Krizman K., Yong Y., Zhou W.
National University of Singapore, Singapore

Modern consumers are becoming more aware of their personal health and demand for food that provide nutritional and health benefits.Consequently,probiotic strains belonging to genera Lactobacillus and Bifidobacterium have been progressively included in various kinds of foods as a result of growing scientific evidence on their health-promoting effects when consumed in sufficient amounts.In the past few decades,dairy products containing probiotics have become a key sector in functional food development.Since growth of monocultures in milk is limited,resulting in slow fermentation,co-culturing became a common commercial approach.However,one of the major challenges faced by industrial producers is low probiotic growth when fast-acidifying starter is used.Consequently,various techniques have been investigated in order to improve starter culture viability.

Studies conducted recently suggested high intensity ultrasound as a promising method.Ultrasound treatment is able to increase microbial viability,promote release intracellular enzymes,reduce fermentation time, and alter carbohydrate metabolism of probiotic inoculated in skim milk.However,to date no studies have been carried out on the effect of sonication on probiotic starter when cultured individually or in co-culture.In this study, the effect of high intensity ultrasound (20 kHz) on fermentative activities and viability of Bifidobacterium lactis (Bb), L.rhmanosus (Lr),L.acidophilus (La), L.paracasei (Lc) and L. delbrueckii subsp. bulgaricus (Lb) in mono and co-culture with S.thermophilus (St) was investigated.Initially, yoghurt starter was exposed to different ultrasound treatment combinations;concurrent sonication or single sonication where one of the probiotic cultures was treated and the other was added subsequently (co-culture) or not (mono-culture),and fermented until a pH of 4.70 is reached.

Results showed that survival of St immediately after the treatment was greater as compared to Lb. However, at the end of the fermentation process, sonicated yoghurts containing only Lb showed an increase in bacterial count when exposed to ultrasound waves for longer periods. Interestingly, St and Lb both release substantial amount of intracellular enzyme β-galactosidase. However, stimulated growth of the later probably resulted from higher quantities of ruptured Lb immediately after sonication,where cellular components might act as growth-promoting factor. Regardless of the treatment and probiotic combination used, fermentation time was prolonged in parallel with their exposure to ultrasound wave.

Similarly, different binary combinations of sonicated St co-cultured with intact counterpart probiotic (Bb, Lr, La or Lc) were assayed.It was observed that extended sonication period was able to stimulate cell growth in milk medium, with Bb population increasing approximately 1 log in the final product. Extended ultrasound treatment also increased lactose metabolism as a result of β-galactosidase released into milk medium and/or higher net growth observed,suggesting potential beneficial effect for lactose intolerant population. At the same time, yoghurt with greater amount of galactose was obtained, which might add to sweetening effect without increasing caloric content. Overall, we demonstrated that total fermentation time was significantly shorter when intact probiotic culture was co-cultured with sonicated St, than when in mono-culture fermentation. Our findings showed that high intensity ultrasound is a promising method when optimal conditions are used and can be applied in certain mono- and co-culture fermentation.

Keywords: High intensity ultrasound, Mono- and co-culture, Skim milk, Streptococcus thermophilus, Viability, Fermentation, Bifidobacterium, Lactobacillus, Lactose, β-galactosidase, Probiotic

Krizman K., Yong Y., Zhou W. (2016). Growth and fermentative activities of probiotic starter culture treated with high intensity ultrasound in mono- and co-culture. Conference Proceedings of IPC2016. Paper presented at the International Scientific Conference on Probiotics and Prebiotics, Budapest (p. 39.). IPC2016

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