Hydroxycinnamic acids are phenolic compounds of the phenylpropanoid family that occur naturally as secondary metabolites in plants. A variety of these derivatives of cinnamic acid can in fact be found in foods, plant-derived products (e.g., beer, wine, olive oil, coffee), herbs, spices, cereals, legumes, fruits, nuts, and vegetables [1
], even in mushrooms [3
], in variable amounts depending on a number of factors including environmental conditions, cultivation technique and plant part [4
]. Scheme 1
shows selected hydroxycinnamic acids. Because these phenolic compounds are ubiquitous in plants, they are usual components of the animal and human diet, albeit in widely variable proportions around the world. Also, although they can be absorbed in the gastrointestinal tract, they are partially excreted unchanged or in derivative forms via urine and faeces [5
Phenols promote health and reduce disease risk, so they are especially relevant to animal and human health, pharmacology, food science and industrial production. For example, the health effects of Mediterranean diet have been widely ascribed to the consumption of phenolic compounds even though the biologically available amount of substances such as flavonoids in foods may be inadequate to account for their action and synergistic effects may be present [6
]. Some major features of fruit production are strongly influenced by the presence of phenols. In fact, phenolic compounds influence fruit quality through a number of sensory properties such as flavour [7
], colour [8
] or texture [9
]. Also, the outcome of storage and other manipulation processes [10
] may be compromised by browning reactions involving phenols and resulting in undesirable colour or taste, or the loss of nutritional value, all of which can have adverse economic impacts.
Caffeic acid (CA) is an especially promising nutraceutical by virtue of its strong antioxidant effects [12
]. Thus, CA modulates redox balance in cells and protects them from reactive oxygen species (ROS) forming in their metabolic reactions [15
]. In fact, CA scavenges free radicals, thereby stopping oxidation; the effect is ascribed to its two hydroxyl groups and their position on the molecule facilitating hydrogen bonding interactions. As a result, CA has some potential health benefits especially prominent among which are anti-carcinogenic action [17
], and cancer prevention and treatment. In addition, CA is an effective anti-mutagenic agent with activity against nitrosamines [20
]. However, it remains as a potential carcinogen for humans on some hazard sheets [21
]. In food technology, caffeic acid has proved effective to reduce aflatoxin biosynthesis during nut storage [22
]. Obviously, high fat contents or the presence of salts can trigger degradation. Finally, because consumers are increasingly favouring naturally occurring products, compounds such as CA may find even wider use.
Micellar systems are colloids with useful properties for industrial and food applications. For example, they can be used to prepare isotropic, thermodynamically stable mixtures of polar and non-polar solvents to solubilize hydrophobic food-related substances such as aromas or all types of preservatives. Also, they can be used to trap unwanted chemicals such as degradation precursors or off-odour compounds. Sodium dodecyl sulphate (SDS), a surfactant that tends to aggregate into micelles, has been widely studied as a model for micellar structures. Whereas European legislation banned the use of SDS as a direct additive for food, the US-FDA has recognised it as a multi-purpose additive [23
In this work, we expanded previous analyses of the influence of temperature and SDS concentration on various SDS–phenolic acid micellar systems [24
]. Micellar properties can be modified by using various additives with a potential impact on micelle uses. For example, some additives may be used in smaller amounts without sacrificing their benefits. In this work, we examined the properties of micellar systems consisting of caffeic acid and SDS.
2. Results and Discussion
The conductance of a solution is a linear function of the concentration of its components. Also, structural changes in a colloidal solution are known to modify their properties. For example, the formation of micellar aggregates interferes with the way electrons flow through the system. For this reason, conductance measurements allow the point where aggregates form to be accurately identified. The critical micelle concentration (CMC) of the SDS–caffeic acid micellar system was determined from specific conductance versus surfactant concentration curves. The graph exhibited two distinct regions differing in slope, namely: the surfactant monomer region and the micellar aggregate region (see Figure 1
). The point where the two branches intersect is taken to represent the CMC of the surfactant. For SDS in water, CMC has been found to range from 8 to 8.3 m mol kg−1
Because interactions between monomers are affected by thermal agitation, which may facilitate or hinder aggregation depending on the nature of the particular molecules, CMC is strongly influenced by temperature [27
]. For ionic surfactants such as SDS, CMC decreases with increasing temperature to a minimum in the low-temperature region beyond which it exhibits the opposite trend. The presence of additives in a surfactant micellar system also modifies monomer interactions in aggregates, thereby altering CMC and conductance. Table 1
illustrates the behaviour of the SDS–caffeic acid micellar system in this respect.
illustrates the decrease in CMC with increasing caffeic acid concentration. CMC also decreased with increasing temperature at each CA level. R2
was invariably higher than 0.9232, so CMC was a linear function of the caffeic acid concentration. Indeed, the degree of micelle aggregation (β) was seemingly independent of the additive concentration and temperature. Because the standard deviation of β was always lower than 10%, its slight decrease with increasing temperature was not significant. We thus took β to be 0.68 ± 0.11.
As noted earlier, ∆G0m
was calculated from the temperature-dependence of CMC (Equation (3)). As can be seen in Table 2
was negative throughout the temperature and CA concentration ranges; therefore, micellization was invariably spontaneous. It should be noted that the absolute value of ∆G0m
increased with increasing temperature and CA concentration, probably as a result of a structural effect of caffeic acid on water.
The influence of temperature on ∆G0m
at different caffeic acid concentrations allowed us to to estimate the changes in standard micellization enthalpy (∆H0m
) and standard micellization entropy (∆S0m
). Figure 3
illustrates the influence of temperature on ∆G0m
and Table 3
shows the values obtained at each CA concentration. R2
was always higher than 0.9985, so ∆G0m
was linearly dependent on temperature.
Micellization in the presence of CA was an endothermic process and ΔH0m
decreased with increase in CA concentration, the two bearing a linear relationship (R2
= 0.9127). Also, ΔS0m
was positive, so the micellization process was entropically controlled. These positive values resulted from melting of “icebergs” or “flickering clusters” around the surfactant leading to increased packing of hydrocarbon chains within the micellar core in a non-random manner [28
also bore a linear relationship to the CA concentration (R2
= 0.8363). Raising such a concentration possibly led to caffeic acid governing the 3D matrix structure of water around the micellar aggregates. The fact that both ΔH0m
were positive testifies to the importance of hydrophobic interactions as a major driving force for micellization.