The effects of gamma irradiation and modified atmosphere packaging on sterilization and quality of some spices

Spices are widely used in food recipes all over the world in order to enhance the color and flavor. Unfortunately, these products are very prone to microbial contamination and threat the safety of the food they are added. Thus, a proper sterilization method, such as gamma irradiation should be used. Even though gamma irradiation is one of the most used methods in spice sterilization, it can result in oxidation and color loss in the food. Combination of irradiation with modified atmosphere packaging (MAP) can decrease the undesired effects of irradiation. In the first part of the study; thyme (Thymus vulgaris), rosemary (Rosmarinus officinalis), black pepper (Piper nigrum) and cumin (Cuminum cyminum) in ground form were packaged under either ambient atmosphere (aerobic packaging, AP) or 100% N2 (0% O2, modified atmosphere packaging, MAP). Packaged samples were irradiated at 7 kGy, 12 kGy and 17 kGy doses in an irradiation facility. Non-irradiated samples were used as control. Samples were evaluated according to their microbial quality, CIELAB color values (L*, a*, b*) and essential oil yield. Changes in the essential oil profile was evaluated by one-dimensional gas chromatography-mass spectrometry (GC-MS) and two-dimensional comprehensive gas chromatography (GC×GC-FID and GC×GC-Q-TOFMS). Gamma-irradiation alone was sufficient to decrease the bacterial load; however, the combination of irradiation with MAP was advantageous. Total mesophilic aerobic count and yeast-mold count were decreased to below 1 log cfu/g after irradiating at 12 kGy and 7 kGy, respectively. Irradiation caused color loss in rosemary and black pepper, bu the application of MAP decreased the irradiation induced color loss in black pepper. The essential oil yield of black pepper and cumin were higher in MAP compared to AP. Thyme essential oil was mainly characterized by carvacrol, and rosemary essential oil was characterized by 1,8-cineole, α-pinene and verbenone, respectively. The major compounds in black pepper were β-caryophyllene, δ-3-carene and valencene. Cumin essential oil was mainly characterized by cumin aldehyde, γ-terpinene and safranal. Gamma-irradiation and O2 content of the packages had varying effects on the essential oils of the spices. The changes in essential oil profile of aerobically packaged samples were higher compared MAP. The monoterpene content in the essential oils of the modified atmosphere packaged samples were higher, whereas oxygenated component and sesquiterpene content were lower compared to aerobically packaged samples. The changes in essential oil content were lower in samples packaged without O2. Differences in essential oil content were also evaluated by GC×GC analysis, and better separation and visualization were achieved by 2D contour plots of new components produced after irradiation. Two of the peaks emerged after irradiation of air-packaged (AP) samples at all doses (7, 12 and 17 kGy). These peaks were tentatively identified as 4-(1-methylethyl)-1,3-cyclohexadiene-1-methanol (CAS number 1413-55-4), and as 2-caren-10-al by using GC×GC accurate-mass time-of-flight mass spectrometry (TOFMS). In addition, a third peak increased in relative abundance after irradiation of the AP samples, and it was identified as cumin aldehyde by using a flame-ionization detector (FID) through comparison with an external standard and also confirmed by TOFMS identification. Although these changes were almost impossible to be noted in 1D GC, GC×GC made more detailed analysis and more reliable comparison of samples possible. It can be said that comprehensive two-dimensional gas chromatography is a useful technology for the comparison of samples with minor differences and where potential volatile molecular changes in samples might not be known in advance. In this specific case, it will be valuable in future studies to detect the irradiation treatment of spices, or by extension, comparison of other samples through their volatile profiles. As the compounds that were the focus of this study were only detectable as resolved peaks in GC×GC, it can be used in the detection and separation of these components in other studies with essential oils. It was observed that irradiation in the presence of oxygen may produce a number of new compounds, or alters the amount of existing compounds in the essential oils of spices. Some of the changes are initiated by oxidation under the effect of irradiation. The extent of changes in compounds attributable to oxidation were significantly lower or in some instances not detected in samples packaged under modified atmosphere. In the second part of the study; the spices (thyme, rosemary, black pepper and cumin) in ground form were packaged aerobically or under 100% N2 (MAP). Packaged samples were irradiated at 6 kGy or 14 kGy doses. Control