Application of Plant Antimicrobials in the Food Sector: Where Do We Stand?

The Special Issue “Plant Extracts Used to Control Microbial Growth: Efficacy, Stability and Safety Issues for Food Applications” explored the potential of plant-based extracts as natural antimicrobial agents in the food industry. Its purpose was to address the growing demand for natural, safe, and effective food preservation methods. The contributions highlighted various plant extracts’ antimicrobial efficacy, including phenolic compounds, terpenes, and other bioactive substances. Research papers and one review were submitted from countries, including Spain, Portugal, Italy, Mexico, Turkey, India, USA, Romania, China, and Lithuania, showcasing a diverse international collaboration. Key topics covered in this issue included the chemical characterization of plant extracts, their stability under different processing and storage conditions, and their safety assessments. Advances were reported in using plant extracts to inhibit spoilage microorganisms and foodborne pathogens, enhance food safety, and extend shelf life. The published papers in the Special Issue studied various food types, including yogurt, catfish fillets, edible Mushrooms, red grapes, herring Fillets, and various food types covered in the review. This diversity demonstrates the broad applicability of plant extracts across different food products. Notable findings included the antimicrobial activities of fermented grapevine leaves, grapefruit seed extract, cinnamaldehyde, clove oil, and other plant-based compounds. In conclusion, this Special Issue demonstrated significant progress in applying plant extracts for food preservation, highlighting their potential to contribute to safer and more sustainable food systems worldwide.


Utilizing the Power of Plant Antimicrobials for Food Safety
The use of natural preservatives with antimicrobial properties has gotten significant interest in food and drug research due to the growing awareness of the negative impacts associated with synthetic preservatives.These impacts include potential health risks to consumers, the emergence of multidrug-resistant microorganisms, and the requirement for alternatives to traditional thermal treatments [1][2][3].Plant antimicrobials offer a promising solution as they are rich sources of multiple bioactive compounds capable of reducing contamination levels of pathogenic bacteria and inhibiting the growth of spoilage microorganisms in various foods.The compounds found in plant antimicrobials are diverse and include polyphenols known for their antioxidant properties; polyphenols also exhibit strong antimicrobial activity [4,5].They can disrupt microbial cell membranes and interfere with the functions of microbial enzymes and proteins [6,7].Essential oils and their constituents, such as carvacrol, thymol, and eugenol, possess great antimicrobial properties [8][9][10].Essential oils can penetrate microbial cell membranes, causing structural and functional damage [11,12].Glucosinolate derivatives found in cruciferous vegetables have been shown to possess antimicrobial activity against a broad spectrum of microorganisms [13].They act by releasing isothiocyanates upon hydrolysis, which are toxic to bacteria [14].Alkaloids are nitrogen-containing compounds with antimicrobial properties that can inhibit the growth of bacteria, fungi, and viruses [15].Alkaloids can interfere with DNA replication and protein synthesis in microorganisms and can attenuate bacterial pathogenesis [15,16].Thiols are sulfur-containing compounds that exhibit strong antimicrobial activity [17].They can disrupt microbial cell walls and membranes, leading to cell lysis and death.
The objective of the Special Issue entitled "Plant Extracts Used to Control Microbial Growth: Efficacy, Stability and Safety Issues for Food Applications" was to present the latest advances in the use of plant antimicrobials to reduce spoilage microorganisms and ensure food safety across different foodstuffs.The papers in this issue highlight the potential of plant extracts as natural preservatives that can be integrated into various food systems to enhance safety and extend shelf life.

Summary of Published Papers
The main studied sources of plant antimicrobials were the herbal plant Potentilla kleiniana, cranberry pomace and grape seeds, essential oils and their compounds, bog bilberry leaf extracts, spice extracts, grapefruit seed extract, and grapevine leaf extracts (Table 1).Among the compounds in the used raw extracts were identified phenolic acids, flavonoids, and terpenes from plant tissues.Tang et al. evaluated the antibacterial activity of the methanol phase extract from the edible herb Potentilla kleiniana against more than 20 pathogenic bacteria.MIC values of Fragment 1 ranged from 6.25 to 50 mg/mL against Bacillus cereus, Shigella flexneri, Staphylococcus aureus, and Vibrio parahemolyticus strains.Oxymorphone and rutin were identified in Fragment 1, and a putative mechanism of action involving the inhibition of energy supply and protein translation, the blocking of signal transduction, and the repression of ABC transporters was proposed [ Contribution 1].An alginate/pectin film containing grape seed extract showed bacteriostatic activity against L. monocytogenes on herring, reducing its load by 3 log cfu/g compared to unpacked fillets stored for 18 days at 4 • C. In addition, the accumulation of histamine, cadaverine, putrescine, and tyramine was significantly reduced starting from 12 days of storage in fillets packed using the active coating [Contribution 2].
Li et al. found that carvacrol displayed the lowest EC50 value against A. alternata, significantly reducing the decay rate when applied to contaminated red grapes.Grapes inoculated with A. alternata showed a decay rate higher than 60%, whereas contaminated fruit treated with carvacrol showed a decay rate lower than 15% after 12  Grapefruit seed extract showed antibacterial action against S. aureus ATCC 6538, P. aeruginosa ATCC 9027, P. fluorescens wild type, Escherichia coli ATCC 8739, with MIC values ranging from 162.5 µg/mL to 650 µg/mL.Rutin, naringin, hesperidin, neohesperidin, and naringenin were identified in the extract.A preliminary trial on mushrooms showed that applying grapefruit seed extract can limit the development of yellowing on Pleurotus eryngii caused by Pseudomonas spp.[Contribution 6].Cinnamaldehyde and clove oil showed in vitro antibacterial activity against S. baltica and A. hydrophila; their application on adsorbent pads in contact with catfish fillets reduced the total bacteria on pads by 3 to 6 log cfu/mL [Contribution 7].
Freitas et al. found that fermented grapevine leaves using Saccharomyces cerevisiae, in both solid and liquid media, preserved the microbiological quality of yogurt in the same manner as potassium sorbate without affecting the viability of lactic acid bacteria.
Further research is necessary to evaluate the antimicrobial effect of fermented grapevine leaves against yogurt spoilage microorganisms [Contribution 8].Finally, Pinto et al. summarized recent findings on applying plant extracts and plant antimicrobial compounds against spoilage and pathogenic microorganisms in different foods.Interesting results were achieved by using combinations of plant antimicrobials, with synergistic or additive effects, and by integrating plant extracts with food technologies, ensuring an improved hurdle effect.The review highlighted the need for further research in fields such as the mode of action of plant antimicrobials, optimization of delivery systems, sensory properties of food including plant antimicrobial compounds, safety assessment of plant extracts, regulatory aspects, eco-friendly production methods, and consumer education [18].

