Survival of potential probiotic lactic acid bacteria on fermented green table olives during packaging in polyethylene pouches at 4 and 20 °C

Highlights

• Green olives were previously fermented by two probiotic strains of Lactobacillus.

• Olives were stored for a year at 4 and 20 °C in polyethylene pouches with brine.

• LAB counts decreased to 4.5–5.0 and 3.5–4.0 log CFU/g at 20 and 4 °C.

• Lactobacillus pentosus B281 was recovered in high levels after 12 months at both temperatures.

Abstract

The survival of selected lactic acid bacteria (LAB) with in vitro probiotic potential was studied during storage of cv. Halkidiki green olives previously subjected to inoculated Spanish-style fermentation. After fermentation olives were packed in polyethylene pouches, covered with freshly prepared brine (9%, w/v, NaCl), acidified with 2‰ (w/v) citric acid and 1.5‰ (w/v) ascorbic acid, and stored at 4 and 20 °C for 357 days. Four packing treatments were studied, namely olives previously fermented by (i) the indigenous microbiota (control); (ii) Lactobacillus pentosus B281; (iii) Lactobacillus plantarum B282; and (iv) a co-culture of both LAB strains. Microbiological analyses were performed on the olives in parallel with physicochemical changes (pH, titratable acidity, salt content, a_w and colour) at the early (day 1), middle (day 197) and final stage (day 357) of storage, as well as sensory evaluation at the end of the storage. The survival of probiotic strains was confirmed by Pulsed Field Gel Electrophoresis (PFGE). LAB decreased throughout storage reaching a final population of ca. 3.5–4.0 log CFU/g and 4.5–5.0 log CFU/g at 4 and 20 °C, respectively. The pH values ranged between 3.90 and 4.61 during storage depending on packaging condition. PFGE analysis revealed that L. pentosus B281 and L. plantarum B282 showed a high survival rate with a recovery of 100 and 96%, respectively, at 4 °C, and less than 20% for both strains at 20 °C. Finally, in the packing treatment with a co-culture of both strains, L. pentosus dominated over L. plantarum throughout storage at both temperatures.

Introduction

Table olives have been explored as a vehicle for incorporating bacterial species with probiotic potential, in an effort to develop a new plant based functional food. In the last years the focus on table olive research has shifted from the spontaneous process controlled by the indigenous microbiota to inoculated fermentations with selected starter cultures of LAB with probiotic potential in order to transform a traditional agricultural commodity into a novel high added value functional food providing new perspectives for the table olive industry (Lavermicocca et al., 2005, De Bellis et al., 2010, Argyri et al., 2014, Blana et al., 2014, Rodríguez-Gómez et al., 2014a). It needs to be noted however that the focus has been given on the production of probiotic table olives whereas there is little information on the survival of probiotic LAB strains on olive drupes during storage of the final product in retail packages. It is thus important to ensure the presence of the inoculated probiotic starter in high numbers not only at the end of fermentation but also during the shelf life of the product. In a recent work (Rodríguez-Gómez et al., 2014b), fermented Spanish style green olives were fortified with a probiotic strain of Lactobacillus pentosus TOMC-LAB2 after being packed in glass jars with brine and stored for 200 days at ambient temperature. The authors reported that the added LAB culture was able to colonize the olive surface and presented high recovery rates at the end of the shelf life, providing thus the possibility of successful enrichment of the microbiota of olive drupes with selected multifunctional starters. In another work (Argyri et al., 2015), green olives subjected to inoculated Spanish-style fermentation with probiotic LAB strains (L. pentosus B281 and Lactobacillus plantarum B282), were packed in polyethylene pouches under modified atmospheres (70% N₂–30% CO₂) and stored at 4 and 20 °C for 12 months. The authors reported that both strains presented high survival rates during storage with L. pentosus B281 exhibiting higher survival rates (94.1%) after six months of storage.

However, the application of modified atmospheres in polyethylene pouches is not a common practice employed by the table olive industry today that still prefers to use brine as a covering liquid in the pouches or other containers. Consequently, the present study is a continuation of a previously published work (Argyri et al., 2015) aiming to evaluate the ability of two Lactobacillus strains, namely L. pentosus B281 and L. plantarum B282, to survive and retain adequate populations during storage of Spanish style fermented green olives packed in polyethylene pouches and covered with brine for an extended period of time (12 months). Both selected LAB strains were investigated for their in vitro probiotic potential (Argyri et al., 2013) and employed successfully as starters in Spanish-style green olive fermentation (Blana et al., 2014).