Preservative is an important food additive that can prevent food from rotting and deteriorating, ensure food quality and extend the shelf life of food. With the improvement of living standards, consumers are more and more in pursuit of nutritious, additive-free, green, safe and long shelf-life food, due to microbial activity and oxidation and other roles, food is easy to corruption and deterioration; and if accidentally eaten expired or deteriorated food may also cause food poisoning, so the storage of food needs an effective preservative method. In addition to traditional preservation methods such as refrigeration, freezing, salting and drying, preservative additives can also extend the shelf life of food.

Natural preservatives are more widely used in food than chemically synthesized preservatives because they are derived from natural resources and have safe, green and efficient preservative properties, thus the importance of natural preservatives is increasing. Research on the application of natural preservatives in food has been carried out and new natural preservatives are being explored.

This paper focuses on three types of natural preservatives: plant preservatives, animal preservatives and microbial preservatives as well as their respective functions and applications in food.

Research status of natural food preservatives

Natural preservatives are a class of functional substances with antimicrobial and antiseptic effects isolated and extracted from plants, animals and microorganisms or their metabolites, which are safer than chemical preservatives.

In the food industry, preservatives are often added to reduce losses, improve quality and extend shelf life. Currently commonly used preservatives are mainly natural preservatives and chemical preservatives, chemical preservatives mainly include sorbates, nitrites and benzoates, etc. Although these preservatives have been in use for many years, but people’s concern about the safety of these preservatives has never been reduced, and most of the chemical preservatives are still some potential hazards to human health.

Mamur et al. found that sodium sorbate was genotoxic to human peripheral blood lymphocytes; the researchers also found that when these preservatives are used together, they can have adverse effects on human health.Binstok et al. reported that the combined use of sorbate and nitrite in meat products should be minimized due to the risk of formation of mutagenic compounds.

The use of chemical preservatives in food preservation may cause certain hazards to human health and the environment, whereas natural preservatives have the advantage of being low in toxins, odorless, safe and highly effective, which are not only harmless to humans, but also have some nutritional value; therefore, the use of natural preservatives is safer for humans and the environment. Therefore, natural food preservatives have received extensive attention in the food industry and require further research and comprehensive development.

Sources and applications of natural preservatives

Natural food preservatives have the advantages of strong antimicrobial properties, high safety and good thermal stability, etc. According to their sources, they can be categorized into plant preservatives, animal preservatives and microbial preservatives. Due to the different sources of various natural preservatives, their compositions and structural characteristics are different, and their applications in food are also different. The following will introduce these three kinds of natural food preservatives respectively.

1. Plant preservatives

Plant preservatives are mostly derived from plant extracts, such as certain natural herbs and spices and essential oils. Because of their unique flavor and antibacterial and antiseptic effects, plant extracts and essential oils have long been used as food preservatives, flavorings and medical preparations.

1) Plant extracts
Plant extracts are usually obtained from the roots, stems, leaves, flowers, fruits, barks and other parts of plants. Some extracts have strong flavors, which can cause sensory discomfort at high concentrations, so the use of their concentrations should be limited. Qualitative and quantitative changes in the content of biologically active phytochemicals in plant extracts lead to changes in their effectiveness, and these phytochemicals also affect the microbial content, texture and flavor of food. Due to their specific phytochemical composition, plant extracts have some antimicrobial activity, and those obtained from herbs, spices, vegetables, and fruits have antimicrobial effects against bacteria, yeasts, and molds. Studies have shown that plant extracts are very effective in controlling the growth of foodborne pathogenic bacteria.

Plant extracts are rich in natural antioxidants, which play an important role in food. Antioxidants are additives that can delay or prevent rancidity of food due to oxidation and can extend the shelf life of food. Most of the plant-based antioxidants exhibit protective effects similar to those of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), and a wide variety of vegetables and fruits can be considered as important sources of natural antioxidants used to fortify and stabilize foods. Researchers have studied the antioxidant effects of plant extracts in foods and found that tomato and garlic extracts delayed lipid oxidation during frozen fish and marinated anchovies.

2) Essential Oils

Essential oils (EOs) are aromatic and volatile liquids obtained from natural herbs and spices that contain high levels of secondary metabolites that have the ability to retard or inhibit the growth of bacteria, yeasts and molds. Essential oils have long been used for centuries in the food, medical, perfume and cosmetics industries as part of spices or herbs.

In the food industry, essential oils are mainly used as flavoring agents. The chemical composition of plant essential oils is not only related to the environment in which the plant grows, but also to many other factors such as the species of the plant, the season in which the plant material is collected, the part of the plant where it is collected, and even the method of extraction of the essential oil. The strongest antimicrobial activity in herbal essential oils occurs during or after flowering. Some of the antimicrobial compounds in essential oils are naturally occurring in the plant, while others are produced by the plant when it is exposed to microbial attack or when it itself is harmed. Since most essential oils have a mature extraction process, strong antimicrobial effect, non-toxic and harmless, it stands out as a natural antimicrobial agent in food preservation.

2.Animal preservatives

Animal preservatives are biological active substances with antiseptic effects artificially extracted from certain animals or their metabolites, which are mainly categorized according to their functions: antimicrobial enzymes, antimicrobial peptides and antioxidant peptides.

