A Whole Chain Approach to Lamb Eating Quality
J E Vipond1, R I Richardson2, E A Hunter3, S Edwards4 and G Simm1
1SAC, West Mains Road, Edinburgh EH9 3JG, UK
2University of Bristol, Langford, Bristol BS40 5DU, UK
3BioSS, James Clerk Maxwell Building, The King's Building, Edinburgh EH9 3JZ, UK
4Newcastle University, Newcastle on Tyne NE1 7RU, UK
Corresponding Author: John Vipond. E-mail: john.vipond@sac.co.uk
Running Head: Factors Affecting Lamb Eating Quality
Abstract
This investigation aimed to find out whether the application of known enhancements across
commercial farms and abattoirs could improve the lamb eating experience enough to differentiate
the product. The effect of increasing carcass weights without increasing fatness was also
investigated. Participating farms selected wether lambs at weaning that were above average in terms
of weight and leanness. These lambs were finished on a planned system that avoided growth
checks in the final period. At slaughter, lambs from participating farms (enhanced farm protocol)
were selected in a target carcass weight range of 16-21 kg for Blackfaces and 21-26 kg for
crossbreds, at a target fat class of 3L. Carcasses from other flocks sent to the abattoir on the same
day, which were generally lighter, were chosen as basal samples. At the abattoir, the carcasses from
both enhanced and basal groups were subjected to either an enhanced or a basal abattoir procedure.
Basal samples were allowed to progress straight to the chiller without any post-slaughter
interventions. Enhanced processed samples were electrically stimulated (either low or high voltage,
depending on the abattoir). At boning, the left carcass side loins were cut, vacuum packed and
conditioned for five days and the right side loins for ten days. Twenty-two farms and three
abattoirs took part. A trained sensory panel at Bristol tested all the meat samples.
Heavier carcasses are shown to be of acceptable texture and flavour, the main components of eating
quality. There was a suggestion that this meat may be less juicy and this needs further
investigation. The enhanced farm protocol had little effect on eating quality. Post-slaughter
enhanced processing and conditioning had a major, positive impact on most attributes of lamb
eating quality. Electrical stimulation had a much greater impact in improving meat texture than the
length of time meat was conditioned. Seasonal effects were minor and whilst texture did not
deteriorate, the trained sensory panel did detect an increased abnormal flavour in late season lamb.
This was significantly higher in crossbred than Blackface lambs.
Keywords: lamb, meat quality, sensory attributes, farm factors, processing factors
Introduction
Many factors across the production and processing chain affect the eating quality of lamb. By
combining best practices across the chain it may be possible to differentiate lamb on eating quality.
Identification of the relative importance of farm and abattoir procedures can direct effort to where it
will have most effect. Small carcass size of lamb relative to beef adds to processing cost and lamb
retail price. Advances in genetics and feeding on farms and semi –automatic processing
development in abattoirs now make larger carcass size attractive. Knowing the eating quality of
the heavier carcasses is important to the uptake of these technologies.
Differences in meat eating quality between lamb breed and sires within breed are generally small;
there is also some evidence for small differences in flavour between breeds but this variation is
mainly attributable to diet (Fisher et al, 2000).
Highest meat eating quality is associated with lambs slaughtered off the mother (Vipond et al,
1995).
In the northern hemisphere, older/heavier spring born lambs slaughtered in the autumn/winter are
not as tender as those finished in the summer. Confounding effects with season of slaughter cannot
be completely excluded, e.g. lower ambient temperature in winter increasing the incidence of cold
shortening. There is also a tendency for undesirable flavours to develop in older animals.
Fatness has a weak positive relationship with eating quality measured in a sensory laboratory, in
particular tenderness. Due to the negative reaction of consumers to fatty cuts, these benefits are
small. Lean cuts of good visual appearance, which avoid difficult trimming of inter muscular fat
from within roasting joints, are preferred.
Shortening of the muscle prior to rigor mortis gives a dramatic toughening effect. It has been found
that the minimum shortening for lamb occurs at chilling temperatures of 5 to 20º C and maximum
shortening occurs at 0º C and 40º C (Cook and Langsworth, 1966; Geesink et al, 2000). Lambs are
particularly susceptible to cold shortening because of the small size of the carcase which enables
rapid heat loss post slaughter. There is conflicting evidence as to whether meat which has been
shortened responds to ageing.
