*3.1. Antimicrobial Assay of Essential Oils*

The method of agar disc diffusion was used to evaluate the antibacterial activity of eighteen EOs against four pathogens. There were differences among these EOs in terms of their antibacterial properties (Table 1). The size of the inhibition zones of cinnamon, oregano, and pomelo oils ranged from 16.33 to 35.18 mm, representing the strongest antibacterial activity among the eighteen EOs. Cinnamon oil had the strongest inhibition effect on SA with an inhibition zone diameter of 30.47 mm. Oregano oil strongly inhibited SA and LM with an inhibition zone diameter of 31.31 mm for SA and 30.01 mm for LM. Pomelo oil demonstrated the highest antibacterial activity against ST, with an inhibition zone diameter of 35.18 mm. Some essential oils including clove, eucalyptus, sweet orange, and blumea oils exhibited moderate antibacterial activity against foodborne pathogens (inhibition zone is 12–20 mm). There was no antibacterial activity against foodborne pathogens from jasmine, sea buckthorn pulp, sweet osmanthus, and citrus oils. Other essential oils showed a low to strong antibacterial activity against four foodborne pathogens. Some reports have demonstrated that essential oils exhibited different antibacterial effects, and different foodborne pathogens have different resistance against essential oils [44]. The antibacterial effect of essential oils depends on the type and content of antibacterial components contained in essential oils [45]. Therefore, the individual components and concentrations of EOs play an important role in the aspect of antimicrobial activity. Some studies have demonstrated that the principal constituents of cinnamon, oregano, and pomelo oils are cinnamaldehyde, thymol, and limonene, respectively [46–48]. Cinnamaldehyde and thymol usually have a broad spectrum of antibacterial activity, and they have a significant antibacterial effect on pathogenic bacteria such as *Escherichia coli*, *Staphylococcus aureus*, and *Listeria monocytogenes* [49,50]. The antimicrobial mechanism of the EOs including cinnamon, oregano, and pomelo oils is related to the disturbance of membrane permeability, which results in the release of cellular contents in the form of some inhibition enzymes such as ATPase, histidine decarboxylase, and amylase [51].


**Table 1.** Diameter (mm) of inhibition zones of essential oils against the four pathogenic strains.

Data are means of diameters of inhibition zones ± standard deviation. Values in the same column not followed by the same lowercase letter are significantly different (*p* < 0.05). LM, *Listeria monocytogenes*; ST, *Salmonella typhimurium*; SA, *Staphylococcus aureus*; EC O157:H7, *Escherichia coli* O157:H7. -, no inhibition zones.

### *3.2. Determination of Minimal Inhibitory Concentration*

Cinnamon, oregano, and pomelo oils were evaluated for their MIC against four pathogens (Table 2). The results show that the MIC of cinnamon oil was 0.313 μL/mL for LM, ST, SA, and EC O157:H7, which is the lowest MIC against the four pathogens of all other tested EOs. The MIC of oregano oil was 0.625 μL/mL against ST and 1.25 μL/mL against LM, SA, and EC O157:H7. The MIC of pomelo oil was 1.25 μL/mL against ST and 2.5 μL/mL against LM, SA, and EC O157:H7. In this study, cinnamon oil has a lower MIC for four foodborne pathogens compared with oregano and pomelo oils. As reported in other studies, cinnamon oil also exhibits strong antimicrobial properties [52]. Cinnamon oil among six essential oils (rosemary, cinnamon, ginger, pepper mint, sweet orange, and tahiti lemon oils) showed the lowest MIC values of 6.25%, 3.12%, and 3.12% (*v*/*v*) for *Staphylococcus aureus*, *Escherichia coli*, and *Salmonella enterica*, respectively [33]. Another research reported that cinnamon oil showed also the lowest MIC values for four fungal species, four yeasts species, and two bacteria species, thereby confirming its higher inhibitory activity compared with clove oils [53]. The mechanism of cinnamon oil inhibiting the microorganism is mainly through denaturing proteins in cell membranes, interfering with the activity of enzymes in cell walls [54]. In many studies, cinnamon oil as a bioactive component positively inhibited microbial growth in food matrices. It also indicated that cinnamon oil could be applied for inhibiting microorganism growth on food and ensuring safety [55]. However, high concentrations of essential oils produce more intense flavour due to their volatility, which might affect the acceptability of food (such as fruits and vegetables) for the consumer. Therefore, a cinnamon essential oil with the lowest MIC was chosen for evaluating further the preservation of fresh-cut potatoes in this study.


