3. Results and discussion
3.1. Expressible water
The expressible water of fresh mackerel muscle was 26.6 2.4%
before cooking and 34.6 3.5% after cooking. This parameter is
related to the fish meat water holding capacity and affects product
juiciness perceived by consumers. Therefore, fish processing should
have no more than a minimum effect on this parameter to retain an
acceptable product sensory quality. After frozen storage for 3
months, expressible water for Atlantic mackerel muscle with no
HPP treatment increased to 38.2% and 48.3% in raw and cooked
muscle, respectively. HPP treatments yielded expressible water
values higher than those for fresh mackerel muscle for any frozen time considered (Table 1). However, values for some HPP-treated
samples were lower than those for frozen controls with no HPP
treatment. Since the three independent variables (pressure level,
holding time and frozen time) showed an effect on the expressible
water of raw samples, a multifactor ANOVA was carried out to
assess their relative influence yielding a significant model
(p < 0.0001). The evaluation of the F-values of the three variables
confirmed that expressible water was highly affected by the pressure
level although an important effect of frozen storage could also
be concluded. The correlation coefficient r2of the model was 0.78.
The prediction of the model obtained for the effect of the two
variables that exerted a higher influence on expressible water
(pressure level and frozen storage time) is shown in Fig. 1. The
employment of HPP as a pre-treatment to freezing and frozen
storage can lead to a significant expressible water increase if high
levels of pressure are selected. However, HPP at 150 MPa yielded
expressible water values lower than 40% reflecting a water holding
capacity sufficient for a desirable juiciness and thus an improved
frozen muscle quality. An expressible water of 38.7% was considered
optimal for low-salt restructured fish products from Atlantic
mackerel (Martelo-Vidal et al., 2012). The effect of HPP pretreatment
and frozen storage on expressible water of cooked
fishes was evaluated by multifactor ANOVA. Although an F-value of
6.17 implied that the model was significant, the correlation coefficient
r2 was very low (0.37).