The effect of steam pasteurisation and gamma-irradiation on the microbial profile of Aspalathus linearis (Rooibos) tea and the investigation of the native lactic acid bacteria profile

Thesis (MScFoodSc)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: Due to the increase in popularity of rooibos amongst local as well as international consumers, as reported by the South African Rooibos Council (SARC), rooibos in recognised as an economically valuable crop leading to an increase in commercial farming (Koch et al., 2012, Joubert et al., 2011). The increase in commercial farming requires the establishment of a commercial process that not only ensures consistent quality of the final product but ensures adherence to both local and international regulations regarding the microbiological safety of herbal foodstuffs. Research in needed to not only determine the quality attributes associated with the products unique polyphenolic profile but to determine processing parameters that allow for standardised production processes with measurable quality control points as well as critical control parameters, thus ensuring a consistent quality as well as safe product for the consumer. As such this study aimed to assess the efficacy of the currently employed steam pasteurisation process as well as an alternate cold pasteurisation process, namely gamma irradiation, at reducing the microbial load of the final product. It was found that the steam pasteurisation process ensured a greater than 3 log reduction of total aerobic microorganism density as well as coliform density, however the organisms remained present at a detectable level. However, Salmonella spp. were detected on 3% of the pasteurised rooibos samples thus indicating that the currently employed microbial reduction step, namely steam pasteurisation, is not able to reduce the Salmonella CFU/g to not detected in 25g as required by international food safety microbial guidelines. In comparison gamma irradiation at a dose rate of equal to or above 7.5 kGy is was found to reduce the microbial load of fermented rooibos so as to adhere to both local and international regulations regarding the microbial contamination of herbal foodstuffs. The results indicate that gamma irradiation at a dose rate of 7.5 kGy is a viable option as an alternative to the current industry standard of steam pasteurisation. Furthermore, gamma irradiation was shown to have the ability to reduce the anaerobic endospore former density to undetectable levels, as compared to steam pasteurisation which only achieved a 0.01 log reduction, thus ensuring a safer product for the consumer as compared to a steam pasteurised rooibos product. The second aim of this study was to determine whether dried fermented rooibos could serve as a matrix for the isolation of lactic acid bacteria. The results indicated that fermented rooibos could serve as an isolation matrix for lactic acid bacteria as the following species were isolated though through culture dependent methods; Lactobacillus, Pediococcus, and Weissella. However, it remains important that the microbiome of rooibos tea is explored as a novel niche for the isolation of robust lactobacterial strains that can be applied in the food processing industry.

AFRIKAANSE OPSOMMING: As gevolg van die toename is die popularitieit van rooibos tee onder beide internationale sowel as nationale verbruikers, soos aangedui deur die Suid Afrikaanse Rooibos tea Raad (SARR), is rooibos tea erken as ‘n ekonomies waardevolle landbou gewas. Dit het gelei na ‘n toemname in veral die kommersiele boer van die gewas (Koch et al., 2012, Joubert et al., 2011). Die toename in kommersiele boer van die gewas vereis dat ‘n kommersiele produseerings prosses bepaal word wat nie net deurlopende kwaliteit van die eindproduk verseker nie, maar ook sal verseker dat die finale produk aan beide internationale sowel as nationale regulasies rakende die mikrobiese kontaminase van kruie en kruiderye kos soorte voldoen. Navorsing rakende nie net die kwaliteits eienskappe geassosieer met die finale produck se unike smaak en reuk profile, maar prossesering prosedures wat ‘n konsekwente kwaliteits beheer punte sowel as kritise beheer punte om ‘n voedsel veilige eindproduk te verseker is druk nodig. Tot gevolg het die navorsings studie gepoog om die effektiwiteit van die stoom pasteurisasie prosses wat huidiglik in die industrie gebruik word om ‘n voedsel veilige produk te verseker te bepaal, en om dit te vergelyk met ‘n alternatiewe koue pasteurisaie prosses, naamlik gamma raduasie, en die prosses se effektiwiteit om ‘n voedsel veilige produk te verseker. Dit is gevind dat stoom pasteurisasie wel ‘n 3 log reduksie in die totale aerobiese mikro-organisme sowel as coliform mikro-organisme digtheid behaal, maar die organismes is nogsteeds waarneembaar deur tradisionele kultuur metodes. Salmonella is waargeem op 3% van die gepasteuriseerde rooibos monsters, wat daarop dui dat die huidige industriele pasteurisasie prosses nie voldoende is om te verseker dat Salmonella spesies afwesig is 25g van die produk nie, soos vereis deur internationale sowel as Suid Afrikaanse regulasies nie. In teenstelling het gamma bestraling teen ‘n dosis van 7.5kGy die kapasiteit gehad om te verseker dan die rooibos monsters voldoen aan internationale sowel as nationale regulasies in verband met voedsel veiligheid. Daarom dui die resultate dat gamma bestraling teen ‘n dosis van 7.5kGy ‘n merkbare alternatiewe pasteurisaie opsie teenoor stoom pasteurisaise. Verdere navorsing het ook getoon dat gamma bestraling teen 7.5kGy die kapasiteit besit om die anaerobiese endospoor vormende mikro- organismes te verminder na onopspoorbaar deur traditionale kultuur metodes. Waaras, sroom pasteurisasie net ‘n 0.01 log CFU/g reduksie kon behaal, daarom dui dit dat gamma bestraling ‘n veiliger produk vir die verbruiker kan verseker. Die studie het ook gepoog om te bepaal of gedroogde ongepasteuriseerde gefermenteerde rooibos tee kan dien as ‘n matriks vir die isolasie van “Lactic acid” bakterie. Die resultate het gedui dat ongepasteuriseerde rooibos welk an dien as ‘n isolasie matriks and die volgende spesies was geisoleer deur kultuur gebaseerde metodes; Lactobacillus, Pediococcus, en Weissella. Dit bleik wel belangrik dat die mikrobioom van rooibos tee ondersoek moet word sodat die unieke groei omstandighede gespkep tydens die produksie van rooibos tee uitgebuit kan word en moontlik kan lei tot die isolasie van robuste organismes wat die kondisies van industriele produksie kan weerstaan.

Masters