**Table 2.** Minimal inhibitory concentrations (MIC) of cinnamon, oregano, and pomelo peel oils.

LM, *Listeria monocytogenes*; ST, *Salmonella typhimurium*; SA, *Staphylococcus aureus*; EC O157:H7, *Escherichia coli* O157:H7. -: no growth; +: minor growth; ++: major growth.

### *3.3. Effects of Cinnamon Oil on the Quality of Fresh-Cut Potatoes*

### 3.3.1. Colour

Colour is a key determinant of consumer acceptability in fruit products. The *L*\* in colour represents the brightness of fresh-cut potatoes. *L*\* is one of the indicators of surface darkening caused by enzymatic browning or pigment gathered during storage [56]. The lower the *L*\* value, the greater the browning. *L*\* and *b*\* showed a significant decrease, and *a*\* showed a significant increase with the extension of storage time (Figure 2A–C) (*p* < 0.05). The *L*\* of fresh-cut potatoes treated with chitosan-based EC was higher than that of control. The *L*\* was higher for fresh-cut potatoes treated with chitosan-based EC containing 0.2% cinnamon oil than for the other groups, and the decline (7.70) in *L*\* was the slowest during storage time. *a*\* is the lower (0.99) and *b*\* is the higher (16.25) on fresh-cut potatoes treated with chitosan-based EC containing 0.2% cinnamon oil compared with that in other groups. The *L*\* of fresh-cut potatoes treated with the chitosan-based EC containing 0.4% and 0.6% cinnamon oil was reduced 23.5 and 25.4, respectively. The *a*\* of fresh-cut potatoes in EC containing 0.4% and 0.6% cinnamon oil was increased 7.71 and 9.08, respectively. The *b*\* of fresh-cut potatoes in EC containing 0.4% and 0.6% cinnamon oil was reduced 7.65 and 8.10, respectively. In addition, the appearance changes of fresh-cut potatoes were observed on the 16th day in this study (Figure 2D). The appearance of fresh-cut potatoes treated with chitosan-based EC, or chitosan-based EC containing 0.2% cinnamon oil, was better than other treatments. The obvious dark browning of fresh-cut potatoes was observed in potatoes treated with chitosan-based EC containing 0.4% and 0.6% cinnamon oil.

**Figure 2.** Changes in colour of fresh-cut potatoes coated with chitosan-based EC containing cinnamon oil. (**A**) *L\**; (**B**) *a\**; (**C**) *b\**; (**D**) appearance on the 16th day. Control: uncoated; EC: edible coating; Cin: cinnamon oil. Bars represent means ± SD (*n* = 3, *p* < 0.05).

When potatoes are cut, the tissue cells are broken, and enzymes such as polyphenol oxidases (PPOs) are liberated and brought into contact with their substrates, causing browning [57]. The browning depends on the characteristics of the samples, amount of endogenous phenolic compound, oxygen condition, and activity of relevant enzymes [58]. The browning has a slight change from *L*\* and appearance of fresh-cut potatoes coated with chitosan-based EC compared with the control. These results were in agreement with other

