Formation and characterisation of stirred yoghurts enriched with avocado pulp : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Auckland, New Zealand

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Avocado stirred yoghurt has huge market potential with the expanding market of fruit flavoured yoghurt and the increasing health concern of the public. However, the avocado is highly perishable due to its high content of monounsaturated fatty acids sensitive to lipid oxidation as well as its polyphenol compounds sensitive to enzymatic browning. Considering the conventional treatment like heating may not be suitable for use, there is a challenge of preserving the physicochemical properties and microbial stability of avocado added into yoghurt during storage. This study aimed at investigating the methods of appropriate treatments of avocado for extending the shelf life of avocado-fortified stirred yoghurt and characterising the yoghurt in terms of various properties. Firstly, to choose suitable treatments, different concentrations (1% and 2%) of citric acid and ascorbic acid, high-pressure processing (HPP) treatment (600 MPa, 5 min) and their combination were used to treat avocado before its incorporation into yoghurt containing 10% avocado pulp. Several parameters, such as pH and colour measurement (CIE L, a*, b* values), were analysed from avocado-fortified yoghurt samples during storage for 4 weeks at 4°C. The instrumental measurement of colour change was recorded for 26 days to determine which method of the treatments was more effective in preserving the colour of yoghurt samples. Results showed that the colour change of avocado yoghurt occurred predominantly in the first week which was caused by a decrease of L* value (lightness) and an increase of a* value. The total colour change (ΔE) of avocado yoghurt containing avocado treated with ascorbic acid was below 2 in 26 days. Lower efficiency was found from citric acid where the ΔE value (colour change) was below 4 compared to the untreated sample around 6. Besides, the pH change of avocado stirred yoghurt was more sensitive to citric acid which decreased the pH of avocado yoghurt from around 4.45 to 4.29 and 4.15 at 1% and 2%, respectively. Meanwhile, no significant influence was observed for the HPP treatment on colour stability as well as the pH value of the sample. The treatments of avocado with ascorbic acid (2%) and HPP (600 MPa/5 min) were further studied about their effects on yoghurt, including microbial quality (lactic acid bacteria and microbial contamination), rheological properties, syneresis, colour and pH. It was found that HPP treatment was the key factor to keep yoghurt syneresis (around 1%) low, minimizing the extra syneresis that could be caused by introduced avocado content. Also, the rheological test illustrated its function to increase and maintain the thickness of avocado stirred yoghurt during storage for 4 weeks. In terms of ascorbic acid, no significant effect on rheological properties was observed by the incorporation of ascorbic acid. Neither of these treatments (HPP or ascorbic acid) influenced the growth of lactic acid bacteria in yoghurt. No yeast and mould or coliform were detected in samples during the whole experiment trials. The effect of those treatments on sensory properties during the shelf-life of yoghurt (4 weeks) was also evaluated. The results showed that the sensory properties of untreated avocado-fortified yoghurt dropped dramatically in 2 weeks for an overall consumer acceptance score from 7.16 to 2.25. On the other hand, HPP treatment and incorporation of ascorbic acid significantly maintained the overall acceptance. HPP treatment specifically affected the texture profile of the sample by maintaining its thickness. Ascorbic acid significantly (p < 0.05) increased the flavour score from 2.72 to 6.89 and the appearance score from 4.79 to 5.55. It was found that the main effect of ascorbic acid to improve the sensory properties was to preserve the green colour and inhibit the development of rancid flavour in the product. Besides, neither of these treatments was observed to significantly influence the sourness of the sample, and the interactive effect was not observed between HPP treatment and ascorbic acid. In conclusion, combining 2% of ascorbic acid and HPP treatment on avocado was found to be most effective in various ways (colour, texture and sensory properties) in extending the shelf life of avocado stirred yoghurt with non-significant influence on pH after 1 week storage.
Figure 4.13 is re-used with the publisher's permission.