There is a growing interest in using plant-derived preservatives in the dairy sector, particularly those derived from vegetable by-products, herbs, and spices. This interest is fueled by the fact that certain plants that can thrive in harsh environments are capable of producing compounds with insecticidal, fungicidal, antibacterial, and antiviral properties [
84]. These compounds are classified based on their main chemical groups, such as terpenes, phenylpropenes, terpenoids, allicin, and isothiocyanates [
85], and they can interact with the bacterial cell membrane and disrupt its structure [
86]. Due to their antimicrobial properties and high antioxidant capacity, these substances effectively preserve food [
26]. The potential of plant-derived preservatives in the dairy industry is very promising and should inspire optimism for the future.
5.1. Plant Extracts
Plant extracts can be either liquid or solid. They can be extracted through various methods such as maceration, digestion, infusion, and decoction or emerging techniques such as Accelerated Solvent Extraction (ASE) and Ultrasound-Assisted Extraction [
87]. The literature has widely reported the antimicrobial activity of these compounds in dairy products [
19,
20,
21,
22]. Moreira et al. [
19] conducted a study to investigate the effects of decreasing the concentration of sodium chloride on the microbiological quality of fresh goat cheese. They achieved this by adding pequi extract (Caryocar brasiliense) to pasteurized milk (CM), cheese mass (CS), and under immersion (CIE). The results showed a significant decrease in LAB counts in CS and CIE samples after 21 days of storage. No activity of Enterobacteriaceae, Staphylococcus spp., E. coli, and bacteria from the coliform group was observed in any of the samples during storage [
19].
In a study by Wanniatie et al. [
20], the effect of adding red ginger extract to goat's milk yogurt was investigated. Results showed that the total bacteria count decreased significantly after adding 2%, but 4% extract increased the LAB count. In another study by Kamel et al. [
21], carrot powder was incorporated into soft buffalo cheese with probiotics. The total bacterial count decreased from 7.5 to 7.3 log CFU/g in the product added with 0.6% of the compound. However, lactic acid bacteria and Bifidobacterium longum count increased at the end of 28 days in dosages of 0.4% and 0.6%. In terms of sensory evaluation, samples with 0.6% carrot powder obtained the lowest score for the flavor attribute on the first day of storage. However, no significant differences were observed in the flavor, texture, color, and appearance scores among all the samples at the end of the storage period.
Ávila Arribas et al. [
22] also obtained satisfactory results when comparing the inhibitory effect of oregano plant extract (Origanum vulgare), savory (Satureja montana), hyssop (Hissopus officinalis) and tarragon (Artemisia dracunculus) under growth of different strains of Clostridium spp. in cheeses. The authors reported that all Clostridium strains were sensitive to EEs in at least one of the concentrations tested and that EEs from hyssop, lavender, and tarragon had lower MICs (<40 μL / mL). EEs from savory, lavender, and tarragon also delayed the appearance of vegetative cells of Clostridium spp. and delayed stewing in 2 weeks without harming the sensorial characteristics of the cheese.
Shehata et al. [
88] conducted a study to evaluate the antimicrobial effect of taro leaf extract (TLE) at concentrations of 250 and 500 mg/L. The researchers found that this compound exhibited excellent activities against various microorganisms, including E. coli BA 12296, Salmonella Senftenberg ATCC 8400, Fusarium oxysporum ITEM 12591, and S. aureus, with inhibition zones of 19.3 ± 1.02 mm, 18.53 ± 0.75 mm, 12.93 ± 1.17 mm, and 11.33 ± 0.84 mm, respectively. Additionally, including the extract enhanced the antioxidant potential, the viability of L. paracasei, and the concentration of polyphenols in the drinks. These findings demonstrated good acceptability by consumers.
Plant extracts have become increasingly important in processing functional foods, and they are being used as alternatives to synthetic antimicrobials. Certain extracts' effectiveness against sporulating bacteria that cause late puffing in matured cheeses is a significant advantage. This defect is challenging to control and can result in significant losses for the food industry. Additionally, when the extracts are derived from industrial waste, they have greater environmental appeal and add value to the products.
5.2. Essential Oils
Essential oils (EO) are volatile hydrophobic liquids important for plants to defend themselves against bacteria, viruses, and fungi [
89]. These compounds are highly effective in inhibiting Gram-negative bacteria, as their hydrophobic nature allows them to easily penetrate the lipopolysaccharide barrier of the outer membrane of these microorganisms. Once inside bacterial cells, they interfere with the transport mechanism of molecules, leading to cell death [
90]. However, it is important to consider their minimum inhibitory concentrations (MIC) due to their pungent smell and potential toxicity at high concentrations [
23].
Several EOs have already been studied in dairy products, such as thyme [
91]; cinnamon [
92]; oregano [
93,
94]; pink pepper [
95]; lemon [
96,
97]; perilla leaf [
98]; rosemary [
99]; tangerine and orange [
96]; mastic [
100]; and, curry and cloves [
101], demonstrating high preservative action, in addition to increasing shelf life and improving physicochemical and sensory characteristics. Jemaa et al. [
91] compared the effect of thyme EO (Thymus capitatos) and its nanoemulsion on the quality of raw milk contaminated by S. aureus, demonstrating greater efficiency of the nanoemulsion in inhibiting the pathogen. However, milk with EO had greater antioxidant capacity and lower protein degradation. Furthermore, there were no statistical differences in the acidity content and peroxide inhibition in both treatments, demonstrating that thyme EO in solution or nanoencapsulated can improve milk quality and extend its shelf life.
