Physicochemical factors of abalone quality: a review.
Article Type: Report
Subject: Collagen (Properties)
Collagen (Physiological aspects)
Abalone fisheries (Research)
Abalones (Physiological aspects)
Abalones (Chemical properties)
Amino acids (Physiological aspects)
Meat (Quality)
Meat (Evaluation)
Authors: Brown, Malcolm R.
Sikes, Anita L.
Elliott, Nicholas G.
Tume, Ron K.
Pub Date: 08/01/2008
Publication: Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2008 National Shellfisheries Association, Inc. ISSN: 0730-8000
Issue: Date: August, 2008 Source Volume: 27 Source Issue: 4
Topic: Event Code: 310 Science & research
Product: Product Code: 2831910 Collagen NAICS Code: 325414 Biological Product (except Diagnostic) Manufacturing SIC Code: 2830 Drugs
Geographic: Geographic Scope: Australia Geographic Code: 8AUST Australia
Accession Number: 184230615
Full Text: ABSTRACT Abalone meat has long been held in high regard for its unique sensory properties of texture and flavor, as well as its appearance. From a physicochemical viewpoint, the concentrations of certain free amino acids (especially glycine and glutamate) and the nucleotide AMP have been implicated as major factors characterizing the taste of abalone, and there seems to be a strong interaction (synergism) between them. The texture of abalone meat is related to the distribution of protein within the foot, and there is a good correlation between the collagen content and the toughness of abalone. These physicochemical factors, which largely define quality, may be influenced by species, season, diet, physiological condition and genetic factors. Protocols for handling and transport, and processing also influence quality; lactic acid is considered a useful post-mortem indicator of "freshness" in abalone meat. This review focuses on the abovementioned physicochemical factors and their link to abalone quality, and briefly discusses market related aspects and objective methods used for assessing quality attributes in abalone.

KEY WORDS: abalone, flavor, free amino acids, quality, texture, post-mortem processing

MARKETS AND THEIR PERCEPTION OF ABALONE QUALITY

Trade in abalone dates as far back as the 1880s in Tasmania, Australia, where Chinese merchants were reportedly harvesting, drying, and exporting abalone product to China (Prince & Shepherd 1992). Hence, abalone meat has long been held in high regard in particular markets for its unique sensory properties of texture and flavor, as well as its appearance. Whereas about two-thirds of the world supply, estimated in 2005 as 40,000 metric tons (FAO data), is as wild-caught product predominantly from Australia and Japan, aquaculture production has risen rapidly with the major producers being China and Taiwan (Gordon & Cook 2004). Despite this, over the last two decades the shortfall in supply relative to demand has increased, primarily as a result of dwindling worldwide catch from abalone fisheries (Gordon & Cook 2004). These factors contribute to the market paying a premium price, typically in the range of US$30-40 [kg.sup.-1] for live product (Gordon & Cook 2004).

The market is predominantly Asia, with China the key major consumer followed by Japan and Taiwan, then the United States (largely in part because of its Chinese population of 1.3 million) (Gordon & Cook 2004, ABARE 2005). Japan is the largest consumer of live, fresh and frozen abalone whereas the Chinese market has a preference for canned product (Freeman 2001). The market differentiates product based on quality, but often the determining factors are difficult to define or understand. In the Chinese market, products tend to be identified by brand name rather than by species. Particular brands, which may include a mix of species, are differentiated by product size, texture and meat color (Oakes & Ponte 1996). Product may also be differentiated according to country of origin. For example, in a survey of Hong Kong abalone traders, 80% mentioned South Africa, the Middle East and/or Australia as producing highquality product, but others thought South African and Australian production was inferior to other countries mentioned. Low grade products were said to be sourced from Indonesia and the Philippines (Clarke 2004).