The key factors in determining shortening are low or high temperatures and pH over 6.2 coupled
with a sufficient ATP level (3.5μ mole/g muscle) for contraction. Chilling to avoid a temperature
below 10ºC within 10 hours of slaughter will avoid cold shortening and hot shortening is rarely an
issue provided carcases are chilled following slaughter. The loin muscle (Longissimus thoracis et
lumborum) because of its shape and location in the carcase is susceptible to cold shortening and a
good marker. The insulating effect of surface fat on the carcase protects against cold shortening,
Smith et al (1976).
Low or High voltage electrical stimulation can improve tenderness and was developed primarily to
allow rapid chilling without the risk of cold shortening. The electrical current applied stimulates
the muscles to contract and hence use up ATP (and hence glycogen). This accelerates the onset of
rigor mortis enabling chilling to take place earlier. It is most effective on tougher carcases thus
helping to reduce variation.
Low voltage stimulation (LVES) must be applied whilst the nervous system is still intact, i.e. when
the animal is being bled and is a less reliable approach than high voltage stimulation. There is also
the risk of a toughening effect of LVES due to hot shortening. Where LVES is effective it seems to
simply prevent cold shortening (e.g. Hildrum et al, 2000; Devine et al, 1999).
High voltage stimulation depends on an intact nervous system. It is applied later on the slaughter
line when the carcase has cooled to an extent where hot shortening is not induced by stimulation. It
has additional benefits through accelerating the ageing process and direct physical modification of
the muscle fibre structure (Dutson et al, 1975; Savell et al, 1977; Savell et al, 1978).
An alternative approach to electrical stimulation is to delay chilling until the muscles of the carcase
have already gone into rigor mortis. It seems likely that all this achieves is the prevention of cold
shortening. Care also has to be taken to avoid hot shortening which could occur if the holding
temperature exceeded 18ºC. Overall this approach adds little to the benefits obtained by control of
chiller temperatures to avoid cold shortening. It can also result in poorer colour stability on retail
display.
The state of contraction of muscles post mortem is a significant factor moderated by the attachment
of the muscles to the skeleton. The tension imposed on any individual muscle in the carcase
depends on the position of the skeleton. Supporting a lamb carcase in a ‘natural’ standing position
improves the loin (longissimus dorsi) and leg (Biceps femoris, semimembranosus and gluteus
medius). Davey and Gilbert (1973) showed hip suspension as a single factor in abattoir processing
yielded the biggest improvement in tenderness.
The storing of lamb at low temperature for a period of time is widely used to increase tenderness.
The rate of tenderisation decreases with time and is due to enzymes breaking down the myofibrillar
structure. Dransfield (1990) calculated that 80% of the tenderisation of lamb longissimus dorsi has
occurred at 7.7 days post slaughter at 1ºC.
Materials and Methods
Overall Design
This study used commercial farms and abattoirs to test packages of measures to improve the eating
quality of the lamb.
Three abattoirs were used (Plant 1, Plant 2 and Plant 3). Blackface (hill breed ) lambs were
slaughtered in January and March at Plants 1 and 2. Crossbred (lowland breed ) lambs were
slaughtered in September and November at Plants 1 and 2 and in September, November, January
and March in Plant 3, Table 1.
At each of the twelve slaughter days, two farms that agreed to use “best practice” methods to supply
“heavy” lambs (enhanced Farm Protocol) were matched with two randomly selected farms (basic
Farm Protocol), supplying sheep to the abattoir on the same day. From each farm two lambs at
MLC score 3L were selected. One lamb from each farm received the basal abattoir treatment and
the other an enhanced treatment. Each carcass was split and one half was conditioned for 5 days
and the other half for 10 days. A summary of the post-slaughter process is given in Figure 1. Thus
participating farms and abattoirs adopted agreed enhanced practices, some of which they did not
already use.
A highly experienced sensory panel was used to test the meat as freshly - cooked hot meat.
