Dairy Products with Modified Lipid Composition
Among major concerns about the use of dairy products as functional food the lipid composition and concentration is of outmost importance, especially for the relatively high concentration of LDL-cholesterol. Therefore, scientific research is devoted to improve/decrease their lipid composition/concentration, mainly through fortification with conjugated fatty acids and use of fat replacers, respectively.
Conjugated Fatty Acids
Conjugated linoleic acid (CLA) and conjugated linolenic acid (CLNA) are a mixture of positional and geometric isomers of linoleic acid (c9c12-C18:2) and a-linolenic acid (c9c12c15-C18:3) with conjugated bonds. The major dietary sources of CLA and CLNA are dairy and meat products from ruminants, because these compounds mainly originate from rumen microbial isomerization and biohydrogenation and endogenous synthesis (especially in the mammary gland) of linoleic and linolenic acids. CLA and CLNA show various, isomer-specific, physiological activities, such as anticarcinogenic, antiatherogenic, reduction of body fat, enhancement of immune functions, and improvement of bone mass (Gorissen et al., 2015). In detail, dietary intake of 3 to 4 g of CLA per day reduces body weight and prevents atherosclerosis in humans, whereas animal diet containing 1% of CLA results in anticancer activity (Bhattacharya et al., 2006). Concentration of CLA and CLNA in dairy products greatly varies from 3.4 to 9.4 and from 0.3 to 2.1 mg/g fat, respectively, depending on species and breed, feeding system, and processing parameters (Dhiman et al., 2005; Akraim et al., 2007; Gomez-Cortes et al., 2009). Health-promoting doses of CLA and CLNA may be achieved through four primarily avenues:
- 1) Increase the level of the precursors polyunsaturated fatty acids (PUFA) by supplementing animal diet with forage, certain oilseed, vegetable, or fish oils (Coakley et al., 2007)
- 2) Use synthetic CLA or CLNA dietary supplements (Yu et al., 2003).
- 3) Add CLA- and CLNA-producing bacteria in fermented foods (Gorissen et al., 2015)
- 4) Use direct food fortification/enrichment (Nazare et al., 2007).
Enhancement of conjugated fatty acids through dietary manipulation shows a disadvantage due to toxicity of added PUFA for ruminal microorganisms, which would cause reduced biohydrogenation (Maia et al., 2010). Distribution of isomers in synthetic supplements could differ from that of natural foodstuff. Thus, a further separation step would be necessary after chemical synthesis, in order to select just the isomers (e.g., c9t11-CLA, c9t11c15-CLNA) whose physiological effect is well-known (Yu et al., 2003).
Food-grade bacteria represent a more natural but yet scarcely explored method for enriching dairy products with conjugated fatty acids. Different bacterial groups (bifidobacteria, lactic acid bacteria, propionibacteria) are able to convert, through FAD- or NAD(P)-dependent linoleate isomerase, linoleic and linolenic acids into the corresponding conjugated isomers (Gorissen et al., 2015). These metabolic pathways are probably a mean for avoiding growth inhibitory effect exerted by PUFA possibly through disruption of lipid bilayer structure or disturbance of membrane potential (Jiang et al., 1998; Maia et al., 2010). The capacity of producing CLA and CLNA is strain-dependent and probably related to the ability of the strain to tolerate PUFA. Bifidobacteria (e.g., Bifidobacterium breve, B. bifidum) are able to produce CLA in laboratory media as well as in milk. The highest concentrations of CLA (0.10.4 mg/mL) may be obtained by adding (0.2-0.5 mg/mL) to milk linoleic acid or monolinolein (a monoglyceride form of linoleic acid) (Choi et al., 2008; Van Nieuwenhove et al., 2007). Other supplements (e.g., sodium salt, short-chain fatty acids, cysteine-HCl, yeast extract, inulin) may cause an increase of CLA concentration with respect to nonsupplemented milk (Gorissen et al., 2015). Usually, lactic acid bacteria (e.g., L. acidophilus, L. brevis, L. casei group, Lactobacillus pentosus, Lactobacillus reuteri, Lc. lactis, Leuconostoc mesenteroides) are able to produce higher concentrations of CLA in laboratory media than in milk. For instance, when using linoleic acid as supplement, strains of L. acidophilus and L. casei produced 0.04-0.1 mg of CLA/mL and 0.08-0.13 of CLA/mL in skim milk and MRS, respectively (Alonso et al., 2003). Among propionibacteria, Propionibacterium freudenreichii ssp. freuden- reichii Propioni-6 was able to convert linoleic acid (0.1 mg/mL) in CLA (0.09 mg/mL) during skim milk fermentation (Jiang et al., 1998). Interestingly, the growth inhibitory effect of linoleic acid on P freudenreichii ssp. shermani was reduced when polyoxyethylene sorbitan monooleate (Tween 80) was mixed (ratio 15:1) with linoleic acid added (1 mg/mL) to whey permeate medium, increasing the CLA yield up to 57% (Rainio et al., 2002).
