Application of Essential Oils in the Dairy Industry: A Review

Essential Oils (EOs) are aromatic volatile liquids obtained from plant materials (from both vascular or nonvascular plants) including buds, flowers, fruits, seeds, leaves, twigs, bark, wood, roots and herbs [1].Essential oils have wide variety of applications. Many essential oils are used as scenting and flavouring agents, some are more or less powerful external or internal antiseptics, others possess an analgesic, heamolytic, antizymatic action etc. In food industry, essential oils are primarily used as natural preservatives as it prevents microbial spoilage. It is also used as flavouring agent in many food products. Antioxidant properties of essential oils are exploited to prevent the oxidative rancidity of fat rich products. For example, citrus essential oils can be used as a preservative, flavouring agent and also as an antioxidant [2]. 

During recent years, consumers are more aware of what they eat and seeks foods that are minimally processed and contains no chemicals. So manufacturers are also behind the natural additives that will give longer shelf life to the products. Essential oils are a good natural food additive to handle this situation. In addition to this essential oils can also gained interest against antimicrobial-resistant pathogens [3].

The essential oil is the product obtained from a vegetable raw material, either by steam distillation or by mechanical processes from the epicarp of Citrus, or ‘dry’ distillation. 

Inorganic solvents like water do not dissolve EOs; however, organic solvents do (ether, alcohol, fixed oils). With the exception of vetiver, sassafras, and cinnamon, they are volatile liquids with a distinct smell and a density below unity. 

A plant can govern and regulate its surroundings (play an ecological function) by using essential oils, which can operate as chemical messages to deter predators, draw pollinating insects, prevent seed germination, and communicate with other plants. 

The presence of essential oils can be found in various parts of aromatic plants, including their flowers (pink, orange, lavender, flower buds in the case of clove and bracts in the case of ylang-ylang), leaves (mint, eucalyptus, bay leaf, thyme, sage, savoury, pine needles), rhizomes (sweet flag and ginger), roots (vetiver), seeds (coriander and carvi), fruits (anise, fennel, and citrus epicarps), and wood and bark (in sandalwood, cinnamon, and rosewood) [4].

Antibacterial Activity: The majority of the time, phenolic chemicals like eugenol, thymol, and carvacrol found in essential oils are what give them their antibacterial properties [5]. These substances have the ability to disrupt the cytoplasmic membrane, the electron flow, the driving force of the proton, and the coagulation of cell contents [6].The primary characteristic of EOs is their hydrophobicity, which enables them to partition into the lipids of bacterial cell membranes, disrupting the bacterial structure and making it more permeable As a result, several biological molecules and various ions are released from the bacterial cell [7]. 

Antioxidant Activity: Essential oils contain phenolic chemicals and other secondary metabolites with conjugated double bonds that often exhibit considerable antioxidant effects [8]. The most significant antioxidant qualities are found in the essential oils extracted from nutmeg, thyme, cinnamon, mint, basil, clove, oregano, and parsley. The phenolic structure of these chemicals influences their activity. The redox characteristics of phenolic compounds make them important for both the oxidation of peroxides and the neutralisation of free radicals [9-11]. Other substances found in essential oils such alcohols, ketones, aldehydes, ethers, and monoterpenes also contribute to the antioxidant activity of Eos [12]. 

Cancer Chemoprotective Activity: Cancer therapy potential is demonstrated by essential oils. According to Edris [13], natural anticancer substances found in essential oils are essential for both cancer prevention and treatment.

Dairy products are more susceptible to contamination major sources of contamination are from the use of raw milk, the dairy environment, and in some cases also vegetable coagulants [14]. Additives like preservatives and antioxidants enhance the overall quality and safety and prolong the shelf-life of products. Even though artificial additives have number of advantages it is responsible for conditions such as allergic reactions, asthma, nausea, diarrhea or in some cases even carcinogenesis in consumers [15].

Cheese is one of the dairy products with the highest commercial value and the greatest global consumption. The dairy industry has long placed a high priority on extending the shelf lives of cheese products because the current short shelf lives of many types of cheese severely restrict their commercialization, particularly in the case of fresh goods. Natural additives, such essential oils and plant extracts, are frequently used in place of artificial ones. Due to these natural compounds' exceptional antibacterial and antioxidant activity, their application as substitute food additives has grown over the past year. 

E. coli O157:H7 and L. monocytogenes are two known harmful bacteria that can spread through cheese. When compared to controls, the cheeses containing bunium persicum essential oil considerably reduced the growth of L. monocytogenes and E. coli O157:H7 during storage. The results of the sensory evaluation revealed that the BEO-added samples significantly outperformed the control samples in terms of colour, odour, flavour, texture, and general acceptability [16]. 

