Vegetable Extracts for Enhancing Nutritional Value of Dairy Food
Vegetable extracts contain vast amount of plant secondary metabolites, among which phenolic compounds and phytosterols may positively affect human health. For this reason, researchers and industries are more and more interested in the functional features of dairy food enriched with vegetable extracts.
In the last few years, increasing attention has been paid to phenolic compounds, as their intake is inversely correlated with some diseases (Garcia-Salas et al., 2010). In detail, phenolics may show anti-clotting, anti-inflammatory, anti-mutagen, and antioxidant activity, which are correlated with a decreased risk of cardiovascular diseases and cancer development (Fresco et al., 2010; Han et al., 2007; Loke et al., 2010; Ostertag et al., 2010). Since dairy products contain very little or no phenolic compound (O'Connell and Fox, 2001), vegetable extracts have been used to increase the phenolic content of fermented dairy food that, for instance, may be used as antioxidant agents.
Fruit in the form of juices, powders, or extracts represents the main dietary source of phenolics (Record et al., 2001). However, due to seasonal production and request of high amount of fruit in the fresh market, alternative sources of phenolics should be considered: use of food byproducts (e.g., olive oil wastewater, artichokes leaves and stems) (Servili et al., 2011; Punzi et al., 2014) and of plant callus/cell cultures (Blando et al., 2004).
Yogurt added with grape callus extract showed higher radical scavenging activity compared to yogurt containing grape berries extract and to standard yogurt. Various phenolic acids (gallic acid, caffeic acid, p-coumaric acid, vanillic acid, gentistic acid, vanillin) and anthocyanins (catechin, epicatechin, transresveratrol, hesperidin, quercetin) were identified by gas chromatography as the main constituents of antioxidant activity of the yogurt added with callus extract. However, the radical scavenging activity of the enriched yogurt decreased during storage (Karaaslan et al., 2011).
Green tea (Camellia sinensis) is another important source of phenolic compounds, known as tea catechins. These compounds are primarily responsible for the beneficial effects of green tea on health, thanks to their protective effect from damage caused by oxidative stress (Manzocco et al., 1998). In addition, some tea catechins (e.g., epigallo- catechin, epicatechin-3-gallate, epigallocatechin-3-gallate) inhibit various pathogenic microorganisms (Hara, 1999). The addition of green tea extract (concentration ranging from 0.3 to 0.9%) to low-fat milk prior to fermentation by either Bifidobacterium bifi- dum or L. acidophilus positively influences the growth and acid production of starter bacteria. It was hypothesized that the phenolic compounds contained in the extract reduced the redox potential of milk, thus favoring bacterial growth (Marhamatizadeh et al., 2013).
Hypercholesterolemia is one of the major risk factors for cardiovascular and brain vessel diseases (Trapani et al., 2013). Phytosterols (e.g., p-sitosterol, campesterol, stig- amsterol), and their saturated counterparts (phytostanols) are sterol compounds found in plants that, being able to block adsorption of cholesterol, have been used for treating hypercholesterolemia since the 1950s (Miettinen, 2001). In the last few decades, phytosterols have been used as food ingredients in order to lower the level of serum cholesterol (Zawistowski and Kitts, 2004). A meta-analysis of published studies concluded that the level of LDL-cholesterol might be reduced by about 10% following regular ingestion of 2.0 to 2.5 g/die of plant sterols or stanols (Katan et al., 2003). The recommended daily intake for consumers who need to lower (7.0% to 11.3%) the level of low- density lipoprotein (LDL)-cholesterol ranges between 1 and 3 g/die (EFSA, 2012). Before being added to milk, phytosterols/phytostanols must be esterified with dietary fatty acids, obtaining the more soluble phytosterol/phytostanol-esters (Noakes et al., 2005). Plant sterols/stanols may be added to milk even before fermentation, since they do not inhibit growth and acid production of the starter lactic acid bacteria (Monu et al., 2008). Recently, the formulation of a yogurt obtained from milk added with phytosterols and the probiotics L. acidophilus LA5 and B. animalis ssp. lactis BB-12 was optimized in terms of concentrations of phytosterols and fat and dosage of probiotics. Phytosterols enhanced the sensory attributes and the cell density of probiotics of yogurt (Parsa et al., 2015).
