Researching materials to keep fruits and vegetables fresh

Situated in the tropical monsoon climate region, Vietnam has one of the most diverse ranges of fruits and vegetables in the world. These fruits and vegetables not only satisfy a domestic demand; they have also become some of the worlds favourite export products. Our country’s annual income from exporting fruits and vegetables runs into hundreds of million USD. However, the ways in which we export our goods still have many shortcomings. The main shortfall is the high volume of rotten produce, caused by too many fruits and vegetables being harvested. Rotten produce can account for as much as 20 percent of total production, obviously presenting a remarkable loss for farmers. Improving the preservation of fruits and vegetables after harvesting would bring many benefits; such as improving the quality of fruits and vegetables, protecting people’s health, meeting the demands of importers as well as consumers and reducing economic damages for farmers. Therefore, maintaining fruits and vegetables freshness after harvesting has become an urgent demand in Vietnam.

For many years scientists in Vietnam have made efforts to carry out research into  advancing food preservation technology. This has been carried out in order to discover ways to effectively preserve fruits and vegetables in a way in which is suitable for climate conditions in Vietnam. Now, research in Vietnam and in the world has produced many methods to preserve fruits. There are two main methods:

1. Preserving fruits by edible cover. Edible cover is a thin layer of material which is applied to the surface of products or used to replace the natural protective membrane of the fruit or vegetable. This provides a barrier from moisture, oxygen and melting substance movement in food. These covers are directly applied onto the surface of fruits by dipping, spraying or coating to create a modified atmosphere (MA). The semi-permeable membrane on the surface of the fruit will reduce the respiration process and control humidity loss as well as provide other functions.

2.  Preserving by modified atmosphere packaging (MAP).  Modified atmosphere packaging is a process which houses food products within an atmosphere designed to protect the product from factors which may cause damage. This helps maintain the high quality of easily rotten food in its natural life or to prolong the expirary date. There are two kinds of packages - vacuum packaging and air exchanging packaging.

Preservation by modified atmosphere packaging is an effective method which shows the potential for real application. Therefore, scientists from the Institute of Chemistry focused their study on creating materials to preserve fruits according to this method.

To preserve fresh fruits by using modified atmosphere packaging effectively, scientists need to consider optimal gas concentration, respiration speed of fruits, gas diffusion through membrane and maintenance temperature.  In addition, in order to have a suitable membrane, scientists must also take care of the preservation ability, durability, healing possibility, purity, processability, label printing possibility and gradient gas created by hermetic membrane.


Measuring gas composition in packaging


When fruits are packaged, the level of oxygen in the package often reduces while carbon dioxide increases. High concentration of carbon dioxide can be harmful for most fruits and vegetables. Therefore, one of the important aspects of the packaging is to let carbon dioxide release faster than oxygen absorption. Ideal packaging usually has the following features: the possibility to change gas absorption nature when temperature increases; controlling moisture absorption speed to prevent the accumulation of over saturated and condensed steam; heat resistance and good ozone; commercial suitability, processability and applicability; effective label printing processes; no reaction with products and no harmful effects to people’s health.

Based on these principles, the Institute of Chemistry under the Vietnam Academy of Science and Technology successfully conducted several materials to preserve fruits such as lychee, longan and plum for a long period of time while still maintaining the fruits’ quality. Remarkably, MAP is altered from low-density polyethylene (LDPE) in combination with several inorganic additives such as silica, zeolit and bentonit. This is one of the results of the project entitled “A research on creating materials to preserve fruits (lychee, longan and plum)” in the state-level science and technology programme KC02.20/06-10, by Dr. Dinh Gia Thanh.


The system of equipment to mix two joint screw of splitting arrangement (the left photo); membrane blowing machine (the right photo)

MAP membrane with different additives


In the research, scientists successfully built a technology process to create MAP membrane with the productivity of 20 kilograms per hour and manufacture 500 kilograms pilot and sample products. Research on preserving three kinds of fruits (lychee, logan and plum) by MAP at 2 to 4 degrees Celsius show that the plum preserved by Z5 membrane sample (zeolit additive contained 5%) gave the best result, enabling preservation for eight weeks without remarkable changes in quality criteria and perceptibility of fruit. Lychee preserved by Z5 and S5 sample (silica contained 5%) at 2 to 4 degrees Celsius brought the highest effect, managing to  preserve the fruit for four weeks with a rotten rate of less than 15%. Longan preserved by Z7 membrane (zeolit additive contained 7%) and S5 showed the best result, managing to preserve the fruit for four weeks with no rotten fruits.

In comparison with MAP CE44 membrane produced in Republic of Korea, the membranes created by scientists from the Institute of Chemistry such as Z5, Z7 and S5 bring preservation effect equivalent from 95 to 100%. Compared with other traditional preservation methods, MAP membrane can preserve fruits for longer periods of time while still maintaining the fruits quality (in quantity, colour and flavour). Preserving fruits by MAP membrane at cool temperatures shows even better results. The membrane also has the advantage of causing no harmful effects to people’s health.

Successfully researching the above mentioned preservation material, scientists submitted an application for patent for MAP membrane production technology. In the future, scientists from the Institute of Chemistry will conduct further research and apply this technology for other fruits and vegetables in order to promote technology transfer nationwide, contributing to increased economic effectiveness for farmers as well as the agriculture sector in Vietnam.




Translated by Tuyet Nhung
Link to Vietnamese version

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