Direct addition of precursor PUFA may be replaced by adding to milk vegetable oils that are rich in PUFA. For instance, fermentation of skim milk added with sunflower oil (0.25%) by rumen isolates of L. brevis resulted in the production of 8 to 10 mg of CLA/g of fat (Puniya et al., 2008). Similar results were obtained upon addition of safflower oil to skim milk and using a mixed starter (L. acidophilus-L. plantarum) (Ye et al., 2013). However, in case the initial concentration of precursor PUFA in milk is not appropriately chosen, no increase in conjugated fatty acids may be achieved (Gorissen et al., 2010). This possible limiting factor may be overcome by moderate lypolitic treatment. Lc. lactis ssp. lactis biovar diacetylactis and Ln. mesenteroides ssp. mesenteroides produced about 0.2 mg of CLA/ml during fermentation of skim milk added (0.6% to 0.8%) with hydrolysed sesame oil (Abd El-Salam et al., 2010).
Appropriate selection of strain, concentration of precursor PUFA and other nutrients, pH, and temperature and time of incubation (considering the growth phase of bacteria) are the main variables to increase the concentration of conjugated fatty acids in dairy products (Gorissen et al., 2015). Increase of CLA was reported in fermented milk beverages due to appropriate strain selection (Yadav et al., 2007; Akalin et al., 2007). Addition of maltodextrin to milk, in combination with a commercial starter (S. thermophilus) and L. rhamnosus LBA, probably stimulated the growth of lactic acid bacteria, favoring the production of c9t11-CLA in fermented milk (Oliveira et al., 2009). Increased concentrations of CLA (6-7 mg/g of fat) were reported in buffalo cheese, due to the use of appropriate strains alone (S. thermophilus) or (L. casei group) in combination with sunflower oil (Van Nieuwenhove et al., 2007). Combination of two probiotic strains (L. acidophilus and L. casei) and sesame oil allowed increased concentration of CLA (8.5 mg/g of fat) in Ras cheese (Abd El-Salam et al., 2011). Concentration of CLA in cheese increased from 0.01 to 0.05 mg/g when linoleic acid converting strains of P. freudenreichii ssp. shermani were used in cheese making, in combination with the lipolytic yeasts Geotrichum candidum and Yarrowia lipolytica (Das et al., 2005). In that study, the major effect on CLA concentration could be represented by the use of lipolytic yeasts, since it would seem that, under relatively low pH and water activity (such as that of the cheese curd), the propioni- bacteria are not able to produce CLA (Das et al., 2005). Concentrations of c9t11- and t10c12-CLA increased in fermented milk when hydrolyzed soy oil and mixed strains of P freudenreichii ssp. freudenreichii were added to milk (Xu et al., 2005).
Given that conversion of linoleic acid into CLA is frequently reported in probiotic bacteria (Gorissen et al., 2015), an alternative strategy for increasing the dietary intake of conjugated fatty acids is to supplement diet with ad hoc probiotic strains, such as L. plantarum PL62 (Lee et al., 2007) and L. rhamnosus PL60 (Lee and Lee, 2009). Finally, the possibility of obtaining enriched yogurt by direct addition of CLA has been recently investigated. The lowest value of complex viscosity, meaning higher creaminess than commercial products, was found for set yogurt added with CLA (0.25%) and fermented for 4 hours (Salamon et al., 2015). Future research should focus on how to enrich dairy products with CLNA and on ascertaining, through in vivo studies, the physiological activities of dairy products enriched with conjugated fatty acids.