Volatile compounds and odour characteristics of five edible seaweeds preserved by high pressure processing: Changes during refrigerated storage

Highlights

• Volatiles of control and high pressure processed (HPP) seaweeds were determined

• The volatile fraction was altered during refrigerated storage, limiting shelf life

• Odour characteristics of control seaweeds were impaired during cold storage

• HPP had no negative effect on volatile fraction or odour after treatment on day 2

• HPP combined with cold storage preserved seaweed odour characteristics for 180 days

Abstract

Edible seaweeds Chondrus crispus, Codium fragile, Himanthalia elongata, Ulva lactuca and Undaria pinnatifida, without any treatment (control seaweeds), were stored at 4 °C for 15, 30, 60, 15 and 30 days, respectively, until microbial counts exceeded 7 log cfu/g. High pressure processed (HPP) seaweeds, treated at 400 or 600 MPa for 5 min on day 2 after collection, were held at 4 °C until day 180. At the start of storage, 133 volatile compounds were detected in control seaweeds, with 89, 31, 45, 79 and 69 compounds found in C. crispus, C. fragile, H. elongata, U. lactuca and U. pinnatifida, respectively. Chemical groups including the highest number of compounds were aldehydes (24), alcohols (23), ketones (18), hydrocarbons (17) and benzene compounds (14). At the end of storage, 131 compounds were detected in control seaweeds, including 18 aldehydes, 28 alcohols, 23 ketones, 14 hydrocarbons and 13 benzene compounds, and 164 compounds in HPP-treated seaweeds, including 23 aldehydes, 31 alcohols, 19 ketones, 23 hydrocarbons and 15 benzene compounds. Increases in the levels of alcohols, benzene compounds and S-compounds and decreases in hydrocarbons, aldehydes and acids were recorded during storage of some control seaweeds, and increases in the levels of alcohols, acids and N-compounds and decreases in aldehydes, ketones and furans during storage of some HPP-treated seaweeds. Odour acceptance fell below rejection threshold as early as on day 15 for control U. lactuca while it remained acceptable until day 60 for control H. elongata and until day 180 for all the HPP-treated seaweeds. HPP treatment at 400 to 600 MPa for 5 min, in combination with refrigerated storage, is a useful tool for seaweed preservation, given its beneficial effects on microbial quality and sensory characteristics.

Introduction

Bioactive compounds and nutrients present in seaweeds such as polyunsaturated fatty acids, phenolic compounds, fibre, vitamins, amino acids and minerals endorse their relevant contribution to the traditional diet of many Asian countries [[1], [2], [3], [4]]. In Western countries, seaweed-derived functional foods, nutraceuticals and ingredients are appealing for consumers on the basis of their health benefits [[5], [6], [7], [8]].

Shelf life of seaweeds is limited by their high moisture content which allows profuse microbial growth in such a nutrient-rich medium [9], in spite of the inhibitory compounds they contain [10,11]. Some pathogenic species have been found among the microbial contaminants of seaweeds [12], and may pose serious risks for consumers [13]. Seaweed preservation by traditional procedures such as drying or salting lowers the water activity value [14,15], but does not guarantee microbiological safety. In fact, the levels of most microbial groups did not decline during refrigerated storage of salted seaweeds [15,16].

High pressure processing (HPP) is a non-thermal procedure which offers advantages in relation to the improvement of the microbiological safety and the preservation of the nutritional properties of foods [[17], [18], [19]]. Treatments at 400 or 600 MPa for 5 min were shown to be successful in assuring the microbiological safety and extending the shelf life of Laminaria ochroleuca up to 180 days. HPP restrained growth of L. ochroleuca microbiota maintaining seaweed characteristics such as pH value, colour, texture, polyphenol content and antioxidant capacity during refrigerated storage [16]. In a later study, satisfactory results regarding the microbiological quality and extension of shelf life were obtained for seaweeds Chondrus crispus, Codium fragile, Himanthalia elongata, Ulva lactuca and Undaria pinnatifida by applying HPP treatments of 400 or 600 MPa for 5 min. From the microbiological point of view, shelf life periods of at least 180 days were achieved by 600 MPa treatments for all the assayed seaweeds [20].

Sensory characteristics of seaweeds, an appealing trait for consumers, tend to decline along refrigerated storage due to decay of exogenous and endogenous origin. Contaminating bacteria and fungi growing on seaweeds affect their aroma, colour and texture during storage [9,15,16]. Concurrently, seaweed physiological processes may contribute to alter their volatile fraction and odour characteristics during prolonged storage [21,22]. Seaweeds release volatile compounds to the surrounding ecosystems which may act as sexual pheromones, deterrents or chemical defenses against herbivores, feeding attractants and incitants, inhibitors of bacteria and fungi, and suppressors of competitive neighbours [23,24]. The volatile compounds found in seaweeds depend on the species, the geographical origin, the time of harvest, and the method used for extraction prior to analysis by gas chromatography–mass spectrometry (GC–MS), such as dynamic headspace extraction [25,26], distillation-solvent extraction [27,28] or solid-phase microextraction [[29], [30], [31]]. In a previous study, HPP of L. ochroleuca at 400 or 600 MPa contributed to minimize changes in its volatile fraction during refrigerated storage for 180 days, maintaining high odour acceptance scores [31].

There is a need for research on the effect of processing and preservation techniques on different aspects of seaweed quality, including their sensory characteristics and volatile compound composition [3,32,33]. Relationships between flavour, odour and volatile profile of some edible seaweeds before and after storage have been reported in previous studies [[31], [32], [33]]. Flavour richness of P. palmata correlated not only with the concentration of free amino acids glutamate and aspartate but also with the levels of volatile compounds 1-penten-3-ol, and (E)-2-hexenal [33]. It must be noted that the perception of umami taste of seaweeds does not only depend on the glutamate content but also on how other tastes and volatile compounds influence the taste experience [34].

Satisfactory preservation of L. ochroleuca odour characteristics was achieved by means of HPP treatments at 400 and 600 MPa for 5 min [31]. However, other seaweed species with different physiological and metabolic traits might behave differently when submitted to HPP followed by prolonged refrigerated storage. Although microbial growth was efficiently controlled by HPP during storage of C. crispus, C. fragile, H. elongata, U. lactuca and U. pinnatifida [20], information on the changes in their volatile fraction and odour characteristics along refrigerated storage is lacking. The objective of the present study was to investigate the effect of HPP and refrigerated storage on the volatile compounds and the odour characteristics of those five edible seaweeds.