Metagenomic and chemical analysis of Tarhana during traditional fermentation process

Highlights

• Variation in microbiota during fermentation elucidated by 16S metagenomic analysis.

• Bacterial communities of ingredients affect microbiota of traditional Tarhana.

• Besides LAB, Clostridium and Bacillus were present in sourdough microbiota.

• Dominant species was identified as Streptococcus peroris in dry product of Tarhana.

Abstract

Tarhana is one of the favourable traditional fermented food consumed as a soup. Different flour, vegetables, spices and yogurt are main constituents and they compose of microbiota of Tarhana. In this study, bacterial communities in each fermentation process and in their constituents were identified by metagenomic analysis. Also, chemical properties (pH, acidity, salt content and dry matter) were analysed in each step. The results showed that in the dough formation, mainly Lactobacillus, Bacillus, Enterococcus and Streptococcus were present and after Day 4, Clostridium and Bacillus became dominant, after drying Clostridium disappeared and in the final product bacterial communities from Bacillus and Streptococcus genus were observed. Chemical analysis showed that pH decreased from 4.94 to 4.46, acidity increased by time at the beginning of fermentation from 7.5% to 22.5% in first 6 days period, then, became stable at 14% in drying process. Salt content increased by time from 1.74 to 3.08 g salt/100 g Tarhana in first 8 days and in drying process salt content was recorded as 2.81–2.90 and dry matter was obtained as 94 g dry matter/100 g Tarhana in the final product. This study elucidated the effects of ingredients, raw materials and how microbiota and chemical properties changes during fermentation steps of home-made traditional Tarhana production and thus preparation methods could be developed to obtain standardized Tarhana products for industrial production in future.

Introduction

Tarhana, a traditional Turkish fermented food, is consisting of flour, yogurt, tomato, onion, pepper and various spices mainly mint and pickling herb. The bacterial communities in Tarhana consists of mainly Lactic Acid Bacteria (LABs) and yeast derived from yogurt and baker (Temiz & Tarakçı, 2017). These bacteria improve aroma via the proteolytic, lipolytic or amylolytic enzyme activities (De Vuyst & Leroy, 2007). Traditional fermented foods can be produced by natural fermentation without addition of starter cultures. In this natural fermentation, raw materials get fermented by their own natural microflora (Tamang, 2010). At the end of natural fermentation process at optimum temperature (30 °C) due to used ingredients or raw materials (vegetables, dairy products, flours and herbs), Tarhana dough is dried and ground to make a powder form. Tarhana powder has many advantageous properties such as high acidity and low moisture content and these properties prevent growth of bacterial contamination particularly pathogenic microorganisms (Kilci & Gocmen, 2014). Tarhana is a healthy food supplement by the way of containing essential nutrients like minerals, organic acids, vitamins and free amino acids owing to the fermentation process (Goncu & Çelik, 2020; O'Callaghan et al., 2019). Tarhana has a mild-acidic, sweet-sour and soup base taste. LABs are generally responsible for forming acid during fermentation. Bakery yeast may be used to increase the typical taste of tarhana. Because yeast's metabolism products are responsible to form the flavour of tarhana (Tamang et al., 2020). In dough fermentation process, intermediate products (Kivanc & Funda, 2017) and end product of Tarhana samples include various LABs were identified as follows: Enterococcus durans, Lactobacillus acidophilus, L. alimentarius, L. brevis, L. casei, L. plantarum, L. paracasei and Lactococcus lactis, Leuconostoc mesenteroides and Pediococcus spp. (Settanni et al., 2011; Şimşek et al., 2017). In addition, LABs show some beneficial biological properties like to decline high cholesterol levels, improve immune system, prevent infection against pathogens, regulation of blood pressure, antimicrobial and anticancer activity, antifungal and antioxidant activities (Coda et al., 2012; Schnürer & Magnusson, 2005). Lb. plantarum, Lb. alimentarius and Lb. brevis could be used as potential starter cultures for tarhana manufacturing (Özel et al., 2020). Özdemir et al. (2018) found the starter potential of Lb. alimentarius co-cultured with Pichia kudriavzevii to obtain desirable volatile and aromatic compounds in Tarhana production. Besides, some L. plantarum strains isolated from Tarhana are reported to be exopolysaccharides (EPS) producers (Sentürk et al., 2020), 4/25 Lactobacillus strains isolated from Iranian traditional fermented Tarhana have a good anti-mutagenic effect (Ahmadi et al., 2014). Three supernatants obtained from LABs (2 L. plantarum strains and P. acidilactici) isolates from Tarhana samples showed antimicrobial properties against indicator microorganisms (Settanni et al., 2011).

The microbiota of this healthy fermented food was formed using local raw vegetables, home-made dairy products and herbs. The aim of this study was to determine how this natural fermentation and used ingredients affect microbiota of Tarhana and to examine changes in microbiota profiles by 16S metagenomic analyses during fermentation process performed a traditional method. The changes in bacterial communities and chemical properties during Tarhana fermentation were also analysed in each step.