4.1. Antioxidant Capacity and Total Phenolic Content
Culinary herbs and spices are, in many instances, cooked prior to consumption, and then undergo digestion. Thus, to further elucidate their biological and nutritional significance, the impact of these processes needs to be understood especially at levels of habitual intake. Chohan
et al., [
20] carried out a study on the impact of cooking on a number of common culinary herbs and spices, namely cinnamon, cloves, fennel, ginger, parsley, rosemary, sage and thyme at amounts used in the preparation of food (0.2–1 g). Microwaving, simmering and stewing all increased the antioxidant capacity probably as a result of heat liberating the antioxidant compounds [
73,
74]. In contrast, cooking techniques that involved dry heating, grilling and frying, resulted in a decrease in antioxidant capacity which was associated with browning and thus may be indicative of the Maillard reaction, or more specifically its products, influencing antioxidant capacity [
74,
75,
76].
Subsequent studies using culinary herbs from the Lamiaceae family: parsley, rosemary, sage and thyme showed that changes in antioxidant capacity due to cooking processes were strongly and significantly associated with changes in total phenolic content [
72,
77]. This finding is not surprising as the assays used to determine antioxidant capacity (Trolox equivalent antioxidant capacity/2,2'-Azinobis-(3-ethyl benzothiazoline-6-sulfonic acid) assay-TEAC/ABTS assay) and total phenolic content (the Folin-Ciocalteau/gallic acid equivalents–GAE-assay) are based on similar redox reactions [
78] however the role of polyphenols as major contributors of antioxidant capacity in culinary herbs and spices is well established [
10,
32,
57,
79,
80]. Chohan
et al., [
71] also showed that the effect of cooking on antioxidant capacity is not always consistent, which is an observation that may be associated with the nature of the food matrix and the type of cooking method used [
81,
82,
83]. Related work by Chohan [
77] also showed that cooking time can also affects the antioxidant capacity (TEAC), total phenolic content (GAE) and polyphenol, specifically phenolic acid, profile of culinary herbs.
Chohan
et al., [
71] also investigated the impact of digestion post cooking on the antioxidant capacity and total phenolic content of these culinary herbs and found that both were significantly increased compared to uncooked and also cooked culinary herbs. Other studies on the impact of digestion,
in vitro, on antioxidant capacity/activity and/or total phenolic content reported a decrease in fruit beverages [
84], either no change or a decrease for herbal teas, prepared from infusions of powdered herbs [
85] or either decreases, increases or no statistically significant change for dietary antioxidant supplements [
86]. The type of
in vitro model (
i.e., the chemical and enzymatic environment within the models) used could account for some of these differences [
87]. However the nature of the food/delivery matrix may have also influenced the outcomes of these studies and thus suggests that the form of the food may contribute to its impact as a dietary contributor of the bioactive properties of its constituents.
Preliminary HPLC analysis of rosemary, sage and thyme (uncooked, cooked and cooked and digested) strongly suggest that significant increases in rosmarinic acid, which is a predominant phenolic acid in these herbs, is most likely responsible for the observed increases in antioxidant capacity following cooking and also cooking and digestion. These results also suggest that to some extent other phenolic acids including caffeic acid, ferulic acid and vanillic acid may also contribute to the increase reported for some but not all of the herbs investigated [
77]. These results certainly strengthen the role of polyphenols in the conferment of antioxidant properties on to these foods. They also highlight the variations that occur in the chemical composition of different batches of foods as there was only partial agreement with data obtained from the Phenol-Explorer database [
11,
12,
13] specifically for the uncooked herbs. These variations could be due to biotic and abiotic factors as stated above [
57,
88] and/or the sensitivity of the analytical technique used.
For culinary spices, Baker
et al., [
70] found that cooking and digestion affected the antioxidant capacity and total phenolic content of cinnamon, cloves and nutmeg-again at levels associated with the preparation of food. However, these changes were not consistent. One possible reason given for this is the behavior of the phytochemical constituents (both polyphenol and non-polyphenolic) within the food matrix during these processes giving rise to additive, antagonistic and/or synergistic effects on the bioactivity of these foods.