Key Advances and Findings
The papers published in this issue highlighted several key advances and findings, demonstrating the potential of plant-based compounds to enhance food safety and quality.The methanol-phase extract from Potentilla kleiniana exhibited a 68% inhibition against over 20 pathogenic bacteria, with MIC values ranging from 1.56 to 50 mg/mL.This study identified oxymorphone and rutin as active components and proposed mechanisms involving inhibiting energy supply, protein translation, signal transduction, and repression of ABC transporters.Alginate/pectin films containing cranberry pomace and grape seed extracts showed bacteriostatic activity against Listeria monocytogenes on herring, significantly reducing the load of histamine and cadaverine during storage.Carvacrol was effective against Alternaria alternata in red grapes, reducing the decay rate to less than 15% compared to over 60% in untreated controls after 12 days at 25 • C. Bog bilberry leaf extracts obtained through ultrasound extraction showed the lowest MIC values against Candida parapsilosis and Salmonella enterica, with high-pressure extracts inhibiting Staphylococcus aureus at sub-MIC levels.Sumac extract significantly reduced the growth of several foodborne pathogens, including Enterococcus faecalis, Campylobacter jejuni, and Yersinia enterocolitica, and decreased the production of biogenic amines.
Grapefruit Seed Extract demonstrated broad-spectrum antibacterial activity against multiple strains, including S. aureus, P. aeruginosa, P. fluorescens, and E. coli, with MIC values between 162.5 µg/mL and 650 µg/mL.Additionally, it showed potential in preventing yellowing in mushrooms caused by Pseudomonas spp.Cinnamaldehyde and clove oil applied on absorbent pads in contact with catfish fillets reduced total bacterial counts by 3 to 6 log cfu/mL.Fermented grapevine leaves used in yogurt maintained microbial quality similar to potassium sorbate without compromising the viability of lactic acid bacteria.
Several studies focused on improving the stability of plant extracts under different processing and storage conditions.Techniques such as encapsulation [19][20][21], inclusion in biopolymers [22][23][24], and spray-drying [25][26][27] were explored to enhance the stability and effectiveness of plant antimicrobials.The research covered a wide range of food products, demonstrating the versatility of plant antimicrobials.These included yogurt, herring fillets, red grapes, catfish fillets, and mushrooms.This diversity illustrates the broad applicability of plant extracts across different food matrices, contributing to enhanced food safety and shelf life.Combining different plant extracts often resulted in synergistic or additive antimicrobial effects.This approach and integration into food technologies provided an improved hurdle effect, enhancing overall food preservation outcomes.

Future Research Directions
The Special Issue highlighted the need for further research in several key areas.Detailed studies are necessary to understand how plant antimicrobials exert their effects, focusing on their mode of action.Additionally, there is a need for the optimization of delivery systems to develop effective methods that maximize the efficacy of plant extracts.Investigating the impact of plant antimicrobials on the sensory attributes of food is crucial to ensure that these natural preservatives do not negatively affect taste, texture, or aroma.Comprehensive safety evaluations are required to ensure consumer health is not compromised.Addressing regulatory challenges will facilitate the commercial use of plant antimicrobials, ensuring they meet all necessary standards and guidelines.Developing sustainable production techniques for plant extracts is essential to promote eco-friendly methods that align with environmental goals.Lastly, increasing awareness and acceptance of natural food preservatives among consumers through effective education campaigns is vital for broader adoption and understanding of these innovations.

Conclusions
This Special Issue has significantly contributed to the field of food microbiology by advancing our knowledge of plant antimicrobials and their applications.The findings emphasize the potential of these natural compounds to revolutionize food preservation, paving the way for safer, more sustainable food systems that meet the growing demand for natural and health-friendly food additives.
Author Contributions: Writing-original draft preparation, L.P., J.F.A.-Z.; writing-review and editing, L.P., J.F.A.-Z.All authors have read and agreed to the published version of the manuscript.
days at room temperature [Contribution 3].Bog bilberry leaf extracts obtained through ultrasound (UAE) extraction showed the lowest MIC values against Candida parapsilosis and Salmonella enterica; high-pressure (HPE) extracts showed the inhibition of S. aureus growth at sub-MIC levels [Contribution 4].Kuley et al. demonstrated that, among four spice extracts, sumac (Rhus coriaria L.) extract reduced by 1-3 log cfu/mL the growth of Enterococcus faecalis, Campylobacter jejuni, and Yersinia enterocolitica in tyrosine decarboxylase broth and the production of histamine by E. faecalis and tyramine by C. jejuni [Contribution 5].

Table 1 .
Plant antimicrobial compounds, sources, target food/microorganisms, and main results were obtained in the published papers in this special issue.