Antioxidant activity of pomegranate (Punica granatum L.) polyphenols and their stability in probiotic yoghurt

Pomegranate is a shrub or small tree grown in different parts of the world that contains higher levels of antioxidants than most fruits. Pomegranate juice (PJ) is well known for its health beneficial compounds, which can be attributed to its total polyphenol compounds and high level of antioxidant activity.

This study was undertaken to characterise the antioxidant properties of the fresh juice extracted from the Australian-grown pomegranate ‘Wonderful’ variety in comparison to those reported in the literature and the imported pomegranate juices (IPJs). Total phenolic compounds (TPC) were determined by Folin-Ciocalteu Colorimetric method and expressed as gallic acid equivalent (GAE), while the antioxidant activity (AA) was measured by ABTS method and expressed as Trolox equivalent antioxidant capacity (TEAC). The TPC in the fresh juice was found to be 2,400 ± 200 mg/L GAE with antioxidant activity of 11 ± 1 mM/L TEAC, while the TPC of the four imported juices was in the range of 1,000 - 2,800 mg/L GAE with AA ranging from 5.5 to 14.5 mM/L TEAC.

In an attempt to further improve the TPC levels in fresh juice different parts of the fruit (arils, chopped peeled or whole fruits) were used for juice extraction using six different extraction methods. The extracted juices showed different yield, colour, soluble solids, titratable acidity, TPC and antioxidant levels. Up to five fold increase in TPC and AA level could be achieved by employing intensive extraction on chopped whole fruits. Pasteurisation did not adversely affect the AA of the resulting samples. Pasteurised juice extracted from arils was concentrated (PJC) to 52 ºB and added into milk used for the production of a probiotic yoghurt containing selected probiotic bacteria. Reconstituted skim milk (16% TS) was supplemented with 6% PJC and inoculated with mixed culture (ABT-5) containing Lactobacillus acidophilus (LA-5), Bifidobacterium bifidum (BB-12) and Streptococcus salivarius ssp. thermophilus (TS-B01). The survival of lactic and probiotic cultures in yoghurts were investigated on weekly intervals during the shelf life of 28 days at 4 ºC. Results obtained indicated more than 90% viability of lactic and probiotic cultures after 28 days storage. To evaluate the effects of PJC on individual culture each strain was inoculated in sterilised RSM (16% TS) and incubated overnight at 37 ºC followed by microbiological analyses. The results revealed that the PJC supplementation adversely affected the population of BB-12, but no significant adverse effect was observed on the number of ST-B01 and LA-5.

The effects of PJC supplementation on TPC, colour parameters, texture and sensory attributes of probiotic yoghurts were also analysed. The TPC of the probiotic yoghurt containing 6% PJC was found to be 1590 ± 34 mg/L GAE against a background TPC in plain yoghurt of 1153 ± 32 mg/L GAE.

Developed probiotic yoghurt was subjected to freeze-drying to evaluate the effects of this procedure on the TPC and probitic bacteria viability. This process raised the TPC of the freeze-dried samples 5.2 folds, and increased the counts of ST-B01, LA-5 and BB-12 by 0.81, 0.64 and 0.74 log cycle.