studies in which chitosan coatings delayed browning in fresh-cut rose apple and litchi in comparison with noncoating [59]. The reason may be that chitosan-based EC prevents oxygen from reaching the surface of the potato and reduces browning [60]. However, the browning has caused a strong change from *L*\* and appearance of fresh-cut potatoes treated with chitosan-based EC containing high concentration of cinnamon oil (0.4% and 0.6%). One study demonstrated that a high concentration of cinnamon oil damages the tissue structure of fresh-cut potatoes and causes serious browning [61]. The reason was may be that the high concentration of cinnamon oil accelerated the browning of fresh-cut potatoes. PPO oxidises phenolics in the presence of oxygen on the cut surface of potatoes, producing quinones, which autopolymerise to form brown-coloured pigments [62]. The other reason was that a high concentration of cinnamon oil might produce phytotoxic effect for fresh-cut potatoes. Several studies reported similar results; the phytotoxic effects of EOs might affect fresh-cut lettuce and fresh-cut apples [36,63]. Therefore, the chitosan-based EC containing lower doses of cinnamon oil would be recommended for maintaining the colour of fresh-cut potatoes.

### 3.3.2. Weight Loss

Weight loss is an important indicator for evaluating the quality of fresh-cut fruits and vegetables during storage times. The weight loss was evaluated in fresh-cut potatoes treated with chitosan-based EC and chitosan-based EC containing different concentration of cinnamon oil during storage time (Figure 3). The weight loss of fresh-cut potatoes significantly increased during storage times (*p* < 0.05). There is no significant difference in weight loss of fresh-cut potatoes among chitosan-based EC, chitosan-based EC containing 0.2% and 0.4% cinnamon oil, and control (*p* > 0.05). However, the weight loss of the fresh-cut potatoes treated with the chitosan-based EC containing 0.6% cinnamon oil was significantly higher than that in other groups (*p* < 0.05). Results also indicate that the concentration of cinnamon oil affected the weight loss. Fresh-cut potatoes treated with a low concentration of cinnamon oil incorporated into chitosan-based EC lost less water than samples treated with that EC with 0.6% cinnamon oil. The weight loss of the fresh-cut potatoes treated with chitosan-based EC containing 0.6% cinnamon rapidly increased after 4 days of storage, possibly due to the fact that the high concentration of EOs can cause potential toxicity of fresh-cut potatoes and accelerate the decay of samples [64].

**Figure 3.** Changes in weight loss of fresh-cut potatoes coated with chitosan-based EC containing cinnamon oil. Control: uncoated; EC: edible coating; Cin: cinnamon oil. Bars represent means ± SD (*n* = 3, *p* < 0.05).

### 3.3.3. Firmness

Fruit firmness is closely related to the cell composition and cell-wall structure. Fruit softening is a consequence of the disassembly of the middle lamella and primary cellwall structures [65]. Fruit softening is a process of starch hydrolysis to sugar and pectin degradation. It is an important factor in the quality of fresh-cut fruits and vegetables and their acceptability to consumers. The firmness of fresh-cut potatoes showed a significant decrease in different treatment groups during storage times (*p* < 0.05) (Figure 4). The decline of firmness is 3.80 N in chitosan coating and 8.20 N in the control during storage time. The result demonstrates that chitosan-coating treatments mitigated the firmness decrease to a greater degree than the control. There is no significant difference in the firmness of fresh-cut potatoes treated with chitosan-based EC and chitosan-based EC containing 0.2% and 0.4% cinnamon oil at 16 days (*p* > 0.05). However, the firmness of fresh-cut potatoes in the group treated with the chitosan-based EC containing 0.6% cinnamon oil was reduced 10.20 N during storage time. In this study, the result agreed with other studies that reported that the application of chitosan-based coatings inhibited the fruit and vegetable softening process [66–68]. Cutting operation might cause the increase in pectinase activity in potatoes' tissue. Under the action of pectinase, pectin in the cell wall is decomposed and tissue is softened. The coating treatments may allow firmness to be maintained by inhibiting water loss due to the activities of pectin-degrading enzymes and by reducing the rate of metabolic processes during senescence [69]. On the other hand, high concentrations of cinnamon oils damage the tissue of fresh-cut potatoes, causing them to be more susceptible to spoilage and fruit softening [64].