samples were not irradiated. The essential oils of the samples were isolated by hydrodistilation, and methanolic extracts of the spices were also obtained. The changes in total phenolic content, 2,2-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, ferrus reducing antioxidant power (FRAP) and percent antioxidant activity (AA%) values (obtained by β-carotene-linoleic acid assay) were examined. The differences in antimicrobial activity was also evaluated by determining minimum inhibitory concentration (MIC) and minimum bactericidal/fungicidal concentration (MBC) against Bacillus cereus, Escherichia coli, Staphylococcus aureus and Aspergillus niger. It was observed that gamma-irradiation increased the antioxidant and antimicrobial activities of the essential oils and the extracts of the spices. Total phenolic contents of black pepper essential oil and cumin extract were increased by irradiation, whereas it was decreased in rosemary essential oil. The DPPH radical scavenging activities of rosemary, black pepper and cumin essential oils as well as thyme extract followed an inclining pattern with increasing irradiation. Some increase due to irradiation in FRAP values of the extracts of all spices was noted. The AA% values of black pepper essential oil were increased with irradiation, where as a reduction with irradiation was observed cumin essential oil. It can be said that aerobic packaging enhanced the antioxidant activity. It was observed that irradiation increased antioxidant properties of aerobically packaged samples. It also noted that thyme essential oil and extract had greater antioxidant activity compared to others in most cases. Antimicrobial properties of spices, especially essential oils of thyme and cumin, were increased with irradiation. It was observed that A. niger was more susceptible to spice essential oils compared to B. cereus, E. coli and S. aureus. In addition, S. aureus was the most susceptible bacteria to the essential oils. Black pepper essential oil did not present any antibacterial effect against the bacteria examined. The antimicrobial effect of black pepper on was also low. Methanolic extract of thyme at 2,5-20 µg/mL concentration was sufficient to inhibit all the microorganisms. However, the extracts of rosemary, black pepper and cumin even at the highest possible concentration (20 µg/mL) did not show any antimicrobial effect on the microorganisms except B. cereus. In the last part of the study, the spice samples which were packaged under 100% N2 or air and irradiated at 6 kGy or 14 kGy were subjected to six months of storage under room temperature. The effects of irradiation and modified atmosphere packaging on microbial quality, CIELAB color values, sensory properties (color intensity, odor, overall liking), essential oil yield, and antioxidant (total phenolic content, DPPH, FRAP, β-carotene bleaching) and antimicrobial activity (MIC and MBC) of the spices during storage were evaluated. Similar to the findings obtained in the first part, gamma-irradiation provided the microbial quality of the spices before and after storage in both MAP and AP samples. It was observed that color values of the spices increased during storage compared to the beginning and lead to color loss. It was also noted that irradiation also caused color change, and this effect was greater at the beginning of the storage. The odor of cumin was higher in aerobically packaged samples at the beginning of the storage, which is believed to be a result of the increase of cuminaldehyde in aerobically packaged samples. There was no other differences in sensory color intensity, odor or overall liking values due to irradiation dose, package or storage. Generally, the essential oil yield, total phenolic content, DPPH radical scavenging activity and antimicrobial activity of spices followed a decreasing pattern during storage. However, FRAP and AA% values of the spices were higher at the end of the storage period compared to beginning. It was observed that the differences noted at the beginning of the storage due to packaging and irradiation were lost after storage. Overall, combination of irradiation with modified atmosphere packaging can be recommended to lower the loses in color and essential oil content of thyme, rosemary, black pepper and cumin. The results are promising that GC×GC can be used as tool to detect irradiation treatment of spices. Irradiation, modified atmosphere packaging and storage generally did not affect the sensory properties of the samples. Gamma-irradiation tended to increase the antioxidant activities of the essential oils and extracts of aerobically packaged spices, and the antimicrobial activities of modified atmosphere packaged the samples. Storage resulted in loss of essential oil yield, antioxidant and antimicrobial properties of samples in most of the cases, and the differences noted due to irradiation and packaging were lost during storage.