The perception of quality and consumer preference varies dramatically between target markets, within and between countries, (Oakes & Ponte 1996, Gordon & Cook 2004). Moreover, abalone reaches the market in a variety of product forms (e.g., live, canned, dried, frozen), and different quality criteria may be applied to each. Also, at the consumer end, traditional abalone recipes use the meat in three general texture forms: tenderized (by cooking, canning or pounding), raw and dried meat. These textural forms of abalone meat have different quality attributes: canned abalone is preferred for a soft, chewy texture whereas raw meat is known for its firm, crisp texture. Therefore the attributes of quality differ depending on the nature and intended end use of the final product.

Hence the relationship between product (species, size, and form), and market preference and discrimination of quality is complex and beyond the scope of this review. In this review, an attempt is made to provide some general understanding of chemical and physical factors that are linked to quality in abalone and describe how they vary under different environmental conditions, management practices and processing methodologies. Some description of methodologies used in assessing quality will also be given.

COMPONENTS OF QUALITY

Taste-Active Components

The sensory properties of abalone relate to their unique combination of taste or flavor and texture; these will be discussed separately in the next two sections. Umami--now recognized as a "fifth-taste" (McCabe & Rolls 2007)--is a Japanese word often used to describe abalone flavor that roughly translates into "robust" or "delicious" and can also be described as heartiness, savoriness, or fullness of the mouth.

The balance and amounts of free amino acids (FAAs) and nucleotides have been implicated as major factors characterizing the taste of abalone, and there seems to be a strong interaction (synergism) between them. In one of the earliest studies examining abalone taste, Konosu (1973) used an omission test with abalone muscle extracts and concluded that glycine, glutamic acid, glycine-betaine, and adenosine-5'-monophosphate (AMP) were essential for producing the abalone flavor. The characteristic umami flavor almost disappeared when either AMP or glutamic acid were omitted from the extract, whereas glycine imparted a sweetness to the extract. The study also found that exclusion of taurine and arginine had only a small effect on taste. However, Bewick et al. (1997) (and references cited therein) have suggested that arginine and taurine--which are quantitatively major FAAs in abalone do contribute to flavor. AMP concentrations can vary significantly in abalone meat, e.g., from 0.2-1.4 mg x [g.sup.-1] wet weight (WW) (Bewick et al. 1997, Chiou et al. 2001). Total concentrations of FAAs are more constant, and represent a major organic component of abalone meat, usually ranging from 21-25 mg x [g.sup.-1] WW (Bewick et al. 1997, Chiou et al. 2001)--or up to 10% of the dry meat weight. Taurine typically makes up half of the fraction, with arginine and glycine ranging from 10% to 25% of total FAAs).

Subsequently, both Watanabe et al. (1992a) and Hatae et al. (1995) assessed seasonal changes in these taste-active components in Haliotis discus in Japanese waters, and found highest concentrations of AMP, glutamic acid and glycine in summer and early autumn (prior to spawning), which corresponded, anecdotally, to the time when abalone were most palatable. Watanabe et al. (1992a) also found higher levels of the taste-active component succinic acid and homarine (a marine alkaloid) in summer than in winter.

Chiou et al. (2001) examined changes in metabolites in small Haliotis diversicolor in Taiwan according to season and also in response to diet. The total amounts of glycine, glutamic acid, and AMP were highest in winter and early spring, which they inferred would make them most palatable at this time. Also from the study, abalone fed on an artificial diet contained substantially more glycogen than abalone fed on the red seaweed Gracilaria sp (e.g., maximum values of [approximately equal to] 7.5% WW, c.f. 1.7%) and concentrations were highest in winter. Other studies have found glycogen concentrations are linked to season and the reproductive cycle; H. discus from Japanese waters had the highest concentration in summer, and concentrations decreased dramatically during, and immediately after, spawning. Studies from our laboratories (unpubl. obs.) found glycogen concentrations of 3 y old cultured H. laevigata in summer 2004 were high (relative to other literature data), averaging 12% of the dry weight (DW), although individuals, values ranged from 5% to 25% of the DW. In the following year, glycogen concentrations within abalone from the same cohort (i.e., as 4-y-old animals) were about one-third, 4% of DW, (range: 0.1% to 15% of DW). The reasons for the high variability between animals harvested at the same time were unclear, though it did not appear to be correlated to the state of gonad maturation. The interannual differences in average glycogen concentration may, in part, have been contributed to by a change in the formulated diet used by the industry grower. Glycogen concentrations in abalone may also reduce dramatically in response to metabolic stresses, such as starvation, elevated temperature, and disease (Braid et al. 2005).

