Transglutaminase and high pressure effects on heat-induced gelation of Alaska pollock (Theragra chalcogramma) surimi

A prior high pressure processing (HPP) treatment has previously been shown to enhance subsequent (at atmospheric pressure) low temperature 'setting' (glutamyl-lysine, non-disulfide protein crosslinking) induced by transglutaminase (TGase; endogenous or added microbial enzyme). This enhanced setting induces greater gel strength and deformability of subsequently cooked surimi gels. We sought to determine if 25 °C incubation carried out simultaneous with, rather than following, HPP might similarly induce a setting effect on subsequently cooked gels. Pollock surimi pastes (± added microbial TGase) were subjected to HPP 200, 300, or 400 MPa; at 5 °C (little or no setting effect expected during HPP at this temperature) for 30 min or at 25 °C (optimal setting temperature for pollock pastes) for 30, 60, or 120 min. Pastes were then directly cooked (90 °C for 20 min) (pressureP°C/time > C), or first allowed to undergo setting at atmospheric pressure (25 °C for 30, 60, or 120 min), followed by cooking (90 °C 20 min) (pressureP°C/time > Stime > C). With no microbial TGase added to the raw paste (e.g., endogenous TGase only), 300MPaP5°C/30min > S25°C/120min > C induced highest gel fracture stress and strain. The same treatment but with HPP at 200 or 400 MPa gave only slightly lower fracture stress (gel strength). Increasing the temperature of the HPP treatment to 25 °C (300MPaP25°C/30min > S25°C/120min > C), even with longer HPP time (300MPaP25°C/60 or 120min > C), gave weaker gels, similar to those obtained by setting and cooking without a prior HPP treatment (S25°C/60min > C). Thus, attempting to induce TGase crosslinking by setting at 25 °C during HPP treatment actually seemed detrimental to gel strength development. However, when HPP was carried out at 25 °C and microbial TGase was added, gel strength and deformability (fracture stress, strain) were enhanced above that of all other treatments tested. All treatments containing microbial TGase evidenced enhanced protein polymerization. Scanning electron microscopy revealed a more dense and fibrous structure in such gels, and reduction of free thiol (SH) groups was noted as a result of microbial TGase addition.