4.2. Anti-Inflammatory Activity and Total Phenolic Content
There is a paucity of data concerning the effect of preparative and digestive processes on the anti-inflammatory activity of culinary herbs and spices. Investigations of the impact of cooking and digestion on the anti-inflammatory properties of culinary herbs and spices demonstrate that this property is not diminished by these processes. Chohan
et al., [
71] reported that at amounts used in food preparation, uncooked, cooked, and cooked and digested rosemary, sage and thyme elicited an anti-inflammatory effect via the inhibition of, and also protection against, the action of pro-inflammatory agents hydrogen peroxide (H
2O
2) and tumor necrosis factor α (TNFα) which resulted in the inhibition of IL-8 release from peripheral blood lymphocytes (PBLs). These decreases were only significant for PBLs exposed to H
2O
2 for the most part which may be indicative of an activity that involves more than the inhibition of a single pro-inflammatory mediator. There was a strong and significant correlation between inhibition of IL-8 release and antioxidant capacity and total phenolic content irrespective of whether the herbs were uncooked, cooked or cooked and digested which indicates that the polyphenols within these foods contribute to this anti-inflammatory activity, and that this activity may be due to their antioxidant properties. However, the findings of Baker
et al., [
70] which were focused on culinary spices at levels associated with habitual intake suggest that the contributory role of polyphenols is not so straight forward (as indicated by Chohan
et al. [
71]): Baker
et al., [
70] reported that the spices cinnamon, clove and nutmeg (uncooked, cooked and digested) significantly inhibited the pro-inflammatory enzyme cyclo-oxygenase-2 (COX-2). The study also reported via correlation analysis that the anti-COX-2 activity was only partially associated with the antioxidant capacities and polyphenolic content of these spices. The partial correlation to phenolic content suggests the involvement of non-polyphenolic compounds, which is supported by the literature. For example, cinnamaldehyde, a major constituent of cinnamon, and the essential oil responsible for its aroma and flavor [
89], has been shown to inhibit COX-2 activity [
90]. The partial correlation with antioxidant capacity suggests that other actions may contribute to the anti-inflammatory properties.
The inflammatory response is a complex one and involves numerous mediators, a number of which may be affected by individual polyphenols and thus by culinary herbs and spices: Yoon and Baek [
91] discuss the inhibitory effect of polyphenolic compounds including phenolic acids and flavonoids on one or possibly several cellular pathways that are involved in the inflammatory process. These pathways include the arachidonic dependent pathways, which involve the action of the cyclo-oxygenase (COX) enzymes, and the arachidonic independent, pathways, which include peroxisome proliferator activated receptors (PPARs), nitric oxide synthase (NOS), nuclear transcription factor κB (NF-κB), which regulates the expression of pro-inflammatory cytokines including IL-8, as well as the non-steroidal anti-inflammatory drug (NSAID) activated gene. Some of these polyphenols include those that are found in culinary herbs and spices. For example, rosmarinic acid has been shown to inhibit the pro-inflammatory PKC/NF-κB pathway [
92]. Curcumin, a predominant polyphenol in turmeric, also inhibits NF-κB [
93] COX-2 has also been shown to be down-regulated and/or inhibited by eugenol (clove) and apigenin (parsley) [
94,
95].
Culinary herbs, including lemon grass, rosemary and thyme are also reported to enhance the activity of the enzyme superoxide dismutase (SOD), which in addition to being an important antioxidant enzyme has the potential to act as an anti-inflammatory agent as it catalyses the dismutation of the free radical superoxide, which is associated with chronic inflammation [
96,
97]. A recent study by Chohan
et al. [
98] identified rosemary, sage and thyme as possessing superoxide dismutase mimetic (SODm) activity which was significantly associated with the antioxidant capacity, total phenolic content and inhibition of IL-8 release. The association with the former (antioxidant capacity) is not surprising but this analysis does indicate that polyphenols in these herbs may be responsible for the SODm activity as suggested by Huaefi and Smetanska [
99] and that it (SODm) may also contribute to the herbs’ anti-inflammatory activity. Thus, this mimetic activity possessed by these herbs may account for some of the antioxidant and anti-inflammatory properties that these foods possess.
In summary, studies on the antioxidant and anti-inflammatory activities of culinary herbs and spices at levels associated with habitual intake show that these properties are not diminished post cooking and digestion. In addition, this work also indicates, via correlation analysis, that polyphenols are significant in conferring both these activities. However, for some spices non-polyphenolic compounds may also have a role to play.