The contribution of glycogen to taste is unclear. According to Konosu (1973), glycogen improves the characteristic taste of abalone, though tasteless itself. In scallop muscle, glycogen has been reported to elevate "continuity, fullness, complexity, and overall preference." However, Carefoot et al. (1993) found an Asian sensory panel could not discriminate between meat of H. kamtschatkana with low or high glycogen content.

Relatively less emphasis has been placed on the lipid profile of abalone meat and its relationship to taste, perhaps because of the low oil content of abalone, typically 1% to 2% of wet weight (Hatae et al. 1995, Dunstan et al. 1996). Concentrations of lipid in wild H. laevigata and H. rubra were more than double in summer (2.5% to 2.7% of wet weight) than they were in other seasons ([less than or equal to] 1.2%) (Su et al. 2006).

Texture

Textural properties of meat, which heavily depend on cellular structures and structural characteristics, such as cell density, size, and uniformity have a strong influence on sensory properties (Alina 1962, Gao et al. 2003). For molluscs, Gao et al. (2003) has added that textural properties are thought to depend on fibrillar diameter, and also other structural parameters including the distance between myofibrils. Abalone has a characteristic tough texture; an attribute that is considered equally important as its taste, especially within Asian markets where abalone meat is predominantly eaten raw (Olaechea et al. 1993, Gao et al. 2002).

There is a good correlation between the collagen (i.e., connective tissue protein) content and the toughness of abalone, as measured by breaking stress assays, with a lower collagen content (more tender) product being preferred by sensory panels (Olaechea et al. 1993). Collagen content was also positively correlated to the weight of the whole animal, hence smaller abalone were more tender (Olaechea et al. 1993). The toughness of different regions of H. discus hannai muscle (with associated differences in muscle structure) was also correlated to collagen content, with upper and middle parts of the adductor muscle being most tender and containing less collagen (Olley & Thrower 1977, Olaechea et al. 1993). In juvenile H. iris, adductor muscle has also been reported as being a more tender than foot muscle (Allen et al. 2006).

Seasonally, textural properties of abalone change but trends can vary according to species. Haliotis discus in summer was most tender and had lowest collagen content, and the reverse was true in winter (Olaechea et al. 1993, Hatae et al. 1995). Textural changes were most profound in the "soft foot" region (i.e., above the foot sole and beneath the adductor), where the toughness increased 4-fold from summer to winter (Olaechea et al. 1993). In contrast juvenile (40-50 mm) H. iris, though reported as more tender than adult H. discus, according to similar objective measurements, had similar toughness in summer and winter, with lower values in spring and summer (Allen et al. 2006). However, interpretation from this latter study could be partly confounded by age-related effects as animals sampled in summer were the oldest and nearly double the mass of those sampled in autumn.

Though not proven, Hatae et al. (1995) have speculated that collagen may be degraded prior to spawning as an energy source, which in turn may contribute to an increase in FAAs (especially glycine, which is a predominant amino acid in collagen) and hence improve the taste (Olley & Thrower 1977).

A common term often used in sensory evaluation of food is "mouthfeel," which in food engineering terms, is defined as an organoleptic property used to describe the overall texture of a product. It is used most often as an in-mouth impression in the beverage industry, particularly for describing the quality of wine, beer, and coffee. In these instances, it is described as a tactile feel; largely a reflection of the physical qualities but can also be significantly affected by flavor elements; and as the viscous feeling in the mouth that provides a measure of the texture. Therefore mouthfeel should be considered a different sense to taste and/or flavor (June Olley, pers. comm.). In many of the studies reported here however, mouthfeel as a sensory attribute has been included into the taste/flavor category.