**Figure 4.** Changes in firmness of fresh-cut potatoes coated with chitosan-based EC containing cinnamon oil. Control: uncoated; EC: edible coating; Cin: cinnamon oil. Bars represent means ± SD (*n* = 3, *p* < 0.05).

### *3.4. Microbiological Analysis*

The population of naturally occurring microorganisms on fresh-cut potatoes treated with chitosan EC with or without cinnamon oil was evaluated (Figure 5). The population of total plate counts, yeast and mould counts, total coliform counts, and lactic acid bacteria counts on the fresh-cut potatoes considerably increased with the prolongation of storage time (*p* < 0.05); the increment is 3.57, 3.37, 2.14, and 1.07 log cfu/g, respectively. This may be because nutrients released from the fresh-cut potatoes after cutting provide suitable growth conditions for microorganisms. Some reports have also shown that pathogens and spoilage microorganisms can grow on fresh, frozen, dried, ready-to-serve, and minimally processed potato products [70,71]. The total plate counts, yeast and mould counts, total coliform counts, and lactic acid bacteria counts during storage time were significantly lower for fresh-cut potatoes treated with chitosan EC than those for fresh-cut potatoes in the control group (*p* < 0.05), the decrease is 1.44, 1.72, 0.57, and 0.56 log cfu/g, respectively. Some studies have also demonstrated that chitosan has antibacterial activity, and involving chitosan coatings reduced microbial growth on mangoes, papaya, and strawberry [72–74]. The mechanism of chitosan EC is mainly the leakage of electrolytes and intracellular protein constituents caused by interactions between chitosan with positive charge and the surface of bacterial cells with negative charge [75,76]. According to the total plate counts, yeast and mould counts, total coliform counts, and lactic acid bacteria counts, the populations were significantly lower in chitosan-based EC containing 0.2% cinnamon oil among the different treatment groups at 16 days (*p* < 0.05), the decrease is 2.14, 1.92, 0.98, and 0.73 log cfu/g, respectively. The population of the decrement of naturally occurring microorganisms on chitosan-based EC containing 0.2% cinnamon oil is more than that on chitosan-based EC. It demonstrated that the combination of chitosan EC and cinnamon oil exhibited a synergetic antibacterial effect against naturally occurring microorganisms. Other studies reported that chitosan coating incorporating several common essential oils can enhance antimicrobial activity. It also showed that the compatibility of cinnamon oil with chitosan in film formation was better than that of other essential oils with chitosan [37]. However, the populations showed a gradual increase with the increase in cinnamon oil concentration (0.4% and 0.6%). It might be that cinnamon oil at a higher concentration of 0.4% and 0.6% damages the cell structure of fresh-cut potatoes. The pulp of fruits and vegetables provides rich nutrient content for microorganism growth [77]. Therefore, microorganisms can easily grow on fresh-cut potatoes treated with chitosan-based EC containing 0.4% and 0.6% cinnamon oil during storage. Interestingly, no coliform nor lactic acid bacteria were observed on the fresh-cut potatoes treated with chitosan-based EC containing cinnamon oil for 4 or 8 days. However, the populations of coliform and lactic acid bacteria on the fresh-cut potatoes in the control group significantly increased after 4 and 8 days. This demonstrates that chitosan-based EC and chitosan-based EC containing cinnamon oil had antibacterial activity against coliform and lactic acid bacteria and inhibited their growth on fresh-cut potatoes. Moreover, according to the standard of the Institute of Food Science and Technology (IFST), 6 log cfu/g of natural microorganisms is considered the limit of acceptance for the shelf life of a fruit product [78]. This is the reason why toxic substances may be produced when microbiological counts exceed 6.0 log cfu/g [79]. In this study, the population of total plate counts on fresh-cut potatoes is less than 6.0 log cfu/g in chitosan EC containing cinnamon oil during 16 days. Therefore, the acceptance of fresh-cut potatoes treated with chitosan EC containing 0.2% cinnamon oil was extended to 16 days.
