Analytical MethodsApplicability of organic milk indicators to the authentication of processed products
Highlights
► Organic dairy products were authenticated by stable isotopes and fatty acids. ► Fresh milk lipid-thresholds for δ13C and C18:3ω3 were confirmed in dairy products. ► Non-organic constituents can be identified by correlating δ13C in protein and fat. ► Maximum δ15N of organic products is lower than of conventional.
Introduction
Procedures for authenticating organic milk continue to be of interest because of its high price and the limitations of available natural resources. The risk of conventional milk fraudulently labelled as organic can be countered with adequate controls to protect consumers and secure fair trade. An obvious starting point for potential procedures is the milk composition, which can vary greatly depending on differences in the diets of cows.
Any specific characteristics from feeding reflected in organic and conventional milk must be distinctive to allow for differentiation despite the seasonal influences on milk composition generally observed. Some studies have reported higher α-tocopherol and β-carotene contents in organic milk than in conventional milk (Bergamo et al., 2003, Slots et al., 2008). An analysis of phytanic acid indicated an increased minimum level in organic milk fat; however, it was insufficient to distinguish organic milk from conventional (Vetter & Schröder, 2010). More promising were reports of elevated levels of α-linolenic acid (C18:3ω3) in organic milk fat (Bergamo et al., 2003, Butler et al., 2011, Ellis et al., 2006, Jahreis et al., 1996, Slots et al., 2008, Ellis et al., 2006), which is in agreement with our previous work (Molkentin, 2009, Molkentin and Giesemann, 2007). Furthermore, we demonstrated that the stable isotope ratio of carbon (δ13C) in milk fat is an interesting variable for organic milk authentication (Molkentin, 2009, Molkentin and Giesemann, 2007), which is related to the varying ratio of C3 to C4 plants in the cow’s diet (Metges, Kempe, & Schmidt, 1990).
Therefore, in 2009, we proposed threshold values for C18:3ω3 and δ13C in milk fat that may be used to identify German organic retail milk (Molkentin, 2009). According to our studies, which systematically included seasonal variability in bulk milk, organic milk always possessed a minimum C18:3ω3 content of 0.50% and a maximum δ13C of −26.5‰. Although conventional milk can sometimes exceed these limits, this procedure differentiates the vast majority of conventional milk from organic milk.
The aim of the present study was to determine whether these established milk fat limits also apply to processed dairy products. Such products may offer a challenge for the food chemist because they are not always produced directly from fresh milk but are often made from previously isolated individual milk components, such as skim milk, cream or casein. During these previous technological treatments, the composition of the milk components may be inadvertently changed, which may impair the authentication of organic milk. Moreover, beyond lipid analysis the authentication of reconstituted dairy products may necessitate an additional analysis of non-lipid components to exclude fraud. Therefore, our investigations into validating fatty acids and stable isotopes as indicators for organic dairy products focus on reconstituted products.
Section snippets
Samples
Dairy products were purchased between August 2007 and September 2009 from retail stores in Kiel, Germany. All products were made exclusively from cow’s milk and were certified as organic according to European Council Regulation (EEC) No. 2092/91 (Council Regulation, 1991) and its amendments. Samples primarily comprised cheeses, most notably curd and cream cheese. Additionally, scattered samples of other dairy products were obtained. More detailed information on the 56 samples is given in Table 1
Results and discussion
Some abnormalities with respect to previous threshold values for C18:3ω3 and δ13C in German milk fat (Molkentin, 2009) occurred among the first samples of organic cream cheese and curd analysed in this study, which required more detailed investigations. Therefore, cream cheese and curd samples were subsequently purchased repeatedly over a period of roughly two years, which explains why these reconstituted products represent the majority of analysed samples (Table 1), and stable isotopes were
Conclusions
The C18:3ω3 and δ13C threshold values previously established for identifying German organic retail milk via lipid analysis are widely applicable to processed dairy products. Noncompliant results may be obtained for certain low-fat organic products or those made from milk produced in countries other than Germany. To confirm the authenticity of reconstituted dairy products, δ13C must be analysed in both the defatted dry matter and lipids. A resulting Δδ13C value of <1.0‰ indicates different
Acknowledgement
The author thanks Birte Fischer and Bärbel Krumbeck for assistance with the analytical work.
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