Farm Protocol
Normal UK specification for lamb is 16-21 Kg. Blackface lambs are often below this weight at 1419 Kg. Crossbred lambs usually meet the specification but are sometimes overweight at 19-22.5
Kg. To obtain heavier carcasses at the correct fatness levels, farmers were identified who were
using improved genetics and asked to implement MLC/QMS best practices, as described below:
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Only wethers of above average weight at weaning were included
Either hill Blackface or lowland crossbred lambs were included
Lambs were suckled for at least 3 months, then weaned according to a protocol to reduce
stress
Lambs were finished on the farms’ conventional finishing system
There was a planned finish – lambs were not subjected to a period of weight loss and were
fed diets checked for adequacy of protein and energy content in the finishing period.
Enhanced
All lambs were suckled for at least 3 months. Ewes were removed from grazing to reduce stress,
rather than moving the lambs. Lambs were weighed, tagged and an “enhanced” group of wethers
with above average weight were identified. Lambs were killed from November onwards and a
minimum of 40 days pre-slaughter, were reweighed and the diets checked to ensure they were
adequate for energy and protein and that lambs were predicted to finish on time using conventional
feeds.
Farmers completed a checklist of procedures on despatch of animals for slaughter, the purpose of
which was to ensure that lambs left the farm and arrived at the abattoir relatively unstressed and in a
clean condition according to transport regulations. Transit time was recorded to ensure no undue
delays occurred and all hauliers were farm assured. Ambient and lairage temperatures were
recorded on arrival and lambs were allowed to rest for one hour before being moved forward for
slaughter with minimum disturbance. Lambs were electrically stunned and killed by
exsanguination in all plants.
Basal
The basal sample was taken from lambs presented at the abattoir from random farms supplied on
the same day and thus reflecting the normal stream of lambs at that time of year.
Within each of three meat plants, eight - 2 farms on each of 4 occasions, selected farms provided up
to 20 heavy lean lambs of which four were filtered off on the basis of carcass weight for taste panel
evaluation. The enhanced carcass weight-range targets were 21-26 Kg for crossbred lambs and 1621 Kg for Blackface lambs at fat class 3L . For basal farms, the carcass weight requirement was
19±1.5 Kg for crossbred lambs and 14.5±1.5 Kg for Blackface lambs also at fat class 3L. Half of
the enhanced farm or basal farm carcasses were subjected to an enhanced abattoir procedure and the
other half to a basal abattoir procedure.
Carcass Processing
In the two abattoirs where LVES was applied (90 volts, for 60sec), carcasses were selected at the
grading station and moved to a separate line. At the third abattoir, HVES was applied automatically
and so it was the non-stimulated samples which were selected and removed from the line so as to
avoid stimulation. At boning, one carcass from each treatment was selected which conformed to pH
criteria (i.e. not greater than pH 5.8 at boning). Both the enhanced and basal carcasses were split.
Loins were collected and the left loins allocated to 5d conditioning and right loins to 10d
conditioning. In this way a very precise measure of conditioning time was built into the design.
Sensory testing (trained taste panel)
Cooking
Loins were thawed at room temperature, stored overnight in a refrigerator at 4º C, then cut into ten
2 cm- thick steaks. These were cooked, turning every 3 minutes, under the grill of a household
cooker until the internal temperature of the lamb reached 75º C in the geometric centre. The lamb
was then placed in a GENLAB holding oven, set at 60º C, until sub-sampled and cut into sections
approximately 2 x 2 x 2 cm, removing fat and connective tissue. These were wrapped in pre-coded
aluminium foil and served to individual assessors.
Sensory Assessment
Ten assessors (all female, age range 25 – 60), previously screened and selected, were asked to rate
samples on an 8 point scale for:
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texture (1 = extremely tough to 8 = extremely tender),
juiciness (1 = extremely dry to 8 = extremely juicy),
lamb flavour intensity (1 = extremely weak to 8 = extremely strong),
abnormal flavour intensity (1 = extremely weak to 8 = extremely strong).
All assessments were completed in a purpose-built panel room, illuminated with red light,
comprising individual booths each fitted with a sensory computer that facilitated direct entry of
results by the assessors. Two samples were tested in each of 8 sessions within a day. The project
required 12 full days in the sensory laboratory.
Statistical design and analyses
The three main design elements were identification of suitable animals, processing of meat samples
for evaluation and the design of the sensory testing procedures.
