Effects of proteolysis and oxidation on mechanical properties of sea cucumber (Stichopus japonicus) during thermal processing and storage and their control

Highlights

• Lactic acid and tea polyphenols affected mechanical property of cooked sea cucumber.

• Lactic acid and tea polyphenols improved mechanical stability of cooked sea cucumber.

• Lactic acid and tea polyphenols prevented protein hydrolysis of cooked sea cucumber.

• Lactic acid and tea polyphenols prevented protein oxidation of cooked sea cucumber.

Abstract

The effects of endogenous proteolysis and oxidation on mechanical properties of sea cucumber (Stichopus japonicus) during thermal processing and storage and their control were investigated. The lactic acid (LA) + tea polyphenols (TP)-treated sea cucumbers showed relatively higher values in texture and rheological indicators than the blank control group after thermal processing. By contrast, the (LA + TP)-treated sea cucumbers also had lower values in water-soluble hydroxyproline, glycosaminoglycans and proteins, trichloroacetic acid-soluble peptide content, and more orderly secondary structure of proteins, indicating that the additives affected the mechanical properties of thermally processed sea cucumbers by preventing the proteolysis of proteins. All texture and rheological indicators of thermally processed sea cucumbers decreased time-dependently during chilled storage. The additives (LA + TP) significantly prevented the progressive deterioration in mechanical properties by retarding the changes in microstructure as well as phase state and distribution of water through preventing protein oxidation.

Introduction

Aquatic products have been regarded as delicacies due to their unique flavor, delicious taste and desirable mouthfeel, thus they are increasingly being recognised as an important source of nutrients for human health. This is due to the presence of peptides, proteins, essential amino acids, polysaccharides, lipids, vitamins and minerals (Qin et al., 2013, Singh and Benjakul, 2018). However, fresh aquatic products are prone to quality deterioration due to microbial spoilage and endogenous proteolysis, hence they predominantly appear in the form of frozen raw or cooked products in the marketplace. Frozen storage significantly prolongs the shelf life of such products, but this requires high costs due to cold chain transportation (Singh & Benjakul, 2018). Therefore, the thermally processed aquatic products which have a long-term storage at normal temperature, such as canned (hard or soft packed), pickled, and dried products, are becoming popular.

The mechanical properties of food such as texture and rheological properties have a significant effect on sensory experience of consumers like mouthfeel characteristics, which further affect consumers' acceptance of foods (Lawless & Heymann, 2010). Among them, texture property is a perceived attribute that is commonly measured and presented as some mechanical indicators such as hardness/firmness, gumminess, resilience, cohesiveness, springiness/elasticity, and adhesiveness (Lawless & Heymann, 2010). Meanwhile, rheological property is defined as the characteristic of deformation and flow of viscoelastic matter under the action of force and presented as some mechanical indicators such as viscosity, consistence, elasticity, and viscoelasticity (Borwankar, 1992). Aquatic products are much appreciated and loved by consumers because of their unique soft, tender and smooth mouthfeel (Qin et al., 2013). Normally, the composition of the food product along with its structure determines its mechanical properties (Borwankar, 1992). The edible part of aquatic products is mainly composed of proteins with both muscle and connective tissues. Therefore, structural changes such as denaturation, degradation and aggregation of proteins in aquatic products during processing and storage causes variations of their mechanical properties (Bi et al., 2016, Chen et al., 2016, Singh and Benjakul, 2018).

During thermal processing of aquatic products, two main factors, namely heat and proteolysis may lead to changes in their proteins (Singh & Benjakul, 2018). Heat treatment can cause the unfolding, denaturation, aggregation, solubilization, fragmentation, cross-linking and gelation of proteins in protein-rich food (Lund et al., 2011, Singh and Benjakul, 2018). Meanwhile, endogenous proteases in aquatic products can be activated during heating process could hydrolyze proteins including myofibrillar, sarcoplasmic, and connective tissue proteins (Singh & Benjakul, 2018). Thermally processed aquatic products undergo texture deterioration during storage, which is manifested as decreased hardness, springiness, cohesiveness, chewiness and recoverability (Chen et al., 2016, Singh and Benjakul, 2018). The changes in texture properties of aquatic products during storage are mainly attributed to the degradation of structural proteins caused by microorganisms, endogenous enzymes and oxidation reactions (Cheng, Sun, Han, & Zeng, 2013). The current research on the mechanisms behind changes in mechanical properties of aquatic products during thermal processing and storage focuses on aquatic muscle foods such as fish, shrimp and shellfish (Singh & Benjakul, 2018). However, to the best of our knowledge, related studies on aquatic products mainly consisting of collagenous connective tissue (such as sea cucumber and jellyfish) have not been fully explored.

Sea cucumbers are important aquatic species with global production of nearly 220,000 tonnes (FAO, 2018), and a gross value of over 7 billion US dollars. Body wall, the major edible part of sea cucumber, is a type of mutable collagenous tissue (Masaki et al., 2016). Therefore, taking sea cucumber (Stichopus japonicus, S. japonicus) as a representative sample, the purpose of this study is to investigate the effects of proteolysis and oxidation on its mechanical properties during thermal processing and storage and the mechanisms involved. To achieve this goal, sea cucumbers were cooked in aqueous solution containing lactic acid (LA) and tea polyphenols (TP), vacuum packaged, sterilized in boiling water, and subsequently stored at 4 °C for different periods. The texture and rheological properties of sea cucumber with various treatments were determined by using a texture analyzer and a rheometer, respectively. Meanwhile, the changing mechanisms of mechanical properties of sea cucumbers during thermal processing and chilled storage were clarified by determining total bacterial count, water-soluble hydroxyproline, glycosaminoglycans and proteins, trichloroacetic acid (TCA)-soluble peptides, moisture content, water holding capacity (WHC), phase state and distribution of water, microstructure, carbonyl content, thiobarbituric acid-reactive substances (TBARS) as well as free radical intensity.