Manufacture and characterization of a yogurt-like beverage made with oat flakes fermented by selected lactic acid bacteria
Introduction
The demand of consumers for non-dairy milk substitutes with high acceptance and functionality is increasing (Mårtensson et al., 2000). Cereal-based beverages have a huge potential either to fulfill this expectation and to act as potential vehicles for functional compounds such as antioxidants, dietary fiber, minerals, prebiotics and vitamins (Kreisz et al., 2008).
A variety of technologies (e.g., cooking, sprouting and milling) are routinely used to process cereals, but fermentation still remains one of the best choice to improve nutritional and sensory properties, and shelf-life (Mattila-Sandholm, 1998). A large proportion of cereals is traditionally and currently processed into foods and beverages through fermentation (Nout, 2009). Although several preparations remain like a house art, especially in African countries, the raw grain materials and/or the type of fermentation are the main criteria to classify cereal-based fermented beverages. Lactic acid bacteria (LAB, Lactobacillus and Pediococcus spp.), Enterobacter spp., yeasts (Candida, Debaryomyces, Endomycopsis, Hansenula, Pichia, Saccharomyces and Trichosporon spp.) and filamentous fungi (Amylomyces, Aspergillus, Mucor and Rhizopus spp.) are mainly used for the manufacture of cereal-based alcoholic beverages (e.g., tchoukoutou, jnard), non-alcoholic beverages (e.g., uji, ben-saalga), porridges (e.g., mawè), and cooked gels (e.g., kenkey, idli, and mifen) (Nout, 2009).
The selection of appropriate starter cultures for each variant of cereal beverage is an industrial need to drive, accelerate and standardize the fermentation (Coda et al., 2014). Selected starters, through their complex enzyme systems, generate metabolites (volatile and non-volatile) that provide peculiar flavor attributes to fermented cereal-based foods (Salmerón et al., 2009, Salmerón et al., 2014). Moreover, the mechanisms by which LAB fulfill the role of efficient cell factory for the production of functional biomolecules and food ingredients to enhance the quality of cereal based beverages, were largely demonstrated (Waters et al., 2013). The failure of the fermentation may lead to spoilage and/or to survival and contamination of pathogens, thereby creating unexpected healthy risks (Coda et al., 2014). Fermentation by LAB is an effective tool to prevent microbial contamination and it may positively affect the nutritional and functional features of cereal-based beverages (Katina et al., 2005). This also allows to develop market strategies to build up nutritional claims that respond to consumer awareness towards healthy diet (Coda et al., 2014).
Oat, mostly as flakes, is included in the human diet because of the healthy status, which is mainly related to the high concentration of β-glucans. Oat is an excellent source of energy and unsaturated fatty acids (Klensporf and Jeleń, 2008), and it contains dietary fibers, high quality proteins and volatile compounds (Heiniö et al., 2001, Heiniö et al., 2002). Different classes of natural antioxidants, like tocols, phenolic compounds and avenanthramides are also largely found in oat (Peterson, 2001). Although still debated in some countries, clinical data showed that oat may be included in the gluten-free diet (Sontag-Strohm et al., 2008), as recommended by Food and Drug Administration in the USA (Sontag-Strohm et al., 2008).
When collected from the field, oat lacks flavor, and the development of the aroma inevitably requires heat treatments (Heydanek and McGorrin, 1981). Without suitable heating, oat products retain flat, green, raw and bitter taste (Klensporf and Jeleń, 2008). The distinctive flavor results from lipid oxidation and n-heterocyclic compounds, which are synthesized during thermal processing of groats (Heydanek and McGorrin, 1986). Simultaneously, the thermal treatment inactivates lipolytic enzymes (Moltenberg et al., 1986), which are significantly more active in oat than in other cereals like barley or wheat (O'Connor et al., 1992). The high lipolytic activity causes the rapid release of free fatty acids, which are further subjected to oxidation, leading to an increase of rancidity (Moltenberg et al., 1986). Flaking is the typical processing for oat. It includes steam stabilization to inactivate enzymes, followed by kiln- or drum-drying to generate flavor compounds (Klensporf and Jeleń, 2008). The use of cereals as ingredient for beverage has been largely proposed in literature, but the selection of suitable fermentation conditions and starters, were only partially investigated.
This study aimed at manufacturing and characterizing the physical, chemical, functional and sensory properties of a non-alcoholic yogurt-like beverage made with oat flakes, which were subjected to fermentation by LAB. Different starters, technology options and enzyme preparations were assessed.
Section snippets
Bacterial strains, culture media and enzymes
Lactobacillus plantarum LP01, LP06, LP09, LP32, LP39, LP40, LP48 and LP51; Lactobacillus casei LC10, LC11 and LC03; and Lactobacillus paracasei LPC02 and LPC16 (Sacco Srl, Cadorago, CO, Italy) were singly used as starters for fermentation. LAB were cultivated in modified MRS (mMRS), prepared with 1% (wt/v) maltose and 5% (v/v) fresh yeast extract, final pH of 5.6 (Oxoid Ltd, Basingstoke, Hampshire, England). Fresh yeast extract was prepared by re-suspending baker's yeast (60 g) in deionized
Selection of starters and beverages
Preliminarily, different percentages (5–40%, wt/wt) of oat flakes flour (OFF) were mixed with tap water, homogenized and assessed based on sensory perception to get a yogurt-like texture. Amount of OFF lower than 20% (wt/wt) gave a too liquid matrix, allowing the fast separation of the solid particles to the bottom. Percentages higher than 30% (wt/wt) gave matrixes similar to semisolid dough. The percentage of 25% (wt/wt) was the optimum and it was used for further experiments.
All LAB strains
Discussion
Traditional cereal fermented beverages or gruels are manufactured worldwide (Blandino et al., 2003). In most of the cases, fermentation occurs spontaneously and involves mixed cultures of yeasts, bacteria and fungi, and the substrates for fermentation are mainly raw or gelatinized flour, and malted grains (Coda et al., 2011, Nout, 2009). The need for suitable LAB starter cultures and cereal-based raw materials has been recognized (Waters et al., 2013). Starter LAB would be suitable for the
Acknowledgements
The authors thank dr. Paola Carnevali (Barilla G. e R. Fratelli S.p.A, Parma, Italy) for technical guidelines, Sacco S.r.l. (Cadorago, Como, Italy) for providing the starter preparations and the Fundación Alfonso Martín Escudero (Madrid, Spain) for the postdoctoral fellowship for J.A. Curiel.
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