As a side-by-side comparison allows assessors to make more precise judgements on differences
between samples, the order in which samples were tested were chosen to maximise precision for
conditioning treatment and enhanced vs basal procedures. At each session, assessors received two
samples from left and right sides of the same lamb (5d conditioning vs. 10d conditioning). Hence,
both samples had received the same processing procedure for both farm and abattoir. In the three
sessions immediately following, they received similar samples, but with either a different farm or
different abattoir processing regime until all four combinations of basal or enhanced farm or
abattoir regimes had been tested. The sequence was repeated for the final four sessions of the
morning for samples from the same abattoir, but a different kill date. Different combinations of kill
date were combined on different days. The order of sample presentation was structured to reduce
the influence of first-order carry over effects. Samples from different abattoirs were tested on
different days.
A model with several random (error) terms, together with fixed terms was fitted using Residual
Maximum Likelihood (REML). The statistical program Genstat was used for this purpose.
The Fixed model was
Order (of testing) + Breed + Abattoir*Season*Farm Protocol*Processing*Conditioning
and the Random model
Assessor/Day/Block/Session/Order+Kill/Animal/Sample
The Random model was shown to be justified by the data using tests of differences in deviances.
Results
Twenty-two enhanced farms supplied lambs killed on four slaughter dates at each of three abattoirs
between September 2003 and March 2004. On two out of twelve occasions one of the two
enhanced farms failed to supply tagged lambs. Fortunately there were sufficient tagged lambs from
the other farm to allow work to proceed. Basal lambs were sourced from two farms at each kill and
their carcasses matched the enhanced carcasses in terms of breed type and conformation/fatness
score, but were lighter in weight.
Table 2 shows lambs achieved the desired distribution and means of carcass weights. Enhanced
lambs sampled for sensory analysis were significantly heavier than basal lambs, by 3.1 Kg for
Blackface and 4.4 Kg for crossbred lambs.
Enhanced lambs had slightly more lambs in fat class 3H and slightly more in higher conformation
grades see table 3 but overall carcass fatness and conformation did not differ significantly.
Farm protocol had no effect on pH of lamb carcasses measured at two hours post slaughter, but the
processing protocol did (Table 4). The results demonstrate stimulation was successful as all
enhanced processed carcasses had a lower pH post-stun than the basal processed carcasses, LVES
reduced pH by 0.3-0.35 units, HVES reduced the pH by 0.6 units. There were no differences in pH
at 24h post-stun when carcasses had virtually reached their ultimate pH.
All three plants had a rapid rate of chill, plant 2 chilling the fastest and to a significantly lower
temperature at 24 hrs of 0.8º C ( plant 1 and 3 achieved 2.4 and 1.4 respectivly sed 0.95). Plant 3
reduced the temperature by some 15º C in the first 2 hours, suggesting the temperature would have
dropped below 10º C in the next 8 hours and some cold-shortening might have been expected.
Table 5 shows weight and enhanced procedures on farms had no effect on eating quality other than
a small reduction of juiciness by 0.17 unit with enhanced weight whereas table 6 shows major
effects of processing treatments. The enhanced treatment produced an increase in the rating for
texture of 1.4 units which is very large in sensory terms, increasing the rating for lamb from slightly
tough/tender to moderately tender on average. There were no effects of processing on juiciness,
lamb flavour or abnormal flavour. There was no suggestion that juiciness declined with increasing
tenderness. Conditioning improved texture (more tender) by around a third of a point across all
treatments and also significantly increased the flavour of lamb.
There were no significant breed effects for texture, juiciness or lamb flavour. Differences in breed
are unlikely to be detected by consumers in terms of eating quality. Overall season had no effect on
texture, juiciness or flavour but there was a significant (p<0.01) increase in the abnormal flavour
strength for old season lamb in January (3.51) and March.(3.19) compared to Sept (2.56) and Nov
(2.83). This was due to differences between crossbred lambs only.
There were no significant interactions between either farm and abattoir processing treatments.
However there were significant interactions between electrical stimulation and conditioning time,
see table 7. Conditioning improved the basal processed lamb by around 0.45 of a unit but improved
the enhanced processed lamb by only 0.21 of a unit. Thus the extra conditioning added little to the
tenderness of electrically simulated lamb..
Plant 3 appeared to produce tougher meat at basal levels of processing than plants 1 and 2, but the
use of HVES compensated for this. HVES increased tenderness by 2.16 units compared to 1.00
units, on average, for LVES.
