Fermentation of reconstituted milk by Streptococcus thermophilus: Effect of irradiation on skim milk powder Irina Stulova a, b, * , Natalja Kabanova a, b , Tiina Kri  s  ciunaite a, b , Anastassia Taivosalo a, b , Tiiu-Maie Laht a, b , Raivo Vilu a, b a Tallinn University of Technology, Ehitajate tee 5,19086 Tallinn, Estonia b The Competence Centre of Food and Fermentation Technologies (CCFFT), Akadeemia tee 15a,12618 Tallinn, Estonia article info Article history: Received 25 July 2012 Received in revised form 28 January 2013 Accepted 1 February 2013 abstract The growth of Streptococcus thermophilus ST12 (ST12) in milk reconstituted from non-irradiated and irradiated at 10 kGy low-heat skim milk powders (RSM and irrRSM, respectively) at 40  C was monitored by microcalorimetry. Statistically significant differences of the growth patterns of ST12 in RSM and irrRSM were found. Distinctively diauxic growth curves in RSM were replaced by one-stage growth curves in irrRSM. The final pH in RSM was 5.56 while in irrRSM samples it was 4.41. The time of initiation of gel formation was about 36 min shorter; however, the gels were considerably weaker in irrRSM than in RSM. The front-face fluorescence spectra were also used to characterise the differences in acidification processes. The microcalorimetric data together with the concentrations of metabolites determined during fermentation, rheological and fluorescence measurements indicated the substantial changes in the growth of ST12 in irrRSM in comparison with RSM. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Starter cultures containing lactic acid bacteria (LAB) have an important role in the manufacture of cheese and fermented dairy products. The starters are selected for rapid lactic acid formation and for their ability to contribute to the development of the aroma, flavour and texture of the products e these features being deter- mined by a diverse range of metabolic peculiarities (Leroy & De Vuyst, 2003). Streptococcus thermophilus is the second most important dairy starter after Lactococcus lactis (Hols et al., 2005; Thomas & Crow, 1984). It is a thermophilic LAB and has traditionally been used in combination with Lactobacillus delbrueckii subsp. bulgaricus or Lactobacillus helveticus for the manufacture of yoghurt and so-called hard ‘cooked’ cheeses (e.g., Emmental, Gruyere, Grana), respectively (Delorme, 2008). S. thermophilus is also used alone or in combination with lactobacilli for the production of mozzarella cheese (Mills, O’Sullivan, Hill, Fitzgerald, & Ross, 2010). The use of S. thermophilus in combination with the mesophilic bacteria L. lactis subsp. cremoris or subsp. lactis as starters in experimental Cheddar cheese has also been reported in the liter- ature (Champagne, Gagnon, St-Gelais, & Vuillemard, 2009; Michel & Martley, 2001). Skim milk powder is among the most abundantly produced dairy products. It is used in a multitude of food applications, many of which require the powder to be reconstituted (Martin, Williams, & Dunstan, 2007). Milk reconstituted from powder is frequently used for investigations of cheese-making processes in pilot-plant conditions (Mohamed, Morris, & Breene, 1982; Peters, 1960; Wang et al., 2012; White & Ryan, 1983) as well as in laboratory practice for studying of activity of starter bacteria (Christopherson & Zottola, 1989) and propagation of mother starter cultures (Horrall, Elliker, & Kensler, 1950). In the early days of the dairy in- dustry raw milk was used as a medium for growing starter cultures at dairies. Considering that raw milk quality variations can affect the uniformity of starter activity, raw milk has been replaced by milk reconstituted from skim milk powder of selected producers for improving product quality and for better control of the pro- duction processes (Whitehead, Ayres, & Sandine, 1993). Skim milk powder, which is specially screened to ensure that it is free of an- tibiotics, is used as the standard medium for growing starter bac- teria and for monitoring the acidification activity of LAB. The powder is reconstituted to milk containing 12% solids and heated at 85e95  C for 30e60 min (Ranken, Kill, & Baker, 1997). Irradiation has been found to be a prospective technology to ensure food safety and sterility with minimum influence on the functional, nutritional, and sensory properties of some products (Chauhan, Kumar, Nadanasabapathy, & Bawa, 2008; Farkas, 2006; Grolichová, Dvo rák, & Musilová, 2004; _ Zegota & Ma1olepszy, 2008). * Corresponding author. Tel.: þ372 6 202 831. E-mail addresses: irina.stulova@tftak.eu, irinastulova@yahoo.com (I. Stulova). Contents lists available at SciVerse ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj 0958-6946/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.idairyj.2013.02.004 International Dairy Journal 31 (2013) 139e149