Sunflower oil-in-water emulsions, like salad dressings and mayonnaise, have a shelf life of 6-12 months at room temperature. The shelf life of these emulsions is usually limited by lipid oxidation leading to the formation of undesirable components such as free radicals and reactive aldehydes which result in rancid notes. Various factors have been reported to induce lipid oxidation in emulsions during storage, thereby limiting the shelf life of oil-in-water emulsions. Antioxidants are added to emulsions to retard lipid oxidation. The effectiveness of these antioxidants can be evaluated over the 6-12 month shelf life period, but this has time and cost implications. There is a need for valid accelerated methods that can be used to measure the efficiency of antioxidants in commercial oil-in-water emulsions. This study is aimed at gaining a better understanding of how antioxidant levels influence oxidation of commercial sunflower oil salad dressing emulsions, when exposed to a combination of oxidation acceleration factors.
Therefore the first part of this study will focus on the effect of high and low concentrations of two antioxidants with different antioxidant mechanisms [gallic acid and ethylene diamine tetraacetate (EDTA)] in a commercial sunflower oil salad dressing emulsion (SOSDE) stored at accelerated deterioration and ambient conditions. SOSDEs stored at accelerated storage conditions contained four pro-oxidation factors (temperature increase to 32.2C, addition of specific potential pro-oxidants FeSO4 and ascorbic acid and adjustment of pH to 5.5). The combination of these pro-oxidants and the their concentrations are based on research to optimise the development of lipid oxidation in oil-in-water emulsions. SOSDEs were examined by sensory (aroma evaluation by a trained sensory panel) and chemical analyses (anisidine value and peroxide value) daily for 20 days for accelerated stored products and bi-weekly for 24 weeks for ambient stored products. Sensory differences between products with high and low concentrations of antioxidants were more apparent for ambient stored than accelerated stored SOSDEs. Accelerated storage resulted in more aroma differences between SOSDEs with different types of antioxidants than between high and low antioxidant concentrations. Peroxide value (PV) differences between products with high and low concentrations of antioxidants were apparent for SOSDEs with EDTA stored under accelerated shelf life conditions, with higher concentrations resulting in significantly lower PV. Anisidine value (AV) differences existed between SOSDEs with different gallic acid concentrations stored in accelerated conditions (with AccLoGal (accelerated storage conditions, low concentration, gallic acid)) significantly lower AV than AccHiGal (accelerated storage conditions, high concentration, gallic acid) and ambient shelf life conditions (with AmbHiGal (ambient storage conditions, high concentration, gallic acid)) significantly lower AV than AmbLoGal (ambient storage conditions, low concentration, gallic acid), and SOSDEs with different EDTA concentrations stored in accelerated shelf life conditions (with AccHiEDTA (accelerated storage conditions, high concentration, EDTA)) significantly lower AV than AccLoEDTA (accelerated storage conditions, low concentration, EDTA). These results suggest that: 1) the accelerated storage model used in this study is more suitable to predict the shelf life changes of SOSDEs with metal chelator antioxidants such as EDTA, rather than free radical scavenging antioxidants such as gallic acid. This may be due to the addition of FeSO4 as a potential pro-oxidant. 2) PV and AV are more sensitive oxidation markers than aroma analysis when using the accelerated storage model, and 3) PV, AV and aroma of SOSDEs stored in accelerated conditions as used in this study, do not clearly predict results for ambient stored SOSDEs.
The second part of the study focused on identifying key sensory attributes associated with the development of lipid oxidation and rancidity that can be used as predictors for end of shelf life during shelf life testing of oil-in-water emulsions. A multivariate accelerated shelf life test (MASLT) was applied to the aroma profiles of SOSDEs with different antioxidant treatments and storage conditions. Addition of antioxidants to the SOSDE decreased the rate constant of change in aroma of lipid oxidation compared to the SOSDE with no antioxidants added. Accelerated storage negatively affected the shelf life of SOSDEs, and all SOSDEs containing antioxidants, stored in accelerated storage conditions had a higher lipid oxidation rate constant compared to those stored at ambient. Shelf life studies require the identification of an attribute that has the highest impact on the quality of the product or shows the most change over the shortest time period, through multivariate analysis it is concluded that pungent aroma is associated with early stage of ambient and accelerated storage and cardboard aroma is associated with later stages of ambient and accelerated storage and therefore the most probable predictors of shelf life in SOSDEs.
Although accelerated storage, using the storage conditions applied here, did not predict ambient storage of SODEs well enough, it seems that MASLT has the potential to screen potential effectiveness of antioxidants, including the most probable aroma shelf life predictors pungent and cardboard, in as little as 20 days before commencing with shelf life trial that may take up to two years. Analysis of more time intervals for longer periods during accelerated and ambient shelf life conditions may lead to better correlation and therefore predictability capacity of the accelerated storage.
Thesis (PhD (Food Science))--University of Pretoria, 2019.