The bioactive properties of food and their relationship to the maintenance of health and protection from degenerative diseases are receiving much attention from nutritionists and consumers. Eating fruits and vegetables is strongly recommended for good health and a desirable intake of antioxidants. Thus the trend in the food industry is toward producing functional foods with increased levels of bioactive compounds.
One of the reasons that Australian native plants have become commercial ingredients is that they personify truly Australian cuisine. There is growing interest in value adding native food plants for local and export markets. There is also an increasing interest in the bioactive components of native plants and as a result identifying a role for native plant foods in the functional food market. However there are a number of commercial issues facing native food producers and food processors in delivering consistent quality products to commercial end users based on a food technology and nutrition platform.
Some specific bioactive compounds were measured. Lycopene, β-carotene and ascorbic acid were determined by HPLC. The method for determining lycopene and β-carotene required optimisation. It was found that the accuracy of measurement of lycopene and β-carotene was improved if the sample was finely blended to release the lycopene from the food matrix to maximise extraction.
Considerable losses were found during the measurement of ascorbic acid by HPLC. An oxygen quenching agent, sodium metabisulphite, was used to delay the rate of ascorbic acid degradation. The rate of addition at 500 μg/mL was found to be very effective. Moreover, since the degradation kinetics can be expressed as a first order reaction, the rate constant k was calculated for each sample. Since loss of ascorbic acid was still observed during long HPLC sequence runs, the k values were used to extrapolate back to the initial ascorbic acid concentration in the samples.
Only Australian native plants currently used as commercial ingredients were selected and used through out the work. These were bush tomato, Kakadu plum, wild lime, finger lime, Davidsons plum, wattle seed and lemon myrtle. The post harvest and handling procedures of all of those above plants were documented. The information was gathered from the farmers, distributors, representatives of Aboriginal community and from the literature. The samples were also collected and tested for bioactive compounds. It was found that post harvest methods of the native plants had an effect on the survival of bioactive compounds. There are two documented post harvest and handling methods for bush tomato, sun and oven drying. Sun drying seemed to give a better appearance however there was lower survival of bioactive compounds. Most of the native plant fruits are frozen for sale commercially. It was found that for the frozen fruit, the levels of bioactive compounds fluctuated probably due to differences in size and species.
Bush tomatoes are one of the most commonly used native fruits and these and other native fruits are frequently made into sauces, chutneys and jams. The stability of bioactive compounds is therefore of importance in these kinds of products. A shelf life study was carried out on bush tomato sauce and chutney. Lycopene was measured in the products together with sensory and colour measurements throughout a period of 9 months. The effect of light and temperature was studied. Lycopene was affected by all of the factors and the study also showed changes in colour and flavour in the products.
The behaviour of bioactive compounds during processing was studied. Samples were collected during the production of commercial sauces. Bush tomato ketchup, chutney and Kakadu plum chilli sauce were sampled from the start, middle and the end of the processing run. The samples were tested for specific bioactive compounds; lycopene for bush tomato sauces and ascorbic acid for Kakadu plum sauce. The studies showed that lycopene content was constant through out the run for both bush tomato sauces. Similarly, for the Kakadu plum sauce, ascorbic acid content was constant. However the losses appeared to have taken place prior to the making of the final sauce.
The effect of heat processing on bioactive compounds was studied on a laboratory scale product. Formulated bush tomato and Kakadu plum sauces were made in the laboratory and also were tested for specific bioactive compounds. Lycopene content increased by 48 % which can be explained by the release of the lycopene from the food matrix. In contrast, there was a large loss of ascorbic acid in Kakadu plum sauce during the heating process.
There are two aspects to the food safety issues in dealing with the use of Australian native plants. First, finding ways to eliminate toxic compounds is essential. For example, it is recommended that bush tomato be collected at the ripe and dried stage when any toxins are at a minimum. Second, during the processing of the products, the critical control points need to be carefully tested in order to protect the products from any contamination.
The antioxidant capacity of native plants using DPPH, TEAC and total phenol assays was determined. A positive correlation was found between the DPPH and TEAC assays but not between either of these with total phenol content. Kakadu plum was found to be the most effective antioxidant containing plant. However, the results showed relatively low antioxidant activity in Davidsons plum even though it contains a high amount of total anthocyanins (polyphenols). This could be due to the colour of the fruit extract interfering with the measurement of antioxidant activity because ABTS radical used in the TEAC assay is purple.
LC-MS was used for the analysis of flavonoids and anthocyanins in a number of native plants. Davidsons plum was analysed differently due to the large amount of anthocyanin detected. A lower cone voltage was found out to give a better anthocyanin M+ molecular ion plus fragment ions for the sequential loss of sugar molecule and finally the anthocyanidin M+ ion. The results shown that 6 anthocyanins were found which are in order of decreasing concentration: delphinidin-hexosepentose, cyanidin-hexose-pentose, petunidin-hexose-pentose, pelargonidin-hexose, peonidin-hexose and malvidin-hexose.