Not only does the texture of abalone meat relate to the distribution of protein within the foot, but also the treatments to which the meat has been subjected (see subsequent section on "Processing") (Olley & Thrower 1977). Other parameters that have been measured and implicated as direct or indirect measures of abalone texture include total protein and composition of the protein fraction, and moisture content (Olaechea et al. 1993, Wells et al. 1998, Watanabe et al. 1992a). Immediately after spawning of H. discus in late summer/early autumn, moisture content was highest (76.5%; c.f. to 60% 6 mo later) and this corresponded to the time when they are considered most watery and unpalatable by local Japanese (Watanabe et al. 1992a). Because glycogen concentrations were inversely correlated to moisture, abalone were therefore most palatable when glycogen concentrations were highest (Watanabe et al. 1992a).

Visual Appearance

Abalone color, specifically the color of the foot, is relevant in the market. Species with lighter pigmentation are usually considered better and obtain the highest prices. The more heavily pigmented species are graded lower and need more trimming, washing or bleaching before sale to reduce the color distinction (Oakes & Ponte 1996). Good quality canned abalone has a creamy/yellow appearance and the surface is free of any localized discoloration. The degree of shell color of certain species can be a quality discriminator in particular markets. For example, redder shells in red abalone, H. rufescens, are preferred by Asian markets.

Whereas a loss of quality between harvesting and delivery to market, or processor, can be avoided by good handling practices, inevitably some damage can occur in a small proportion of animals (e.g., chipping of shell, splitting of foot). Such product, in addition to catch that may be discolored, is still marketable, though often packed as second or third grade (James & Olley, 1974). The level of responsiveness, or "vitality" is also a major quality indicator in live transported abalone. Another criterion that affects marketability of live export abalone is meat to shell weight ratio, with higher being better.

Size

Size also affects marketability; currently for cultured live abalone the premium size is 80-100 mm (Oakes & Ponte 1996, Sales & Britz 2001), though market trends suggest that in future more demand will be on larger sized abalone, 120 mm or above (Gordon & Cook 2004). Also, size preference is strongly linked to the target market, the species and the product form. For example, with cultured H. rufescens produced by Abalone Farms Inc. in 1996, live product was sent to the Japanese market in the range 100-110 mm, whereas their canned product for the Chinese market was 65-95 mm (Oakes & Ponte 1996). For IQF (individually quick frozen) meat of different weight ranges, premium price for H. laevigata is paid for 1-2 pieces/lb product, whereas for H. rubra, the 3 5 pieces/lb product attracts a better price (Dion Edmunds, Streaky Bay Marine Products P/L, Australia, pers. comm.).

MANAGEMENT AND PROCESSING FACTORS AFFECTING ABALONE QUALITY

Influence of Diet

Studies with cultured abalone have demonstrated that diet can have a significant effect on quality-related factors such as chemical composition, taste, texture and color (Dunstan et al. 1996, Chiou & Lai 2002, Allen et al. 2006). Chiou & Lai (2002) found that small abalone H. diversicolor fed artificial diets contained less (on a percentage basis) taurine and arginine, but more glycine, glutamic acid, proline, AMP, and glycogen than similar abalone fed algae. In sensory tests, cooked meat (steam cooker for l0 min) from abalone fed on artificial diets were preferred to those fed macroalgae; the authors attributed this to their differences in the taste-active components such as glycine, glutamate, and AMP (Chiou et al. 2002). Haliotis iris fed on a formulated diet (Makara[TM]) contained seven-times more glycine and double the ATP than animals fed local macroalgae (Bewick et al. 1997). More recently, a sensory panel indicated a preference in texture and acceptability of cultured H. iris fed formulated diets than wild-caught abalone, though the study found no difference in flavor between the groups (Preece 2006).

