By Dr Phillip Strydom, Kealeboga Mosimanyana, Magdeline Magoro, Jane Boikhutso, Animal Production Institute: Agricultural Research Council
Freezing is one of the preservation technology used to preserve fresh meat (pork, beef, chicken) during long-term storage (Alonzo, et.al, 2013). This also normally applies for imported meat as it travels far distances over long durations and are often then stored for further use when needed. It is important for the meat industry to have a thorough understanding of the physical and chemical changes caused by freezer storage.
Freezing is often perceived by the consumer and even processors to reduce meat quality even though this perception is not clearly supported by scientific evidence. While most chemical processes are slowed down during freezing, prolonged freezing could affect various quality characteristics such as off flavours and taste. Hansen et al. (2004) showed that frozen storage can affect the oxidative stability of pork which in turn can affect taste and consumer acceptability of fresh and processed meat. In addition, freezing affects the functional properties of meat that could affect colour and water holding capacity but also meat tenderness and the characteristics of processed meat (Lagerstedt et al., 2008).
We simulated the process of importing frozen pork loins and ribs that are then stored frozen for various lengths of time and evaluated the effect of freezing time on various quality parameters. The research was done at the Meat Science Centre of the Animal Production Institute of the ARC.
Fresh pork loins (derind, defat, deboned, n=36) and ribs (n= 36), were purchased from a meat retailer over a period of 18 months. Six cuts of each were purchased at 18, 12, 9, 6 and 3 months before evaluation of quality was conducted. The loins and ribs were overwrapped with PVC (Polyvinyl Chloride), frozen and stored at -20°C for 3, 6, 9, 12 and 18 months, respectively. At the time of evaluation another six loins and ribs were purchased and used as fresh reference samples to study sensory quality, oxidative stability and moisture characteristics.
What did we find?
Cuts were thawed under controlled conditions before cooking or recording of colour.
The amount of drip/thaw was expressed as a percentage of the original weight of the cuts and only loins were used for this measurement. For fresh samples only the moisture formed in the bag from the time of purchase to testing was measured. As expected, the thawing loss of frozen cuts were much higher than the drip loss in fresh samples (Table 1). However, it was not expected that cuts frozen for 18 months showed slightly lower drip loss than other durations of freezing. It was more likely that longer freezing durations would cause more damage to the muscle structure due to growth in crystal size over time as reported by previous studies. Increased times of frozen storage did increase the cooking loss and the effect of freezing started to increase between nine and ten months of freezing with a difference of seven percentage units between fresh samples and those frozen for 18 months.
Colour was determined by means of a Minolta chromometer. The various colour variables were used to calculate chroma and Hue angle. Chroma measures the vividness of meat colour, while hue angle measures the amount of discolouration (higher value relates to more discolouration or browning). Lightness measures the light reflection on the meat and higher values indicate paler meat. Neither freezing or length of freezing any effect on colour or discolouration. All colour parameters were within normal specifications for pork., i.e. lightness values (L*) higher than 50 – 57 indicate PSE meat (Pale, soft and exudative).
Freezing or duration of freezing had no effect on oxidative stability as measured by TBARS. Value of 1.0 and higher indicate levels of oxidation that are likely to be sensed by consumers, e.g. rancidity. This finding was also surprising as many off-flavours were detected by the sensory panel.
Sensory attributes that were significantly affected by freezing and duration of freezing are presented in Figures 1- 4. A ten-member trained sensory panel was used to evaluate the oven broiled loin chops and ribs for typical pork aromas and flavours, off-flavours and aromas, juiciness and tenderness. A mechanical measurement of tenderness was also performed on the loin chops by means of an Instron machine equipped with a Warner Bratzler sher device. This shear device attached to the Instron consists of a blunt blade cutting or shearing through cores of cooked loin sample of standard diameter. The Instron is equipped with a load cell and therefore records the resistance of the balde shearing through the sample. Higher values (expressed in kg force) indicate tougher meat.
Typical pork fat aroma deteriorated slowly between three and nine months of freezing but significantly at 12 months of freezing and longer (Figure 1). Freezing had the same but the inverse effect on rancid fat aroma and “fishy” aromas, as scores for these aromas increased at 12 to 18 months.
Typical pork aroma of the meat portion followed the same trend as that of typical pork fat aroma, although the effect was only significant after nine months (9 vs. 12 months). The mean scores for the “negative” aromas (rancid, musty, fishy, sour) were much lower than those of the “typical” aromas since the panelists only scored for these attributes when they were recognised. Nevertheless, the significant differences among treatments (freezing times) indicate consumer will most likely distinguish between freezing durations. Duration of freezing had the same effect on typical pork flavour as on typical pork aroma and was scored lower for loins frozen for 12 or 18 months compared to fresh loins or loins frozen between three and nine months. Freezing or duration of freezing did not seem to create off-flavours in loin chops.
Loins frozen for nine months or longer was significantly tougher than fresh samples, while duration of freezing also showed a slight downward trend (Figure 2). Warner Bratzler shear force complimented the tenderness scores, measuring more than 2kg higher for loins frozen for 12 months than for fresh samples (Table 1).
Quality attributes that were significantly affected by freezing and duration of freezing of pork ribs are presented in Figures 3 and 4.
The panelists scored higher for “caramelised” aroma in fresh pork ribs compared with ribs frozen for nine months or longer in correspondence with the scores for loin samples. In contrast, ribs frozen for 12 and 18 months scored higher for rancid aroma than fresh samples and those frozen for six months.
Caramelised pork flavor was higher in fresh ribs than in any frozen rib portion. However, as duration of freezing increased, scores for caramelised pork decreased. Freezing or duration of freezing had no effect on perceived texture or juiciness of pork ribs.
In contrast to pork loins, panelists noted an increase in rancid after taste as duration of freezing progressed. Fresh ribs and those frozen for three and six months recorded lower scores than ribs frozen for 12 and 18 months. Musty after flavours were also higher in ribs frozen for nine to 18 months compared with fresh ribs and those frozen for six months. Metallic flavour after taste was scored higher in ribs frozen for three, 12 and 18 months than for fresh samples.
Pork loins and ribs were overwrapped with PVC sheets and stored for three, six, nine, 12 and 18 months at -20°C. Frozen loins and ribs were compared with fresh cuts with regards to sensory quality, colour, oxidative stability and moisture properties.
Freezing had no effect on colour but drip loss of frozen samples were higher than that of fresh samples but duration of freezing did not affect drip loss. Favourable aromas and flavours, such as caramelised and typical pork flavour and aromas in meat and fat (loin only) of rib and loin chops were negatively influenced by freezing duration but not by freezing itself (frozen vs. fresh). Favourable flavours and aromas were replaced by off-flavours and aromas such as rancid, fishy and sour, while musty and rancid after states were recorded in pork ribs. When positive flavours/aromas were replaced by off-flavours/aromas during freezing, the effects became evident between nine and 12 months.
For some attributes, samples at nine months of freezing started to deteriorate but at 18 months the negative effects were always significant. This means that in most cases frozen storage up to six months did not pose any risk to sensory quality except in the case of flavour of ribs where fresh samples scored significantly higher than al frozen cuts. Fresh loin chops were more tender and juicy than frozen chops although the negative effect of freezing only became significant at nine months of freezing. It is interesting to note that rancid aromas and flavours were noted by taste panelists despite the fact that TBARS, indicating oxidative stability, did not differ among treatments.