Preliminary observations on the effects of milk fortification with conjugated linoleic acid in yogurt preparation

The fortification and enrichment of food with health benefic natural or natural identical substances creating new functional foods became an important issue for food researchers and processors. However, often occurs that the obtained products (despite of their health benefic activity) cannot be marketed due to strange or accustomed taste and/or texture. The aim of the research was to elucidate the effect of conjugated linoleic acid (CLA) enrichment of raw milk on the rheological properties of the obtained yogurt. The results show that the values of the complex viscosity at 50 rad.s-1 (correlated with the thickness and sliminess of the food gel structures) of the CLA-enriched yogurt was the lowest among the studied samples, meaning the enriched yogurt is more creamy than the commercial products. These observations gave us the hope that, in this case, the texture of enriched product will not present any drawback related to consumer quality judgment.


Introduction
Conjugated linoleic acids (CLAs) form a group of positional and geometric isomers of conjugated dienoic derivatives of linoleic acid. The cis-9,trans-11 isomer of CLA, also known as rumenic acid, accounts for 90% of the total CLA intake and is mostly found in beef and dairy products. The cis-9,trans-11 isomer in CLA has been considered to be the most biologically important in terms of anticarcinogenic activity, protection against athero-scolerosis, cachexia, and treatment of non-insulindependent diabetes [1]. The level of CLA in milk fat commonly falls between 3 and 6 mg/g [2]. To receive the recommended 3 g/day, approximately 30 servings per day (about 250 mL) of milk with naturally increased levels of CLA would be necessary. Direct fortification of milk with CLA oil is a more feasible and realistic solution to obtain maximum health benefits [3]. The aim of the paper is to determinate the influence of the CLA fortification on the rheological properties of the obtained yogurt products and to correlate the rheological data with the yogurt sensory attributes (thickness, stickiness).

Yogurt production and control
A volume of 400 mL of whole milk (pH = 6.79, fat content 2.7%, total solid content 12.9%, density 1032 kg.m -3 ) was used in each sample. A Hanna HI 99161 instrument was used for pH measurements, fat content was determined with Gerber method and the total solid content with Kern MLB 50-3N thermobalance. For CLA fortification 1 g of CLA supplement (Vitking) from the market was used. After fortification the milk was pasteurized at 72 ºC, 14 s, cooled to 43 ºC and inoculated with 0.2% mixed-starter culture of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus YC-380 (Chr. Hansen's). The inoculated milk was mixed and incubated at 43 ºC. Fermentation was stopped at a pH=4.52 then stored at 4-5 ºC for 12 h, before evaluation. Control samples obtained without a starter culture and without CLA addition were prepared in parallel with fortified and starter culture added yogurt samples.

Rheological measurements
The rheometric investigations were performed on an Anton Paar, Physica MCR 501 modular rheometer provided with a Peltier-system for the temperature control. A solvent trap was used to avoid the solvent evaporation. Parallel plate geometry with serrated plates was used to avoid slippage of the sample. The upper plate from stainless steel was 50 mm in diameter. The working temperature was 5 o C, and the gap between the plates 0.5 mm. The obtained data were processed with the Rheoplus software. Amplitude sweep tests were carried out for all four samples in the range of strain 0.1-100%. Based on the obtained data the linear-viscoelastic range (LVE) was estimated and the critical share stress was calculated (both in terms of deformation and shear stress).

Results and discussion
Firstly a stain test (amplitude sweep -AS) (constant frequency, ω = 10 rad.s -1 and variable oscillation amplitude between 0.01-100%) was performed to determine the LVE and to study the mechanical stability of the yogurt structures [4,5,6]. The variation of the dynamic moduli (G'storage modulus; G"loss modulus) is presented in figure 1. From the graphs (figure 1) the LVE limits were determined and the values of the critical shear stress was calculated. The limit of the LVE region indicates the maximum deformation tolerated by the sample before the internal super-structure is destroyed. All samples exhibit a well developed and stable structure even for higher deformations. Starting with the crossover point of the curves G'(γ) and G"(γ), hence for G'= G", the gel character of a system turns into a liquid character and this point is referred to as "flow point". The sample P 1 with lower values both for G' and G" has the lowest rigidity and consequently is smoother and creamy with a pleasant sensation for the consumer. The frequency sweep (figure 2) is widely used as a standard test in polymer rheology. In this test a sinusoidal strain with a constant amplitude (depending on the LVE range limits for each sample, γ = 10% in our case) was applied and the oscillation frequency was varied (between 10 -2 and 10 1 rad.s -1 ). The evolution of the storage modulus with frequency evidenced the smoothness and creamy structure of the sample P 1 .
The complex viscosity (*) is related to the consistency of the gel structure [7,8]. The results of the measurements at low amplitude are presented in figure 3. This measurement has a particular importance, as the mimics the oral conditions during mastication of foods and the value of the complex viscosity correlate well with the slimness, creaminess and the mouth feel of the food products. Furthermore the oral release of the taste and aroma compounds from jellified structures highly depends on the gel fragmentation [9]. The values of the complex viscosity at 50 rad.s -1 are presented (figure 4). Because the P 1 sample possess the lowest value for complex viscosity can be concluded that the P 1 sample is the best from the rheological point of view. Complex viscosity at the angular frequency of 50 rad.s -1 obtained in small amplitude tests correlates with the thickness, sliminess verbal description of the in-mouth texture.

Conclusions
The rheological studies allow to establish good correspondence between the rheological parameters and the sensory properties of the CLA-fortified yogurts which exhibited better mouth feeling characteristics as compared with the control probes. To elucidate the effects of processing parameters on the quality of the final products more studies are needed.