In a study of lipids in juvenile abalone H. laevigata, Dunstan et al. (1996) found animals fed on artificial diets contained approximately double the lipid than animals fed on macroalgae, and observed differences in fatty acid and sterol compositions that were related to the profiles in diets. Though levels still remained low (i.e., 1.45% of DW) compared with other seafood products, lipids are an important factor contributing to the flavor and odor of seafood (Lindsay 1988). Hence artificial feeds incorporating fish oils may give cultured abalone a "fishier" flavor than those fed diets containing vegetable oils, and, potentially, lead to a subtle change in texture (Dunstan et al. 1996). For this reason, Dunstan et al. (1996) suggested that it may be beneficial to feed cultured abalone on macroalgae for some period immediately prior to harvest to ensure market acceptability and to maintain product quality.

Meat and shell color may be influenced or indeed manipulated by their diet. Cultured H. iris fed on diets containing algae had a distinct darkening of the foot, compared with individuals that were fed a pelleted diet (Allen et al. 2006). Changes in the banding and coloration of wild abalone shells seemed to be mainly associated with seasonal diet shifts in macroalgae and their associated pigments (Olsen 1968). In culture, juvenile H. asinina fed formulated diets produced shells with light-blue green color, whereas those fed natural algal diets produced brownish shells (Bautista-Teruel & Millamena 1999). Shells of the red abalone H. rufescens turned red after feeding on diets containing red algae (T. S. Suskiewicz, Moss Landing Marine Laboratory., unpubl, observ.), a characteristic which is preferred by the Asian markets.

Meat quality can be negatively affected by the metabolic activity and stress associated with harvest and transport in a variety of meat animal species, including abalone (James & Olley, 1970, Bosworth et al. 2007). These impacts on meat quality are usually associated with an increased anaerobic metabolism, which leads to decreased muscle glycogen reserves, a lowering of muscle pH, and an increase in protein muscle degradation after slaughter (Olley & Thrower 1977, Rathgeber et al. 1999). Generally, for meat products of a variety of species, this can result in a soft texture, reduced water holding capacity, lower shelf-life and poor consumer acceptability (Bosworth et al. 2007).

Abalone adhere tightly to the solid substratum via their foot muscle, and therefore may be difficult to remove from the wild or farms. Hence, mechanical removal is often required e.g., using a blunt spatula, which can result in physical damage or death because of bodily fluid loss, a slow rate of healing and an increased likelihood of bacterial infection and stress (Genade et al. 1988). In the farming environment, removal can be aided by the use of suitable anaesthetics (e.g., Aqui-S or magnesium sulphate; Sales & Britz 2001), which can reduce muscle stress, but care must be taken with regard to the choice of anesthetic and its concentration, otherwise significant mortality may occur during recovery (White 1995).

For transport from point of harvest to the processor or live market, abalone are taken out of their natural seawater environment, and placed within insulated boxes that are humidified to reduced gill dessication, and often with added oxygen to reduce aerobic stress (Sales & Britz 2001). According to Wells and Baldwin (1995) "abalone have a unique biochemistry which enables them to continue metabolic processes at a reduced level during air exposure, despite the inability to irrigate their gills when removed from water." Animals are usually fasted for 2-3 days prior to harvest, to prevent them producing feces inside the box (Sales & Britz 2001), moreover, at least for juvenile H. discus discus, starved abalone survive better during transit than fed abalone (Watanabe et al. 1994). Nevertheless, the whole process constitutes a major stress on abalone. Transit time may be up to 36 h and is associated with water loss of between 3 15% of the total weight (James & Olley 1970, Vosloo & Vosloo 2006), with evaporation and mucus production the main routes of water loss (Vosloo & Vosloo 2006). With losses above 10%, the animal usually dies (James & Olley 1970). This is believed to occur through internal bleeding because in the latter stages the fluid appears to be pure blood, as evidenced by the deep blue color of oxygenated hemocyanin (James & Olley 1974).

