The evaluation of plant extracts as natural preservatives on the chemical, microbial and sensory quality of boerewors

Abstract

Nowadays, consumers prefer the use of natural preservatives over chemical preservatives. Boerewors, a typical South African fresh sausage, is usually preserved with sulphur dioxide (SO2), which is associated with negative health effects in humans. When partly replacing a preservative with another preservative, it is essential to maintain the same product quality and safety. The aim of this study was, therefore, to investigate the effect of plant extracts as natural preservatives on the chemical, microbial and sensory quality of Boerewors.

Extracts of green Rooibos, honeybush and Rooibos consist of many beneficial uses, as they all provide antioxidant and antimicrobial activity in food products. The concentrations of use of each preservative should first be determined before it can be included in a food model. In the first part of the study, three concentrations of each of the three plant extracts were evaluated in vitro, using the disc diffusion assay, for microbial activity against five strains each of 𝘌𝘴𝘤𝘩𝘦𝘳𝘪𝘤𝘩𝘪𝘢 𝘤𝘰𝘭𝘪 and 𝘚𝘵𝘢𝘱𝘩𝘺𝘭𝘰𝘤𝘰𝘤𝘤𝘶𝘴 𝘢𝘶𝘳𝘦𝘶𝘴. The plant extracts with the most promising inhibition zones, were 0.25% Rooibos (R025), 0.50% honeybush (H050) and 2% green Rooibos (GR2). The GR2 showed the best inhibitory effect against both bacteria.

In the second part of this study, R025, H050, GR2 and combinations of each with low SO₂ (S100 = 100 mg/kg SO₂) were evaluated as preservatives in eight Boerewors models over a period of 6 days at 4 ºC. The treatments included a negative control (NC), which had 0% preservatives and a positive control (S450 = 450 mg/kg SO₂). Physico-chemical analysis was conducted in terms of pH, water activity and lipid stability at 4 oC on days 1, 3 and 6. Lipid stability, measured by thiobarbituric acid reactive substances (TBARS), were also analysed after 90 days of storage at -18 ºC. Microbial analysis in terms of total bacterial count (TBC), Gram-positive bacteria (lactic acid bacteria and 𝘚𝘵𝘢𝘱𝘩𝘺𝘭𝘰𝘤𝘰𝘤𝘤𝘶𝘴 𝘢𝘶𝘳𝘦𝘶𝘴), Gram-negative bacteria (𝘌𝘴𝘤𝘩𝘦𝘳𝘪𝘤𝘩𝘪𝘢 𝘤𝘰𝘭𝘪), 𝘌𝘯𝘵𝘦𝘳𝘰𝘣𝘢𝘤𝘵𝘦𝘳𝘪𝘢𝘤𝘦𝘢𝘦, coliforms, yeasts and moulds were performed. The sensory quality of these treatments in terms of colour stability, the evaluation of sensory attributes (colour, taste, texture, and overall acceptability) by a sensory panel, as well as thaw-, cooking- and total losses, were analysed.

No significant differences in the pH, water activity and TBARS values of the eight treatments were observed. The H050 + S100 showed the best inhibition against 𝘚𝘵𝘢𝘱𝘩𝘺𝘭𝘰𝘤𝘰𝘤𝘤𝘶𝘴 𝘢𝘶𝘳𝘦𝘶𝘴 and the total bacterial count in the Boerewors. Lactic acid bacteria and 𝘚𝘵𝘢𝘱𝘩𝘺𝘭𝘰𝘤𝘰𝘤𝘤𝘶𝘴 𝘢𝘶𝘳𝘦𝘶𝘴 were inhibited more effectively by S450 and the plant extract combinations with S100 near the end of shelf-life (day 6). The GR2 + S100 treatment had the best inhibition against the mould counts over the 6-day storage period at 4 ºC. In terms of CIE colour stability, S450 was the best in preserving the colour of Boerewors. The H050 and R025 treatments had the best colour among the treatments when evaluated by the sensory panel. The GR2 and GR2 + S100 treatments had significantly higher losses during thawing and cooking. In this research, partial replacement of SO₂ with plant extracts such as honeybush and Rooibos have been shown to be possible.