Vacuum Insulated Food Jars Retain Heat Better than Lunch Kits Pay Heed to Usage & Consumption Time to Avoid Food Safety Risks

14 January 2021
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Many office workers are inclined to bring packed lunch to work during the pandemic, to avoid risks while dining out. As such, thermal food containers have come in handy. The Consumer Council tested 15 models of thermal food containers, including 8 vacuum insulated food jars and 7 vacuum insulated lunch kits. The results revealed that all vacuum insulated food jars passed the heat retention test whereas the heat retention performance of vacuum insulated lunch kits is generally mediocre, with 2 models dropping to under 50°C over the course of 6 hours. The Council reminds the public that the food-safe temperature is above 60°C for hot foods and below 4°C for cold foods, to avoid bacterial growth which will increase food safety risks. Food in the thermal food containers would better be consumed within 6 hours to avoid health risks such as stomach aches, diarrhoea and vomiting. This poses a particularly high risk to children, pregnant women, the elderly, patients receiving treatment or those with a weaker immune system and should be regarded with caution.

A number of vacuum insulated food jars also claim to be able to cook simple ingredients. Although the results revealed that all 8 tested food jars met European standards, with a 6-hour heat retention efficacy of over 62°C, vacuum insulated food jars in fact do not have heating functions, and their thermal retention drops once the hot water comes into contact with the ingredients and metal surface of the inner wall, as well as over time. Thus, the heat energy may not be sufficient to cook through the ingredients or kill bacteria. Consumers are recommended to precook the ingredients thoroughly, especially raw foods such as meat, seafood, fungi and eggs, before putting them into the food jar for further stewing.

15 vacuum insulated food jars and vacuum insulated lunch kits were tested for several parameters, including heat retention, actual weight and capacity, and assessment of the convenience of use and labelling information, etc. The claimed capacities of the models ranged from 550ml to 840ml and had a considerable price difference. The 8 insulated food jars (i.e. without inner compartments, generally able to cook simple ingredients) ranged from $130 to $449 in price, with a $190 model and the most expensive model which cost $449 receiving a tied rating of 4 points. The 7 vacuum insulated lunch kits (with 2 or 3 compartments) cost $169 to $468 in price per set, amongst which a $181 model and another $409 model were simultaneously rated as best performers in this category, receiving 3 points, outperforming the most expensive model ($469) which only received 2.5 points. Results proved that price and quality do not necessarily go hand in hand.

Both vacuum insulated food jars and lunch kits are made with a stainless steel shell and interior with a vacuum insulation layer in between to retain the temperature of the content. The heat retention test was conducted with reference to the European standard EN12546-1:2000, during which the samples were first preheated by filling with 95°C hot water. The samples were then refilled with hot water and the temperature was measured after 6 hours, 8 hours and 12 hours respectively. As for the vacuum insulated lunch kits with 3 compartments, it is generally claimed that the topmost container can be used for room temperature foods. As such, besides the hot water test conducted on the middle and bottom container to test the heat retention, the top container was tested with 4°C cold water, to simulate the temperature of common side dishes, such as kimchi, smoked food or pickled food, which are normally packed into the vacuum insulated lunch kit directly from the fridge.

The 8 vacuum insulated food jars recorded water temperatures ranging from 76.4°C to 63.6°C after 6 hours, meeting the requirements of the European standard (above 62°C) for containers of the relevant capacity (601ml to 800ml). The tests results revealed, however, that the heat retention performance dropped over time, with recorded temperature ranges of 72.3°C to 58.6°C for 8 hours and 65.6°C to 48.6°C for 12 hours respectively.

Of the 7 vacuum insulated lunch kits, 2 out of the 5 dual-compartment models recorded water temperatures ranging from 49.1°C to 40.3°C across both compartments after 6 hours, lower than the European standard (above 50°C) for containers of the relevant capacity (201ml to 400ml); another 2 models’ top compartment temperatures were lower than 50°C; only 1 model had a measured temperature of above 50°C for both compartments. However, all 5 models of the dual-compartment vacuum insulated lunch kits showed a temperature drop at 8 and 12 hours, with recorded temperatures of 54.6°C to 37.0°C (8 hours) and 44.4°C to the lowest of 29.6°C (12 hours) respectively.

As for the 2 models of vacuum insulated lunch kits with 3 compartments, the middle and bottom compartments recorded temperatures over 50°C, yet the top compartment’s temperature rose from 4°C to 32.6°C and 31.2°C respectively, indicating that the temperature of the top compartment increases as a result of heat conduction from the lower containers or the surroundings.

When selecting food containers, the capacity is one of the major factors to consider. According to the European standard, the capacity of food containers is defined as the volume of water it holds up to 10mm below the rim, to ensure there is no overflow after the lid is fastened, and also to prevent the rubber sealing rings from touching the food. The actual capacity of the 15 models were measured and 14 were revealed to be lower than claimed. All 7 vacuum insulated lunch kits’ capacities were 15.6% to 32.1% lower than claimed, and 7 vacuum insulated food jars’ capacities were 2.5% to 10% lower. Only 1 type of vacuum insulated food jar had an actual capacity larger than declared.

However, the larger the container, the heavier and bulkier it may be. The test calculated the samples’ weight to capacity ratio. The 7 vacuum insulated lunch kits were rated only 1.5 to 3.5 points for their lower portability, due to the extra space inefficiently taken up by the materials of inner containers instead of for storing food. All 8 vacuum insulated food jars scored 4 to 5 points.

The review in product labelling showed that 2 vacuum insulated lunch kits had Japanese instructions only, 2 vacuum insulated food jars had either English or Chinese instructions, and 1 food jar’s Chinese and English bilingual instructions lacked details. Consumers might not be able to understand the instructions provided on the labels and result in misuse.

Consumers should bear in mind the following when using vacuum insulated food jars and lunch kits:

  • Vacuum insulated food jars can only keep food warm, but not cooking. Consumers are reminded to precook fresh ingredients before putting them into the food jar to further stew the food for more flavour and softer texture;
  • Food should be kept at above 60°C to prevent bacterial growth, thus it is recommended to consume the food within 6 hours. If the food is not finished in one sitting, the remainder should not be kept for later consumption to prevent food safety risks;
  • Preheat the interior with boiling water before use, then remove the water and place the food or soup inside to retain the temperature for a longer period;
  • The container should only be filled with soup or food up to 1 to 2cm below the lid, to ensure no overflowing of hot liquids after replacing the lid which may cause scalding, and also to prevent the rubber sealing ring from touching the food;
  • After every use, disassemble the container’s lid and parts and clean with warm water, detergent and a soft cloth. Use the cleansing powder specially designed for food jars if there are stubborn stains. Never use cleaning detergents with bleach or chlorine content to prevent chloride damage to the stainless steel surface.

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