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Effects of bacterial microflora on the quality of frozen plant products

Freezing is one of the best known methods of storing vegetables and fruit. It allows to preserve their nutritional values, such as minerals and vitamins, while preventing spoilage of plants that occurs as a result of attack of mould, fungi, bacteria and viruses.

Published: 2-08-2019
bacteria in frozen food

It turns out, however, that not all types of microorganisms die at low temperatures, going into hibernation instead. The presence of bacterial microflora has a significant impact on the quality of frozen plant products, both in technological terms and for food safety reasons. Therefore, in processing operations in the food industry, it is often necessary to perform procedures aimed at reducing the amount of microorganisms found on the surface of plants.

Microflora in plants and its activity

Plants are inhabited by a large number of different bacteria that can affect the plants in a positive or negative way depending on the type of strain. In the operations of fruit and vegetable processing, the stages occurring after the harvest are particularly important as the existence of microorganisms leads to the transformation of vegetable and fruit ingredients. This in turn causes browning, slackening, softening, shape change and fermentation.

A characteristic feature of plant products is that their interior, in terms of microbiology, is almost sterile or only slightly populated with microorganisms. The infection usually starts from the surface and usually reaches the highest concentration on the surface.

Another important fact is the specificity of indigenous microflora, which ensures the proper development of plants during their growth (e.g. Lactobacillus covering green parts of plants). It also serves another function, namely, by dominating, it protects the surface from being settled by other types of bacteria. On fruit and vegetables it is almost in a spore-like state. This means that it is not vegetatively active and its activation takes place only during contact with the juice of damaged tissues.

Soil bacteria vs. plant microflora

Soil bacteria are found in the close vicinity of a plant, and the proof of such symbiosis is the composition of the rhizosphere, that is, the root zone. It is located within a radius of several to a dozen or so millimetres from the root, and the density of microorganisms in this soil layer can be even nearly hundred times higher than outside the root zone. This is due to the presence of root secretions that nourish the microflora naturally. The metabolism of this microflora leads to greater bioavailability of nutrients for the plant and protects the root from the harmful effects of parasites.

On the other hand, due to the intensification of agricultural production, fertilization and irrigation with contaminated water, the soil microflora profile is often so disturbed that in most cases it can be classified as a foreign microflora.

Nevertheless, one can notice a certain systematics of infecting plant products with soil microflora: the further away from the soil the harvested part of a plant is located, the lower is the infection with the soil microflora. And thus, root vegetables are the most contaminated with microflora (from 105 up to 108 cfu / g), while fruit growing high above the ground show the presence of microflora within the limits from 106 up to 109 cfu/g of bacteria and from 103 up to 104 cfu/g of fungi.

carrots - dirty vegetables

To sum up, the total number of microorganisms on plants depends on various factors:

  • the climate in which the plant is grown,
  • the location of the harvested part of the plant, i.e. height above the soil or depth in the ground,
  • the quality of water used for crop irrigation,
  • the surface structure of the edible part of the plant,
  • the fertilization method,
  • the air quality,
  • the presence of rodents and insects,
  • the errors resulting from non-compliance with “Good Agricultural Practice” (e.g. lack of composting processes or sanitization of manure used for fertilization).

Microflora reduction – does freezing suffice?

The most common plant pathogens includes:

  • bacteria: E. coli., S. aureus, Salmonella, Listeria, Shigiella, Campylobacter jejuni, Campylobacter coli, Yersinia enterocolitica,
  • enteropathogenic viruses, causing gastroenteritis: Rotavirus, Adenovirus, Norovirus, Astrovirus, Sapovirus, Coronavirus and Archivirus.

In the processing operations of plant products, one of the methods to increase their durability and extend the storage time is cooling and freezing. This method also allows to preserve the high nutritional values of vegetables and fruit. The faster the plants are frozen after the harvest, the higher the values are.

It should be borne in mind that the freezing process itself effectively inhibits microflora development, but only slightly reduces the amount of microorganisms on the surface of a product. Moreover, there is a lot of data confirming that the majority of microorganisms transform into a spore form during freezing, and when favourable conditions arise, the microorganism return to the vegetative form. In addition, rotaviruses after being frozen have even higher survival rate than under standard conditions, and pH in the range of 3-10 is a convenient hibernation environment for them.

Microbiological reduction of microorganisms with the use of disinfectants

The health safety of products of plant origin, which are subject to freezing in processing operations, is ensured, among others, through the implementation of microbiological reduction system. This way, the microorganisms living on the outer parts of the plants , which can be harmful when ingested or which can cause spoilage of the product, are removed. The technological process of reducing microorganisms uses, e.g. disinfectants, such as HYSEPTA M1 FGTM.

