Smart packaging technologies are attracting increasing interest because they monitor changes in food or the inside the packaging and thus meet the need for an effective management of the supply chain. This is very important because real-time monitoring of changes in packaged foods is crucial to ensure their safety.

In particular, the development of indicators for gases resulting from food deterioration and microbial growth is of interest. A review written by W. Heo et al. of the Department of Food Science & Biotechnology at the Gachon University, South Korea (Foods 13, 2024, 3047) summarizes the working principles and characteristics of gas indicators, both commercially available and under development. Indicators are applied in forms such as films, labels, sachets and devices; they show gas presence through colour changes while sensors provide quantitative data by converting this information into electrical signals.

The gases detected are usually oxygen, carbon dioxide and ammonia, which are the main indicators for food quality. The study classifies indicators according to detection method and sensitivity; this classification highlights the unique characteristics of each technology and its potential in ensuring food quality and safety. Indeed, not all indicators can be applied to every type of food due to compatibility problems with the food being monitored. Therefore determining the appropriate indicator for each specific product is crucial.

Oxygen indicators

These indicators are particularly useful in case of modified atmosphere packaging (MAP), which is commonly used for ready-to-eat foods requiring an extended shelf life. MAP packaging is designed to block air; however, when oxygen from the outside enters into the packaging, micro-organisms, enzymes and other organisms within the package or food proliferate accelerating spoilage and triggering enzymatic browning reactions that are particularly harmful to fruits and vegetables. Additionally, oxygen contributes to the oxidation of vitamin C and lipids.

Therefore, oxygen is a critical factor in food deterioration, and indicators that can detect it are extremely useful. Most commercial oxygen indicators are based on dyes that detect the presence of oxygen by a colour change. An example of these indicators is methylene blue in an alkaline solution. In the presence of oxygen, methylene blue retains its blue colour, while in the absence of oxygen the indicator remains colourless. Unfortunately, however, conventional indicators are synthetic products that pose safety concerns; for this reason, new indicators using natural compounds are being developed, such as haemoglobin and myoglobin. These complexes change colour upon oxygen binding.

Carbon dioxide indicators

Metabolic processes that occur in food, caused by the presence of enzymes or micro-organisms, may generate carbon dioxide. Food deterioration due to microbial metabolic activity can be both desirable and undesirable: While fermentation can be beneficial in some cases, as in the production of certain fermented foods, it is not desirable during food storage because it affects their freshness and quality. For this reason, carbon dioxide is regarded as an important indicator of food spoilage.

Its presence can be detected using an indicator based on Whey Protein Isolate (WPI), which reveals the presence of carbon dioxide through changes in transparency. The principle of this indicator is based on the reaction of carbon dioxide with water, i.e. the water-soluble solvent of the indicator, which, being acid, leads to a pH decrease. Under neutral conditions, WPI is transparent; however, its solubility decreases with the pH and near pH 5.5 it becomes opaque. Hence, in the presence of carbon dioxide and therefore of carbon acid, WPI becomes opaque.

Even if the pH increases again, the opacity does not change because the transparency change is irreversible. This feature provides the advantage of permanently indicating the presence of carbon dioxide. The same principle applies to a chitosan-based colorimetric indicator, the solubility of which varies with pH. The indicator is prepared by encapsulating an edible blue pigment with chitosan, which is insoluble under neutral (pH 7) or basic conditions, making the indicator opaque. When carbon dioxide is produced, the pH decreases, causing the chitosan to dissolve in water, releasing the blue pigment encapsulated within. Using only chitosan to indicate the presence of carbon dioxide can be problematic, because if the pH increases again, the chitosan reaggregates, making it difficult to accurately determine whether carbon dioxide was generated. 

Ammonia indicators

When high-protein foods like meat and fish deteriorate, volatile amines are destroyed by enzyme and microbial activity. Volatile amines such as ammonia, dimethylamine, trimethylamine and biogenic amines are produced, which are referred to as total volatile basic nitrogen (TVBN). All of these compounds contribute to the formation of off-flavours and serve as indicators of food spoilage. Given that ammonia is a key component of TVBN and a significant marker of spoilage, extensive research is being conducted to develop ammonia indicators for improved food quality control.

As an indicator of ammonia, a low-density polyethylene (LDPE) film has been developed by embedding the natural pigment curcumin into it. This indicator detects the presence of ammonia gas through a colour change in the curcumin dye. The detection mechanism relies on the colour change of the dye in response to pH changes caused by ammonia gas. In fact, as the meat spoils, ammonia gas is released – which is basic –  causing a pH increase.

This change in pH results in a color change in the curcumin dye within the film, indicating meat spoilage. The advantage of this method is that it is non-toxic and cost-effective. In conclusion, gas indicators can provide easily interpretable information about harmful substances and food quality without opening the packaging. This capability is valuable for food safety management. However, applying these technologies to commercial food packaging presents several challenges: Current indicators need higher accuracy and precision, and improvements in material stability and safety are also required.

References W. Heo et al., of the Department of Food Science & Biotechnology of Gachon University in South Korea (Foods 13, 2024, 3047).