Individual extracts of cinnamon, garlic, lemongrass, cress, rosemary, sage, and oregano each prevented L. monocytogenes from proliferating in processed cheeses [17]. 

The antibacterial and antioxidant potencies of EO/extract ingredients may potentially be impacted by interactions with dietary components. Dairy products' phenolic chemicals in particular appear to interact with proteins, reducing their antibacterial effectiveness against food borne infections. Additionally, because they absorb the hydrophobic components of EOs, lipids can impact how evenly EOs is distributed throughout the food matrix. Contrarily, it appears that the activity of EOs and extracts in the food matrix is unaffected by the presence of carbohydrates [18]. 

Fungi development is a regular issue for the consumer, the retailer, and the cheese maker when the cheese is ripening or being stored. These fungus not only cause financial losses but also cheese deterioration and the majority of them create mycotoxins [19]. Additionally, certain plant-based substances have demonstrated promising outcomes in preventing the growth of pathogenic fungus [20]. 

 The findings demonstrated the superiority of clove oil emulsion over thyme oil and peppermint oil emulsions in inhibiting the growth of all fungi, with clove oil emulsion inhibiting all fungi growth at a concentration of 0.5% and thyme oil emulsion inhibiting all fungi growth at a concentration of 1%, respectively [21]. 

The concentration of Essential Oils (EOs) added to cheese plays a key role in improving its sensory characteristics [22,23].

 Table 1: Antimicrobial actitivity of aromatic plants and essentiaal oils added to cheeses 

Source: Gouvea et al., [20]

Application of Eos to Fluid Milk: One of the greatest natural alternatives to additives or pasteurisation to stop food spoiling from bacteria in a variety of food matrices is Essential Oils (EOs) [24,25] claim that adding essential oils from the Thymus capitatus plant can prevent bacterial development in pasteurised milk, extending the shelf life of the product. This led to the conclusion that Thymus capitatus EO improved the effectiveness of pasteurisation in maintaining the quality of raw milk. Encapsulated EO's may be a fantastic choice for milk industries as a natural and efficient antibacterial and antioxidant agent in order to monitor and maintain the quality criteria. [26] looked at how EO, which is made up of terpenoid compounds, aldehydes, ketones, alkanes, esters, alcohols, and benzene compounds, affected the sensory characteristics of milk (2008). The sensory characteristics of milk were altered by the addition of this EO at a concentration of 1.0 l/L. The milk had a greater taste of mint, was sweeter, and had a higher concentration of EO (1.0 l/L). There is no question that the EO addition was responsible for the odd flavours. Consequently, the 0.1 to 1.0 l/L range represented the threshold concentration for the perception of the flavour associated with the addition of EO. 

There hasn't been any research done yet on the interaction between probiotic soy milk and essential oils made from herbs. Therefore, [27] used Lactobacillus plantarum A7 combined with C. cyminum essential oil to study the effects of probiotic soymilk in streptozotocin-nicotinamide induced diabetic rats. They found that the probiotic soy milk group that contained C. cyminum essential oil had substantial reductions in Fasting Blood Glucose (FBS), total cholesterol, and low-density lipoprotein cholesterol, with an increase in high-density lipoprotein cholesterol, as compared to other groups. Thus, it was determined that the combination of probiotic soy milk and herbal EO ingestion may be significant in the management of diabetes. 

Utilizing Eos with Yoghurt: Essential oils are employed to enhance the lactic acid products' nutritive content, organoleptic qualities, and health benefits [28]. One of the most popular dairy products with better nutritional value, flavour, and health benefits is yoghurt. Concentrated yoghurt (labneh) was given a 600 μL\kg dose of three essential oils: wheat germ, eucalyptus, and cinnamon. These findings demonstrated that EOs cause a reduction in the numbers of bacteria, yeast, and mould. None of the Labneh samples made by adding the corresponding essential oils contained either Coliform or E. coli. Because even a small amount of an EO can influence the taste of a product, it is crucial to choose the right EO and account for its concentration. Compared to wheat germ oil used in this study, cinnamon and eucalyptus oils have better antiseptic, antibacterial, and antifungal properties because they contain various secondary metabolites, such as polyphenolic and monoterpene compounds that inhibit the growth of pathogenic microorganisms, particularly gram-positive ones [29]. 