1) Antimicrobial enzymes
Enzymes can exert antibacterial activity through many different mechanisms. For example, lysozyme and other antimicrobial enzymes mainly catalyze the cleavage of cell surface polymers or cell walls, inducing bacterial decomposition and thus have antimicrobial activity; Lactoperoxidase (LPO) and other peroxidase systems catalyze oxidation to play an antimicrobial role, and LPO has an inhibitory effect on the growth of both Gram-positive bacteria and negative bacteria.

Lysozyme is widely used in the preservation of meat, meat products, fish and their products, milk and dairy products, as well as fruits and vegetables to prevent food spoilage and fruit senescence. Lysozyme shows antimicrobial activity against a certain range of bacteria and fungi, especially against Gram-positive bacteria, and the combined use of lysozyme and other preservatives can enhance its antimicrobial activity, and it was found that lysozyme and streptococcal lactic acid have a synergistic effect on Gram-positive bacteria.

Lactoperoxidase is an important natural antimicrobial agent with bactericidal or bacteriostatic properties in food packaging.Mohamed et al[19] added lactoperoxidase system (LPOS) directly to chitosan membranes at different concentrations and found that the number of spoilage bacilli, fluorescent bacilli, and cryophilic and neutrophilic bacteria was significantly reduced in the LPOS group during the storage period as compared to the control group.

2) Antimicrobial peptides and antioxidant peptides
Antimicrobial peptides (AMPS) are oligopeptides consisting of 20-60 amino acid residues with broad-spectrum antimicrobial activity against bacteria, fungi, protozoa, and some viruses, and can be isolated from other organisms such as plants, insects, crustaceans, and marine organisms.

Animal-derived antimicrobial peptides are mainly derived from mammals, amphibians, fish, etc. Some potential animal-derived antimicrobial drugs are lactoferrin, fish protein and chitosan. Lactoferrin has iron binding ability and polycationic nature, and thus has antibacterial activity, which is mainly found in milk and other animal secretions; Ichthyoglobin is a cationic antimicrobial peptide, which is mainly composed of polyarginine in the molecule as well as a few other amino acids in a certain structure or form to bind with the bacterial cell wall, inhibiting the formation of the cell wall, and thus inhibiting the cell’s metabolism and causing the cell to die.

Ichthyosperm is mostly found in the sperm cells of fish, birds and mammals, and is now mainly used in foods such as milk and bread, which has antibacterial activity against bacteria and molds, especially stronger for Gram-positive bacteria; chitosan is mostly found in the exoskeletons of crustaceans and arthropods, and has antimicrobial activity against bacteria, fungi and viruses.

The antioxidant activity of peptides mainly depends on the binding properties of metal ions and free radicals, and antioxidant peptides mainly include proteins and their hydrolysates, peptides and amino acids. The antioxidant activity of proteins is caused by the amino acids in their structure and the active peptides obtained by enzymatic hydrolysis.Centenaro et al. added the enzymatic hydrolysis products of fish and chicken to minced beef, and found that the enzymatic hydrolysis products had antioxidant effects and were able to prevent lipid oxidation of beef, and the inhibition rates of lipid oxidation were 93% and 80%, respectively.The antioxidant activity of proteins and their metabolites mainly depends on the binding properties of metal ions and free radicals.

3. Preservatives of microorganisms and their metabolites

Many compounds produced by bacteria can prevent the growth of spoilage bacteria or pathogenic microorganisms in food. Food-grade microorganisms can form a large number of substances to inhibit the growth of other microorganisms, thus achieving a natural balance in the microbial ecosystem.

Lactic acid bacteria (LAB) among microorganisms have the ability to inhibit the growth of microorganisms and are considered to be better candidates. In the production of fermented foods, lactic acid bacteria are used to improve the taste, flavor and texture of foods as well as to prevent the growth of pathogenic microorganisms, and LAB are harmless when consumed.The antimicrobial constituents of LAB are mainly bacteriocins, organic acids, hydrogen peroxide, carbon dioxide, and diacetyl; among them, bacteriocins, which are natural preservatives or biopreservatives synthesized by LAB and other strains, are effective against Staphylococcus aureus, Listeria monocytogenes, Listeria monocytogenes, and Clostridium botulinum. Listeria monocytogenes, Clostridium botulinum and other foodborne pathogens with good bactericidal or bacteriostatic effects, and also has good antimicrobial activity against Gram-positive bacteria, and bacteriocins have stronger antimicrobial activity at lower pH values.

In addition, bacteriocins can be used in combination with other antimicrobial compounds to enhance the inactivation of bacteria. Bacteriocins can be used in bioactive packaging and in recent years, researchers have worked on adding bacteriocins to food packaging films to control the growth of food pathogens.Antimicrobial packaging films are used to prevent the growth of microorganisms on the food surface by bringing the packaging material into contact with the food surface.

Conclusion

Food is an important source of nutrients and energy for human intake from the outside world, and its safety and hygiene are particularly important, and preservatives can achieve the effect of preventing food spoilage and extending the shelf life of food, thus food preservatives have an extremely important role in the preservation and storage of food. Compared with chemically synthesized preservatives with side effects, carcinogenic and other shortcomings, natural preservatives are not only non-toxic and harmless, can prolong the shelf life of food, but also conducive to human health; there is no doubt that, in addition to the existing natural herbs, animals or microbial additives components, there are more new natural food preservatives yet to be discovered.

Therefore, the research and development of safe, reliable, inexpensive and efficient natural preservatives is a major research direction for the development of food science in China, and an important measure to enhance the efficiency and safety of the food industry.