Drying and fat oxidation may not be the only reasons frozen foods change in flavor. In a 2016 study, karaage, Japanese-style fried chicken, stored in an opened bag in a household freezer for a month took up volatile compounds from its surroundings, and off-odors were actually perceived in some cases. The amounts taken up also varied with how the bag was closed.
The researchers examined sorption: the transfer of compounds from the household environment to food during frozen storage. The term covers substances adhering to the surface or being taken into the interior. They combined surveys of compounds in freezers with comparisons of foods and packaging methods after opening.
Storing opened fried chicken in a household freezer
In the first test, one edge of a commercial frozen-fried-chicken package was cut open, and the food was stored at approximately −18°C for a month in the freezer of an employee volunteer’s household. A special film that takes up surrounding volatile compounds was placed alongside it, and compounds appearing in both the food and the film were analyzed.
After storage, the chicken was heated as directed and compared with an unopened product by six expert panelists. When other foods were also present in the freezer, all six detected off-odors reminiscent of resin or chemicals. When the test chicken was the only food present, none detected a clear off-odor.
The opened product showed no confirmed moisture loss or increase in acid value or peroxide value, measures related to fat oxidation, in this test. The authors therefore considered it difficult to explain the flavor change solely through drying or lipid oxidation and suggested that compounds taken up from the surroundings were involved.
Detecting compounds is not the same as identifying the cause of an off-odor
A similar one-month test in seven other households found that at least one of the six panelists detected an off-odor in samples from every household. Descriptions included resin-like, citrus-like, paint-like, and detergent-like notes; the number of panelists detecting them and the compound profiles varied among households.
The study focused on p-dichlorobenzene, or p-DCB, and styrene. Both were detected in the chicken and collection films from all seven households. The authors used these substances as indicators to investigate whether compounds originating from the living environment or refrigerator materials were taken up by food even during frozen storage.
Importantly, neither compound’s concentration correlated with the number of panelists detecting an off-odor. Detection provided a clue for tracking transfer, but did not directly indicate off-odor intensity. The study did not identify the actual compound or combination of compounds responsible for the off-odors.
In another comparison, collection films were placed inside freezers and in nearby kitchens in five households. In every household, the freezer film took up more p-DCB. This was a comparison of concentrations in films. Because low temperatures and air exchange affect sorption, it cannot be directly reinterpreted as evidence that the air concentration itself was higher inside the freezer.
Different foods took up different amounts
The next test used five foods: fried chicken, hamburger steaks, imagawayaki filled cakes, edamame, and mixed vegetables. The opened ends of the bags were folded and secured with rubber bands, and concentrations of p-DCB and styrene were tracked for up to 91 days.
No marked increase in either compound was found in edamame or mixed vegetables. Concentrations increased in fried chicken, hamburger steaks, and imagawayaki, with particularly large increases in the chicken. A sharp increase occurred around days 28–35 in this test, but that duration cannot be treated as a universal storage limit for all foods.
The authors suggested that affinity for fats and differences in the surface structure of coatings or dough might contribute. However, food composition, surface properties, and original packaging all differed together, so the experiment did not isolate the effect of fat alone.
Closing the full width of the opening reduced sorption
Finally, the researchers compared five ways of storing the fried chicken for a month: unopened; left open; folded and secured with a rubber band; the opened package placed in a resealable polyethylene bag; and the opening folded and secured across its full width with a clip.
Compared with the bag left open, concentrations in the rubber-band condition were approximately two-thirds as high for p-DCB and three-quarters as high for styrene. This reduction was relatively modest. With a full-width clip closing the opening, both concentrations were approximately one-fifth as high.
When the opened package was placed in a resealable polyethylene bag, p-DCB was approximately one-third and styrene approximately one-quarter of the open-bag concentrations. In unopened products, increases from the pre-storage levels remained small. The authors interpreted the findings as reflecting both closure of the opening and the packaging material’s resistance to permeation by the compounds.
This comparison involved a particular fried-chicken product and packaging, including a 0.04 mm-thick polyethylene bag. The numbers represent concentrations of the two compounds under those conditions, not equivalent proportional reductions in the overall off-odor. No tests assessed health effects or determined food safety.
Summary
Opened frozen foods can take up volatile compounds from their surroundings and change in flavor even while frozen. In this study, off-odors were detected in stored fried chicken, and the five-food comparison found greater uptake of the indicator compounds in foods such as chicken than in edamame or mixed vegetables.
Closing the fried-chicken package across the full opening with a clip also resulted in lower sorbed concentrations of p-DCB and styrene than leaving it open. Freezing does not necessarily stop the movement of aroma compounds, and packaging after opening can influence flavor. Identifying the actual compounds responsible for the off-odors remains an outstanding question.
All four authors were listed as affiliated with the Food Safety Research Center, Quality Assurance Division, Nichirei, at publication. No separate research-funding or conflict-of-interest declaration section could be found.
Reference
Kurakazu, T., Suzuki, W., Ito, C., & Shimabara, Y. (2016). Study on sorption of volatile organic compounds (VOCs) to frozen foods in home freezer. Japanese Journal of Food Chemistry and Safety, 23(2), 95–101. doi:10.18891/jjfcs.23.2_95 (Full original paper)