In a study conducted by Abbes et al. [
92], the antimicrobial efficacy of cinnamon essential oil (EO) in milk was examined. The results showed that Salmonella Hadar was completely inactivated at 3μg mL−1. Similarly, Hao et al. [
94] investigated the antimicrobial properties of oregano EO, which is rich in carvacrol, against Escherichia coli and Staphylococcus aureus in milk. They used nuclear magnetic resonance (NMR) to study the mechanism of action and observed bacteriostatic and bactericidal effects, which were associated with changes in the cellular morphology of both microorganisms. In a previous study, Campos et al. [
93] analyzed the inhibitory effect of oregano EO on filamentous fungi (Aspergillus flavus, Fusarium oxysporum, and Penicillium citrinum) as well as Escherichia coli and Staphylococcus aureus during the ripening of cheese. Oregano EO at a concentration of 0.02% w/w inhibited the growth of the tested strains over the 30-day maturation period. It reduced 7 logs (CFU/g) of S. aureus in the first hour of maturation and suppressed all viable cells of E. coli after three days of maturation. There were no changes to the pH and humidity of the cheese.
Listeria monocytogenes is a difficult pathogen to control in cheese production. In a recent study conducted by Dannenberg et al. [
95], the efficacy of pink pepper essential oil (EO) as an antioxidant and antimicrobial agent was evaluated in Minas Frescal Cheese. The study also tested the oil against 18 bacteria in-vitro. The study showed a reduction of 1.3 log CFU/g of L. monocytogenes within 30 days, along with the inhibitory activity against seven spoilage bacteria, six pathogenic bacteria, and three bacteria with technological applications. In addition, there was a reduction of free radical DPPH. Another study by Fancello et al. [
97] evaluated the effect of lemon EO (Citrus limon var pomia) on ricotta salata cheese stored at 5°C. The study showed that the EO had bactericidal effects on L. monocytogenes DSMZ and bacteriostatic effects on a mixture of L. monocytogenes strains. The chemical analysis of the liquid phase of this EO revealed that its main active compounds were linalyl acetate, limonene, and two isomers of citral.
Numerous studies have shown that essential oils (EOs) can enhance the quality of yogurts in terms of their microbiological, physical, chemical, and sensory characteristics [
98,
99,
102,
103]. For example, He et al. [
98] investigated the impact of perilla leaf EO on the production of volatile compounds and the microbiological quality of yogurt. They found that adding 0.04% of this EO resulted in longer shelf life, the production of 69 volatile compounds (mainly limonenes), an increased concentration of terpenic substances, and greater sensory acceptance than the control yogurt. Similarly, Kamel et al. [
99] examined the properties of rosemary EO as an alternative to synthetic preservatives in yogurt. They reported that rosemary EO exhibited antimicrobial activity against various microorganisms such as Escherichia coli, Staphylococcus aureus, Salmonella marcescens, total coliforms, yeasts (Candida albicans), and fungi (Aspergillus flavus). Moreover, adding rosemary EO enhanced the viability of LABs (lactic acid bacteria) and improved the overall acceptance of the yogurt.
Ice creams also represent a dairy matrix favorable to adding essential oils, as they are among the most consumed dairy products and are highly accepted among different age groups [
104]. For example, EOs obtained from fruit residues from industrial processes are sustainable alternatives for incorporating natural antimicrobials into edible ice creams [
96]. Tomar and Akarca [
96] evaluated the microbiological, physical, chemical, and sensory properties of ice creams produced with EOs from lemon, tangerine, and orange peels in different concentrations (0.1%, 0.3%, and 0.5%). In conclusion, the authors did not identify the presence of total coliforms, Salmonella spp., Staphylococcus aureus, Escherichia coli, and Listeria monocytogenes in any of the samples. Furthermore, the ice cream added with 0.5% EO and orange peel stood out with the lowest counts of aerobic mesophilic bacteria (3.80 log CFU/g), yeasts and fungi (3.71 log CFU/g), psychrophilic bacteria (4.01 log CFU/g) and Pseudomonas spp. (2.04 log CFU/g).
Mitropoulou et al. [
100] researched ice cream to determine the minimum inhibitory concentration of microorganisms and the main active compounds of mastic EO (Pistacia lentiscus) by GC/MS. The study found that ice cream containing 0.2% (w/w) of EO gradually decreased the counts of Escherichia coli, Listeria monocytogenes, and Pseudomonas fragi during the first seven weeks of storage. On average, the count was reduced by 3 log CFU/g compared to control samples (p<0.05). The primary constituents of EO that contributed to this reduction were a-pinene (67.7%), myrcene (18.8%), and β-pinene (3.0%).
The use of EOs in dairy-based sweets has also been reported. Badola et al. [
101] added EOs from curry leaves (0.05–0.15 ppm) and cloves (0.15–0.25 ppm) and evaluated the microbiological, antioxidant, physicochemical, and sensory characteristics. In addition to the greater antioxidant activity and greater presence of total phenolics, there was a reduction in the count of total bacteria, molds, and yeasts.
Although the numerous technological and antimicrobial properties of EOs are reported, their incorporation into the formulation of dairy products can also be limited by several factors [
107]. For example, volatile organic compounds can quickly decompose in adverse environmental conditions, leading to loss of flavor, aroma, and antioxidant and antimicrobial efficacy [
108]. Furthermore, antimicrobial phytochemicals in high concentrations can present toxicity and allergenic potential in sensitive individuals [
109], in addition to possible interactions with food components, damaging their quality [
110].
Although essential oils (EOs) have been shown to have antimicrobial properties, their use in certain dairy products should be limited. This is because they can negatively affect the product's sensory qualities [
100,
101]. In some cases, the undesirable flavor may be due to the presence of certain phytochemicals such as 3-carene, caryophyllene [
105], and eugenol [
106]. This issue is particularly noticeable in dairy products with high-fat content.