Effect of high power ultrasound and ageing on the physical properties of bovine Semitendinosus and Longissimus muscles

Abstract

Tenderness is an important meat quality parameters and the use of high power ultrasound to disrupt muscle structure may prove effective for reducing both myofibrillar and collagenous toughness. The experiment was carried out with Longissimus lumborum et thoracis and Semitendinosus muscles from 3 to 4 year old steers. Uncooked beef samples (60 × 40 × 20 mm) were treated with high power ultrasound (24 kHz, 12 W/cm²) for up to 240 s, and aged for up to 8.5 days before evaluation of pH, drip loss, cook losses Warner–Bratzler shear (WBS), compression hardness, and colour. Ultrasound treatment significantly reduced WBS force and hardness, but significantly increased pH. Ageing significantly reduced hardness and WBS force, but there was no significant interaction between ultrasound treatment and ageing time. Ultrasound treatment did not affect any of the colour parameters (L^∗a^∗b^∗, chroma and hue) but the ageing time significantly increased the lightness, chroma and hue. There was no significant effect of ultrasound treatment on drip loss, but it did significantly reduce the cook and total loss. During ageing, cook loss and total losses significantly increased. The results suggest that high power ultrasound is capable of reducing objective texture measurements of beef without compromising the other quality parameters investigated.

Introduction

Tenderness is one of the most important quality attributes affecting consumer satisfaction and positive perception of beef. Inconsistency in beef tenderness has been rated as one of the major problems faced by the meat industry (Koohmaraie, 1996). Tenderness of meat is determined by two major components of the skeletal muscle: contractile tissue, which is largely the myofibrillar fraction, and connective tissue fraction which determines the fixed or “background toughness” (Tarrant, 1998).

A variety of chemical, physical and mechanical methods have been introduced for tenderizing meat. While all these methods have been shown to tenderise meat, they have both disadvantages and advantages. Traditional ageing relies on endogenous proteases (Koohmaraie, 1994), however it is time consuming and its effectiveness varies between animals. Electrical stimulation is primarily used to prevent toughening due to cold shortening (Hwang, Devine, & Hopkins, 2003), but it does not improve the tenderness beyond its baseline level. Tender stretch (pelvic suspension) of carcasses (Sørheim & Hildrum, 2002) alters muscle conformation. Treating meat with exogenous enzymes (Morrissey & Fox, 1981) may cause over tenderising and needle damage when injecting enzyme solutions. Injection of brines containing sodium chloride, calcium chloride, polyphosphates and acids (Berge et al., 2001, Eilers et al., 1994, Lawrence et al., 2004) affects meat flavour and causes needle damage when injecting the brines. Mechanical tenderization methods such as blade tenderization (Hayward, Hunt, Kastner, & Kropf, 1980) cause mechanical disruption affecting both texture and appearance. High pressure processing (100–800 MPa) (Cheftel & Culioli, 1997) causes colour changes that are related to protein denaturation. In the hydrodyne process (Solomon, Long, & Eastridge, 1997) the use of explosives presents some unique operational difficulties.

The importance of tenderness in determining meat acceptability, and the need for processes that give consistent and rapid improvements in tenderness, mean that other processes must be evaluated. One option is the use of ultrasound which can cause physical disruption of materials through cavitation related mechanisms such as high shear, pressure and temperature and formation of free radicals. The ability of ultrasound to cause cavitation depends on ultrasound characteristics (e.g. frequency, intensity), product properties (e.g. viscosity, surface tension) and ambient conditions (e.g. temperature, pressure) (Williams, 1983). Growth of bubbles in ultrasound fields often involves a nucleation site, such as an existing micro-bubbles (Leighton, 1998, Williams, 1983), however cavitation can still occur in degassed samples though a higher ultrasound intensity is required (Williams, 1983). Cavitation effects have been demonstrated in a wide variety of biological samples (Williams, 1983).

Ultrasound is largely distinguished on the basis of frequency. Low frequency ultrasound (from 20 kHz to about 100 kHz) is used as a processing method because at high enough intensity it can disrupt structures, accelerate crystallisation and mass transfer, and modify chemical reactions (Mason, Paniwnyk, & Lorimer, 1996) through stable and transient cavitation causing effects such as creation of high shear and pressure on cavity collapse, microjet formation, formation of free radicals and microstreaming (Lamminen et al., 2004, Leighton, 1998). High frequency ultrasound (over 1 MHz) is used as a non-destructive sensor for materials analysis (Kundu, 2004), medical imaging (Wells, 1999), fat thickness assessment in animals (Fisher, 1997) and food process monitoring (Coupland, 2004). The ultrasound intensity required to cause cavitation increases markedly above about 100 kHz (Williams, 1983), and so cavitation is unlikely at the frequencies used for sensors. When used as a sensor, ultrasound reduces or avoids damaging changes through operating mode (e.g. use of ultrasound pulses, instead of continuous operation), frequency and by minimising the ultrasound intensity.

While a number of workers have investigated the use of low frequency ultrasound in meat tenderisation (Jayasooriya, Bhandari, Torley, & D’Arcy, 2004), the findings have been mixed. Some studies show increased tenderness with low frequency ultrasound (22–40 kHz) treatment (Dickens et al., 1991, Dolatowski, 1988, Dolatowski, 1989) or an increased tenderness with short ultrasound treatments (25.9 kHz; up to 4 min), but decreased tenderness at longer treatment times (8–16 min) (Smith, Cannon, Novakofski, McKeith, & O’Brien, 1991). Other studies showed ultrasound did not tenderise meat samples. This may be due to the use of relatively low intensity ultrasound baths (0.29–1.55 W/cm²) (Lyng et al., 1997, Pohlman et al., 1997b), or high intensity ultrasound (62 W/cm²) applied to individual regions of the meat sample for short treatment times (15 s) (Lyng et al., 1998a, Lyng et al., 1998b), which may have been insufficient to produce a tenderising effect. Raw meat treated with ultrasound (20 kHz) and subsequently convection oven cooked (Pohlman et al., 1997a, Pohlman et al., 1997c) was the same tenderness or was less tender than meat convection cooked without ultrasound treatment, cooked in boiling water or cooked using ultrasound energy. Differences in cook losses between wet (boiling and ultrasound cooking) and dry (convection) cooking techniques and cooking time may account for these results.

One study of the effect of high frequency ultrasound on meat texture has been reported. In this study pre- and post-rigor meat were treated with high frequency, high intensity ultrasound (2.6 MHz, 10 W/cm²) (Got et al., 1999). Pre- and post-rigor ultrasound treatments had small effects on raw meat texture, with ultrasound treated meat having a slightly softer raw meat texture after three to six days ageing. The difference between control and ultrasound treated sample texture had disappeared after 14 days ageing.

The continuing demand for meat tenderisation techniques and inconsistent results in previous studies ultrasound tenderising of meat mean further research is needed to clarify the applicability of ultrasound for meat tenderisation. The objectives the current study was firstly to determine the length of high power ultrasound treatment required to tenderise different muscles, secondly to assess the impact of ultrasound treatment on the ageing of meat, and thirdly to determine if ultrasound treatment was adversely affecting other meat characteristics (colour, drip, cook and total losses, and pH).