What happens when we coat strawberries with nanoparticles?

An article published in the journal Polymers investigated the use of nanoparticles to extend the shelf life of strawberries. The research was carried out by scientists from Al-Azar University in Egypt and Taif University in Saudi Arabia.

Study: Impact of starch coating incorporated with silver nanoparticles on storage time of strawberries. Image Credit: MarcoFood/Shutterstock.com

Post-harvest problems with fruit

Fresh fruit is an essential part of a healthy diet, providing several essential vitamins and minerals. However, after harvest, the quality of soft fruits such as strawberries in particular can deteriorate very quickly. Rapid softening and decomposition pose storage and consumption problems. Fungal infections of strawberries occur frequently during storage, and their fragile tissues make post-harvest handling difficult.

Transportation and storage can affect soft fruit quality due to physical damage and physiological processes. Loss of nutrients, water, color and textural changes can occur, which makes the fruit unattractive and can lead to lower prices and sales volumes, and therefore economic loss for growers, manufacturers and retailers.

Size, shape and distribution of starch-silver particles examined using TEM. Image credit: Taha, IM et al., Polymers

Strategies to improve the quality of delicate fruits

Several strategies can be used to maintain the quality and nutritional profile of delicate foods, including quick freezing methods. Another method that can be employed is to coat the surface of the fruits with various biodegradable and non-toxic substances to protect them against environmental factors.

A biodegradable polymer, starch is found in plants. This polysaccharide is non-toxic, inexpensive and abundant, making it an ideal material for coating fruits. However, due to its limited flexibility and brittleness, processing starch can be difficult.

In recent research, the addition of nanoparticles to coatings has been explored to improve their properties. Their ability to protect against microbial activity has been established by numerous studies. These nanoparticles can be prepared using physical and chemical methods and for many decades silver nanoparticles have been used as food additives and materials used for food contact.

Uses of food colors containing silver nanoparticles include chocolate and confectionery coatings and liqueur additives. In some countries, however, the use of silver directly or in food contact materials is restricted or prohibited. The EU, for example, has classified it as a suspect additive. The US FDA, on the other hand, has authorized its use.

In recent years, several new applications have been explored using silver nanoparticles. They have been proposed as an alternative to sulfur dioxide because of the health risks of sulfides, or to increase the antimicrobial properties of this substance. Silver nanoparticles have also been studied as functional ingredients to provide anti-caking properties and to clarify liquids. Several studies have investigated how silver nanoparticles can improve food quality and shelf life.

DPPH free radical scavenging activity (%) of St-AgNPs relative to ascorbic acid.

DPPH free radical scavenging activity (%) of St-AgNPs relative to ascorbic acid. Image credit: Taha, IM et al., Polymers

The study

The article published in Polymers used strawberries as a food model to investigate how coating fruits with nanoparticles can help improve their quality and shelf life. Differences in shelf life of untreated and treated strawberries were examined, as well as differences in physical, microbiological and chemical properties. The authors synthesized and characterized several silver nanoparticles in the study.

Nanoparticle coatings were prepared from a solution of starch and silver nitrate. Ethanol was used to precipitate the nanoparticles, this precipitate being washed to remove by-products and unreacted components, then separated, dried and characterized. The characterization methods used in the research were TEM, FTIR and UV-Vis analysis. In addition, the antioxidant activity was characterized by the authors and compared to ascorbic acid.

Analysis of the effect of the washing process on the removal of silver by the inductively coupled plasma optical emission spectrometer (ICP-OES).

Analysis of the effect of the washing process on the removal of silver by the inductively coupled plasma optical emission spectrometer (ICP-OES). Image credit: Taha, IM et al., Polymers

The fruits were selected on the basis of the same size and the absence of damage and fungal infection. They were then coated with the new nanoparticles and glycerol was used as a plasticizer. A dipping method was used to coat the fruit samples. The samples were then stored both at room temperature and under refrigerated conditions. Samples stored at room temperature were tested every other day, with refrigerated samples tested every four days until the fruit deteriorated.

Quality criteria were assessed including weight loss, soluble solids content, visual degradation, anthocyanin content and microbial count. The number of days they remained marketable was used to assess the shelf life of nanoparticle-coated strawberries. Some results of the study analyzes indicated good antioxidant activity, reduced weight loss in coated samples, lower proportion of visual spoilage, reduced microbial activity, and significantly increased shelf life of coated strawberries. The effect of washing was also studied.

The new coating process offers significant advantages for post-harvest handling and storage of delicate fruits such as strawberries, and is a low-cost, simple, fast, non-toxic and environmentally safe strategy. The authors said this technology should be encouraged for use in food processing and packaging.

Further reading

Taha, IM et al. (2022) Impact of starch coating incorporated with silver nanoparticles on storage time of strawberries [online] Polymers 14(7) 1439 | mdpi.com. Available at: https://www.mdpi.com/2073-4360/14/7/1439

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