeria monocytogenes. After displaying the products for three days in the case with doors, with temperatures maintained below 41°C (5oC) and above freezing, no pathogen growth was observed during the entire period.
Operational Energy Costs
We evaluated the operational energy use of the display cases, which included the energy consumption by the condensing units, the evaporator fans, and the LED lights. Anti-sweat heaters were not used for the glass doors, which would have contributed to the energy use. We determined that overall energy consumption can be reduced by as much as 69 percent with the display case with doors under typical door-opening frequencies and durations (every 10 minutes for 6 seconds).
Recently, some retailers took actions toward displaying produce behind glass doors. The latest awardees of the 2014 and 2015 Retail Sustainability Award from Produce Business (Wegmans and Weis Markets) have pilot programs in place to
retrofit open cases with doors for produce, and according to the reports, the initiative has been positive for the stores and accepted by their customers. The opportunity of a truly win-win situation at the intersection of food quality, food safety, and energy efficiency is at hand for the retail industry.
About The USDA Agricultural Research Service (ARS)
The ARS works to ensure that Americans have reliable, adequate supplies of high-quality food and other agricultural products. ARS accomplishes its goals through scientific discoveries that help solve problems in crop and livestock
production and protection, human nutrition, and the interaction of agriculture and the environment.
The research was funded by USDA-National Institute of Food and Agriculture (Specialty Crops Research Initiative Award No. 2010-51181-21230) and USDA-ARS (Project No. 1275-43440-004-00D).
J. Atilio de Frias and Bin Zhou are post-doctoral research associates at USDA-ARS. Yaguang Luo is Research Food Technologist in the Food Quality Laboratory and Environmental Microbial and Food Safety Laboratory at USDA-ARS.
Patricia Millner and Xiangwu Nou are Research Microbiologists in the Environmental Microbial and Food Safety Laboratory at USDA-ARS.