The samples of yoghurt that had been treated with eucalyptus oil had the best organoleptic qualities. Increasing the EO concentration reduced the levels of syneresis. Increasing the concentration of EOs also improved the total phenolic content, antibacterial activity, and antioxidant activity. The highest levels of phenolic content and antioxidant activity were found in yoghurt that contained 0.9% eucalyptus oil. With an inhibitory zone of 20.63 mm, Eucalyptus oil at the same quantity demonstrated the greatest antibacterial activity against S. typhimurium before E. coli (the inhibition zone was 19.43 mm). On the other hand, yoghurt supplemented with myrrh oil had the maximum antibacterial effect against L. monocytogene by 0.9%, and the inhibition zone was 19.21 mm. The outcomes indicated that eucalyptus and myrrh oils could be used [30]. 

Application of Eos to Ice Cream Ice-Cream: The physiochemical, microbiological, and sensory qualities of ice creams made with the essential oils from lemon, mandarin, and orange peels were studied by Tomar et al., [31]. Limonene, -pinene, and -terpinene were found to have an antimicrobial effect with lemon peel. The most promising technique for encapsulating peppermint essential oil, according to their research, is ca-alginate. According to sensory analysis, adding peppermint oil to ice cream up to 0.3% (w/w) could be a good way to enhance the functional qualities of the herb without compromising the texture. 

Eos Used in Cream: In the study conducted by [32], lycopene and essential oils from Echinophora platyloba (EEO) were used to extend the shelf life of pasteurised cream. Lycopene concentrations were 20 and 50 ppm whereas EEO concentrations were 0.10% and 0.50%. They were combined and added to pasteurised cream, which was then examined for microbiological traits, lipid stability, and sensory qualities while maintained at 4°C and 25°C for two weeks. The pasteurised cream with the combination of the aforementioned ingredients in higher quantities performed best in terms of microbiological characteristics, chemical analysis, and stability when combined to control in storage conditions, according to the results of microbial tests and chemical analysis. Analysis of sensory studies revealed that all of the employed therapies were broadly accepted. The researchers discovered that creams with lower concentrations of the EEO and lycopene combination had the best sensory qualities. The study's conclusion suggested that EEO and lycopene may be used together as a natural preservative in dairy products with high fat content like cream and butter. 

Application of Eos to other Dairy Products: According to Boroski et al., [33], oregano extract and essential oils serve as powerful antioxidants for use in dairy beverages that are supplemented with linseed oil at a dosage of 2 g/100 g. Omega 3 fatty acid oxidation caused by light and heat was the main issue with dairy beverages, however a colour was also seen during storage. Linseed oil was used to address these issues, and as a result, the problem was much lessened. Therefore, it was determined that oxidation can be greatly decreased by adding a natural antioxidant to dairy beverages that are fortified with omega 3 fatty acids. In a study on the well-known Indian confection Burfi, which has a short shelf life, Badola et al., [34,35] succeeded in boosting both antibacterial and antioxidant activities by utilising herbal essential oil. For that purpose, 0.15-0.25 ppm of both the essential oil from Clove Buds (CLVB) and Curry Leaves (CRYF) was used. The findings demonstrated that as the amount of EO from herbs in Burfi increased, antibacterial and antioxidant activity also increased at the same time, lowering sensory characteristics. The characteristics of the physicochemical system did not alter much. When the relationship between burfi samples and quality criteria was calculated, 81.5% was reported. Therefore, the ideal concentration on khoa basis was discovered to be CRYF (0.10 ppm) and CLVB in order to maximise the stability of burfi during storage while taking into account its sensory characteristics (0.20 ppm).

The bioactive components of essential oils have been authorised by the European Commission for use as commercial flavouring agents. Before registering natural agents, the European Union (EU) conducts toxicological and microbiological testing to verify their safety profiles [36]. The Everything Added to Food in the United States (EAFUS) list is then updated with the tastes that have been registered. Generally Recognized or As Safe (GRAS) food additives are also included on this list, which has been recognised by the Food and Drug Administration. There are a few exceptions, such as estragole, which is on the EAFUS list but is restricted on the EU list because of its genotoxic and carcinogenic properties.

Essential oils are secondary metabolites found in plants that provide them the ability to defend themselves against pests like bacteria, fungi, insects, and herbivorous animals. It has been discovered that essential oils are beneficial against pathogenic and food deterioration agents. 

Numerous studies have documented the positive effects of combining essential oils with dairy products; nevertheless, further in-depth research is required to fully understand the potential synergistic and antagonistic interactions between the constituents of essential oils and dairy products. If essential oils are used in dairy products in larger concentrations, then maintaining the flavour could be a challenge. Additional research on toxicity and safety is also required.

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