Some dairy products added with phytosterols/phytostanols (0.3% to 3.0%) are commercially available (e.g., Becel Pro-activ, Benecol, Danacol) (Willems et al., 2013) and their label reports the health claim (“Plant sterols/stanols contribute to the maintenance of normal blood cholesterol levels”) approved by EFSA (EC n° 432/2012). Recently, a lipidomic analysis performed on overweight and moderately hypercholesterolemic subjects demonstrated that the regular intake of milk enriched with phytosterols not only lowers serum LDL cholesterol but also elicits lipid effects (Padro et al., 2015). In detail, LDL-glycerophospholipids (especially the phosphatidylcholine and lysophosphatidyl- choline subclasses) were significantly reduced, and LDL was less susceptible to oxidation. The variations of such additional lipid markers could contribute to the protective role of phytosterol-enriched milks from cardiovascular disease (Padro et al., 2015).
Among the numerous vegetable species usable as source of bioactive compounds, Aloe vera (Aloe barbadensis Miller) has received much attention due to its millenary use in the traditional medicine of China, Japan, India, and countries of British West Indies (Foster et al., 2011). Aloe vera gel, obtained from the center of leaves, contains over 75 biologically active compounds, which, individually have not been correlated with effects on health (Habeeb et al., 2007). Leaving aside the dermatological uses of Aloe vera gel, anthraquinones (e.g., aloin, barbaloin) are secondary metabolites that, as such or as derivatives, (i) alleviate inflammatory responses in inflammatory bowel disease (Langmead et al., 2004), (ii) have anticancer functions (Lin et al., 2009; Muto et al., 2007), and (iii) possess antibacterial (Ferro et al., 2003) and antiviral activity (Li et al., 2014). The radical scavenging activity of Aloe vera gel is due to a-tocopherol, carotenoids, ascorbic acid, flavonoids, tannins (Hamman, 2008), and polysaccharides (polymannans) (Kang et al., 2014). Phytosterols isolated from Aloe vera indirectly favored the reduction in intra-abdominal fat and improved hyperlipidemia (Misawa et al., 2012). Plasma and tissue cholesterol, triglycerides, free fatty acids, and phospholipids decreased upon oral administration of Aloe vera gel extract to diabetic rats for 21 days (Nassiff et al., 1993). Visceral fat mass and level of blood glucose decreased in hypercholesterolemic rabbits upon administration of phytosterols from Aloe vera (Dana et al., 2012). Cholesterol level decreased in hypercholesterolemic rats following dietary administration of probiotic L. rhamnosus GG combined with Aloe vera gel. In addition, the gel caused an increased cell density of lactobacilli in feces, probably because of growth-promoting components (e.g., mannose, glucose, L-rhamnose, vitamins, amino acids, and trace elements) present in Aloe vera (Kumar et al., 2013).
Despite the great potential for health, currently few studies focus on the enrichment of fermented milk with extract from Aloe vera (Pushkala and Srividya, 2011; Panesar et al., 2014; Basannavar et al., 2014). When Aloe vera gel powder was added (0.5 or 1.0%) to skim milk inoculated with the potential probiotic L. casei NCDC 19, degree of proteolysis and ACE-inhibitory activity of the enriched fermented milk increased compared to control (not added with Aloe vera) (Basannavar et al., 2014). The authors attributed this to the increased growth and metabolic activity of lactobacilli during fermentation, as well as to bioactive compounds (e.g., aloin, acemannan, phytosterols) contained in the Aloe vera extract. The observed increase in viable counts of lactobacilli in the fermented milk enriched with Aloe vera was attributed to the growth-stimulating effect exerted by acemannan, barbaloin, and some amino acids (Basannavar et al., 2014).