The entire process consists of the following steps:

  • collection and delivery of plants and/or vegetables,
  • sorting,
  • washing, cleaning and peeling,
  • cutting, fragmentation,
  • blanching,
  • fragmentation,
  • freezing,
  • dispensing and packaging.

The use of HYSEPTA M1 FGTM is recommended for the third and fourth step of plant products processing, thus during cleaning and fragmenting. Disinfectants can also be introduced at the stage of freezing in the form of disinfecting glaze, which protects ready-made products against the development of microorganisms.

harvest vegetables

For a good start: rinsing

The first step of vegetables and fruit processing in the food industry is rinsing. Microbial contamination may already occur at this stage due to the use of rinsing water in closed circuits. A new portion of microorganisms coming from cleaned plants goes into the washer all the time, and the microorganisms have favourable conditions for development. For this reason, disinfectants are dispensed into the washing liquid continuously, which allows to inhibit the development of microflora (at a concentration of 0.1%) and reduce the level of microbial contamination to a lower level than the contamination of the processed plant product (0.2%).

Knife disinfection = safe fragmentation

One of the most vulnerable stages in the processing of plant products is the fragmentation stage. The data show that during the cutting and slicing processes, the level of microorganisms in the plant tissue may be even seven times higher. For this reason, it should be ensured that the knives are disinfected as often as possible during the production process. For this purpose Hysepta M1 FG TM is used at a concentration of 0.3%, by spraying or immersion, depending on the fragmentation method and the design of the equipment and tools, and workers’ gloves are disinfected as well.

Disinfecting glaze – how does it work?

Freezing vegetables and fruit can be carried out in various manners; however, one of the most frequently used methods is freezing in tunnels. The complex design of freezing tunnels makes it difficult to properly maintain their hygiene, therefore, cross-contamination of the processed product often occurs during the freezing process.

Nevertheless, it is possible to effectively prevent microorganism contamination by spraying the surface of a frozen product with HYSEPTA M1 FGTM at a concentration of 0.2%. It causes the formation of glaze with disinfectant properties on the surface of fruit and vegetables. According to the technical documentation of the preparation, the activity of the applied biocide decreases with time and temperature.

The disinfecting properties of the glaze produced result from the interaction of three factors:

  • modification of the chemical composition of the surface of the product – Aspergillus Niger has been proven to die at a temperature of -2o C, at the availability of glucose, while the same strain in the presence of 40% glucose solution, dies only at -20o This is why the disturbance of the chemical composition of the product’s surface reduces the resistance of microorganisms to freezing,
  • increase of oxygen molecular pressure up to 0.35 bar, thus outside the optimum for the microflora being 0.12 bar,
  • perlactic acid attack on cell structures: cell wall, cellular proteins, functional enzymes and nucleic acids.

Fogging as a way to keep the air clean

The regulations governing the GHP system for the fruit and vegetable industry require maintaining air purity in the packaging zone within 200 cfu/10l of air, including mesophilic oxygen microflora. At the same time, the condition regarding the level of fungi must be met. Their level should not exceed 15% of the total amount of all microorganisms.

To ensure adequate air purity in the packaging halls, air fogging that effectively purifies the air from harmful microorganisms is used. For this purpose, it is recommended to perform fogging using 1% solution of the Hysepta M1 FGTM every day after production completion.

Disinfection of sprouts

Plant sprouts are products particularly susceptible to infection with microorganisms. Infection of sprouts is mainly due to the presence of microflora on seeds subjected to germination. For this reason, it is necessary to disinfect the grains themselves, in order to obtain satisfactory microbiological purity after germinating. Thus, it is recommended to use Hysepta M1 FGTM at a concentration of 0.2% in the initial phase of grain hydration. Such a process additionally causes microbiological stabilization of the water used for germination. After the germination process is completed, plants should be rinsed with a 0.15% solution of disinfectant to finish the process.

vegetables sprouts

What should you remember during the procedures to remove bacterial microflora?

Effective microbial removal requires the use of disinfectants containing ingredients that can affect processed plant products. Therefore, one should strictly observe the recommended doses of preparations, which will allow to avoid the negative consequences of overdosing. For example, HYSEPTA M1 FGTM is an oxidant and using excessive amounts of it can lead to the degradation of dyes and cellular structures in vegetables and fruit. Moreover, you should be aware that the use of preparations reducing bacterial microflora cannot be treated as a substitute for Good Practices at the production plant. It can however, be an effective supplement to those practices and to the systems ensuring food safety.

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