International Dairy Journal 17 (2007) 657–665 Survival and activity of selected probiotic organisms in set-type yoghurt during cold storage O.N. Donkor, S.L.I. Nilmini, P. Stolic, T. Vasiljevic, N.P. Shah à School of Molecular Sciences, Victoria University, Werribee Campus, P.O. Box 14428, Melbourne, Vic. 8001, Australia Received 14 October 2005; accepted 11 August 2006 Abstract The growth and metabolism of two probiotic organisms (L. acidophilus LAFTI s L10 and Lactobacillus casei LAFTI s L26) and a regularyoghurtculture(L. delbrueckii ssp. bulgaricus Lb1466and Streptococcus thermophilus St1342)werestudiedinyoghurtcontaining 0.5%,1.0%,and1.5%(w/v)ofhighamylosecornstarchpowder(Hi-maize s )orinulin.Viablecellcountsofprobioticorganisms,their metabolites and proteolytic activities, and viscosity of the yoghurts were determined during refrigerated storage for 28d at 4 o C.Inthe presence of inulin, cultures showed better retention of viability (8.0logcfug 1 ) in comparison with that of Hi-maize, which had a reductionbyonelogcycle.Lowerconcentrationsof0.5–1.0%Hi-maizeimproved(P o0.05)theproductionofpropionicacidandalso increasedproteolyticactivityofprobioticorganismssubstantially.Agreaterreleaseoffreeaminoacidsmayhavesustainedbettergrowth of the organisms in yoghurts. Supplementation with either Hi-maize or inulin increased the viscosity of probiotic yoghurts significantly (P o0.05). r 2006 Elsevier Ltd. All rights reserved. Keywords: Prebiotics; Probiotics; Proteolytic activity; Viscosity 1. Introduction The presence of live microorganisms, in particular lactic acid bacteria (LAB), in food has been traditionally associated with certain health benefits. Previous investiga- tors have attributed such health effects to shifts in the intestinal microbial balance (Schrezenmeir & De Vrese, 2001). This shift is attributed to probiotics, live micro- organisms that, when added to foods, beneficially restore microbial balance in the gut flora of the host (Gibson & Roberfroid, 1995). In order to exert any beneficial health impact, the concentration of probiotics in a product that serves as a delivery system needs to be high. However, no general agreement has been reached on the recommended levels,andsuggestedlevelsofprobioticsfrequentlyreferred to as therapeutic levels ranged from 10 6 (Kurmann & Rasic, 1991) to over 10 7 or 10 8 cfumL 1 (Lourens- Hattingh&Viljeon,2001).Themainobstacleforachieving andmaintainingrequiredlevelsisthestrain-dependenceof probiotic organisms and poor survival due to increased acidity and oxygen tension (Dave & Shah, 1997; Shah, 2000a; Talwalkar & Kailasapathy, 2004) and nutrient depletion (Dave & Shah, 1997; Shah, 2000b). In order to overcome these problems, different approaches have been used, including culture selection (Lourens-Hattingh & Viljeon, 2001; Tuomola, Crittenden, Playne, Isolauri, & Salminen, 2001), microencapsulation (Capela, Hay, & Shah, 2006), and addition of prebiotics (Bruno, Lankapu- tra, & Shah, 2002; Capela et al., 2006; Corcoran, Ross, Fitzgerald, & Stanton, 2004). Prebioticsarenon-digestiblecomplexcarbohydratesthat selectively stimulate the growth and/or activity of bacteria inthecolonandalsobeneficiallyaffectthehost(Gibson& Roberfroid, 1995). They may exert a protective effect towards selected probiotic bacteria in improving their survival and activity during storage of the product containingprobioticsaswellaspassagethroughtheupper parts of the gastrointestinal tract (GIT). A range of oligosaccharides has been tested in this regard (Kaplan & ARTICLE IN PRESS www.elsevier.com/locate/idairyj 0958-6946/$-see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.idairyj.2006.08.006 à Correspondingauthor.Tel.:+61399198289;fax:+61399198284. E-mail address: Nagendra.Shah@vu.edu.au (N.P. Shah).