Lamb is a popular meat source in New Zealand which contains of protein, fat and other micro-elements. The lamb meat is frozen after slaughter to increase its shelf life and to decrease the cost on transportation. However, the frozen lamb is considered as low quality meat because it has less stable tenderness, more drip loss and shorter colour stability than chilled lamb. High Pressure Processing (HPP) and Pulsed Electric Field (PEF) are both new technologies which are used for preserving food products. HPP treatment inactivate microorganisms and maintain microbial safety with effective cost, availability and efficiency without causing chemical reactions resulting in loss of nutrients and formation of off-flavours. And PEF is a food preservation technique where foods are exposed to a pulsed high voltage field so that the microorganisms are inactivated.

The aim of the present study was to characterize whether HPP and PEF technologies increase the nutritional values of frozen lamb in terms of the digestibility, measured as free amino acids (FAAs) released from meat proteins. Digestibility of different lamb cuts under HPP and PEF processing at three different simulated digestion phases (prior to digestion, oral (37 °C, 5 min), gastric (37 °C, 120 min), and intestinal (37 °C, 180 min)) were studied. The storage effects on the digestibility of PEF treated frozen lamb were also investigated. It was the first time to study the FAAs digestibility of HPP and PEF treated meat, particularly in lamb, by measuring the release of FAAs. In addition, the colour and moisture were also measured. A static in vitro digestion model was used to simulate the digestion process of HPP and PEF treated lamb. ANOVA and mixed-linear model analysis using R were applied for statistical analysis in this study.

Nineteen FAAs (nine essential and ten non-essential amino acids) were identified and quantified in both of HPP and PEF samples by using a commercial EZ-Faast kit and gas chromatography. For HPP samples, the total FAAs in three digestion phases of two lamb cuts (flat and inside) were significantly different from one another (P<0.0001). The total FAA contents of flat and inside cuts increased from 0 MPa to 300 MPa and decreased from 300 MPa to 500 MPa throughout the digestion. For individual FAAs, similar trends were also detected. The FAAs' digestibility of different cuts were also affected which inside cut contained higher total FAA contents after 300 MPa pressure processing than the flat. The instrumental color of HPP lamb differed largely (P<0.001). PEF processing significantly (P<0.001) increased the total amount of FAAs at all three digestion phases. The storage period for 7 days had significant effects (P<0.001) on improving total FAA contents as well. The moisture content of lamb under 200 MPa treatment was the highest after cooking in both cuts. A combination of PEF processing and 7 days of storage contributed to reach the highest value of total FAA contents at the end of the intestinal phase. It suggests that PEF associated with frozen storage for 7 days had the best effects on improving total FAA contents in lamb compared to other processing conditions. The color and moisture content of PEF samples were significantly different (P<0.01) in different muscle cuts with respect to the storage times. Among seven PEF processed cuts, the full back contained the highest total FAAs at the end of the intestinal phase, while the bolar contained the least.

To sum up, both HPP and PEF technologies have shown satisfactory characteristics as meat preserving technologies, which not only maintain microbial safety of lamb, but also promote favorable nutritional values and digestibility of lamb meats.