Discussion
It was hypothesised that by combining best practices across the chain it may be possible to
differentiate lamb on eating quality. In this study, enhanced pre-slaughter protocols had little effect
on sensory attributes compared to those of the basal sample. It is well known from other studies that
low growth rates, growth checks and stressful handling of animals, can negatively affect meat
quality. Apart from the weight of lambs they submitted, basal farms were not deliberately chosen
to have contrasting management practices to enhanced farms. Basal farms were intended to reflect
typical throughput for that abattoir, at that time. The fact that no significant effect of the preslaughter enhanced protocol was detected here may be due to good practice in the basal farms
supplying the participating abattoirs.
Useful insights into the relative benefits to eating quality of the various post slaughter treatments
were revealed, conditioning slightly reduced the juiciness of basal processed lamb, but increased
juiciness of enhanced processed lamb. Also, it increased lamb flavour of enhanced processed lamb
whilst reducing abnormal flavour. Taken overall, the extra conditioning gave favourable responses
to enhanced processing producing the best combinations of sensory attributes as assessed by a
trained test panel.
Five day conditioned samples with enhanced processing were much more tender than ten day
conditioned samples from basal processing. It is probable that electrically stimulated meat with no
conditioning would be more tender than the non-stimulated meat with 10d conditioning. Thus it
would be viable for plants to install a simple LVES system and retain their rapid throughput,
producing more tender meat than they do now, which could be retail packed at cutting thus saving
on chiller space. Selected carcasses could be conditioned to produce even more tender meat for
specialised markets.
With relation to the design of the taste panel evaluations, the order of treatments meant the most
precise comparison was for conditioning time as this was a comparison done within sessions.
Enhanced and basal treatments, both on-farm and in-plant, were the next most precisely compared
as they were tested in adjacent sessions within the same half of the day. Two kill dates were tested
within the same day from the same plant. The least rigorous test was between plants as these were
always tested on different days. Tentative comparisons can be made as the panel is highly
experienced and when tested with repeat samples some weeks later have produced very similar
results. The larger increase in tenderness produced by HVES than LVES is almost certainly real.
The overall average difference in texture between stimulated and non-stimulated carcasses for
plants 1, 2 and 3 was 0.88, 1.13 and 2.16 respectively. Plant 3 operated a HVES unit and could
therefore move carcasses to chill immediately and employ an initially faster chill (Table 5) without
the danger of inducing cold shortening. Hence, this would explain why basal carcasses from this
plant tended to be tougher than from the other two. They may have experienced some cold
shortening, which would not normally have occurred, as all carcasses would normally be
stimulated. With HVES they produced some of the most tender meat, although the average
tenderness was not statistically significantly different between plants.
Conclusions
Key findings are that enhanced processes applied across the whole of the meat chain significantly
improve eating quality of lamb but that the effects post slaughter are much greater than on farm
procedures.
Electrical stimulation had a much greater impact in improving meat texture than the length of time
meat was conditioned. Of all the factors considered in this study, it has the greatest influence on
meat quality. HVES or where applicable LVES plus a 10 day conditioning period give the best
outcomes. Conditioning also enhanced HVES carcasses to a lesser extent.
The study also showed it was possible to produce heavier carcasses of over 16 kg for Blackface and
over 23 kg for crossbreds with acceptable fatness (3L) without deleteriously affecting texture or
flavour, the main components of eating quality. There was a suggestion that this meat may be less
juicy and this needs further investigation.
Seasonal effects were minor and whilst texture did not deteriorate, the trained sensory panel did
detect an increased abnormal flavour in late season lamb. This was significantly higher in crossbred
than Blackface lambs. There was no clear evidence from this project that an increase in abnormal
flavour over the season was caused by a specific diet. This problem needs to be investigated further
with a specifically designed experiment.
The improvement in eating quality due to post-slaughter enhancement occurred across all breeds,
abattoirs and pre-slaughter treatments.
Acknowledgements
This project was commissioned by REERAD. This project was made possible by the enthusiastic
co-operation of the farmers and abattoirs directly involved. We thank the large team of people that
were involved at each of our organisations for their contribution to the success of this project. In
particular we thank Alejandro Lombardi, who managed the data collection, and Geoff Nute who
managed the sensory testing process.
Kim-Marie Haywood of Quality Meat Scotland acted as the industrial liaison officer for this
project. We thank her for her enthusiasm and tireless efforts throughout.
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