Early studies with H. rubra showed factors that influenced the survival rate during transport were size, temperature and time out of water (Olley & Thrower 1977). The optimum temperature was 6[degrees]C; below this, animals suffered cold shock and contracted and died, and above this mortality was directly related to temperature. Based on metabolic studies, Wells and Baldwin (1995) concluded that larger individuals of H. australis and H. iris were less susceptible to anaerobic stress than smaller individuals, and therefore should live longer out of water. Species may also differ in their tolerance to transport; a survey by the authors of the major Australian abalone growers suggested that H. laevigata suffered greater mortalities than H. rubra during routine transport (e.g., 24-36 h) to market.

Little information is available on the impact of air exposure during transport on taste-active components in abalone. Concentrations of AMP in the adductor and foot muscles of H. iris and H. australis were significantly higher in 24 h air-exposed individuals (1.3-2.3 [micro]mol [g.sup.-1] WW) than in water-immersed individuals (0.2-0.3 [micro]mol [g.sup.-1] WW) (Wells & Baldwin 1995). Chiou et al. (2002) examined changes in glycogen, FAAs, nucleotides and sensory characteristics in small H. diversicolor during 3.5 d of air-exposures at 5[degrees]C, 15[degrees]C, and 25[degrees]C. All animals died during this period, though the timing of death was not monitored. Glycogen tended to decrease during storage, though at 5[degrees]C and 15[degrees]C changes only became apparent after 2-3 days. Also after 2.5 d storage at these two temperatures, total FAAs increased (from 2.1% to 2.6% of WW) with those associated with taste e.g., taurine, arginine, glycine, glutamic acid and alanine increasing by between 10-110% of initial value. In all treatments, AMP concentration increased approximately 5-fold after 1 day. The onset of initial decomposition--as determined by sensory (odor) assessment--occurred after 3.5 days at 5[degrees]C, 2.5 days at 15[degrees]C and 1 day at 25[degrees]C, and was also accompanied by an increase in volatile basic nitrogen. The authors postulated that the enriched amounts of the taste-active FAAs might be beneficial to palatability up to the point of initial decomposition, though no sensory data was presented to support this.

Post-mortem Effects

Significant changes in the chemistry, taste and texture of abalone can occur postmortem, depending on the specifics of storage conditions and also the physiological state of the animal at harvest (Olley & Thrower 1977, Watanabe et al. 1992b, Wells et al. 1998, Gao et al. 2003). James and Olley (1970) noted that postmortem glycolysis, and the associated production of organic acids and lowering of pH were implicated in the meat texture quality during thermal processing of the Tasmanian species H. rubra, specifically in the case of canned product increasing its toughness and reducing its water-holding capacity. The quality of canned product as assessed by a taste panel differed significantly according to the time and temperature profiles of product prior to processing (James & Olley 1970). Fresher material (i.e., live abalone) had a higher initial pH and higher moisture content as compared with canned product; attributes that were associated with an increased tenderness.

The most comprehensive analysis of postmortem biochemical changes in abalone was undertaken by Watanabe et al. (1992b). Their study with H. discus hannai concurred with previous studies of Olley and coworkers, documenting production of organic acids, especially D-lactic acid (the main end product of anaerobic glycolysis) which increased linearly in muscle from [approximately equal to] 1-10 [micro]mol x [g.sup.- 1] WW over 7 d of storage at 5[degrees]C. Among a number of metabolites and indices measured, D-lactic acid was assessed as being the most useful indicator of "freshness". Of the ATP-related metabolites, at temperatures of 0-5[degrees]C, significant reduction in ATP concentration with concomitant increases in AMP occurred over the first 7 d of storage, i.e., AMP increased from 0.25 to between 1.3 and 2.4 [micro]mol x [g.sup.-1] (WW). After 7 d, levels of AMP dropped, with levels of degradation products inosine and hypoxanthine increasing. Polyamines associated with putrefaction, e.g., cadaverine, putrescine and agmatine, were also detected in increasing concentrations after 7 d. Total amounts of FAAs increased after 1 d of storage at 5[degrees]C, and after 5 d of storage at 0[degrees]C. They concluded that "it is probable that (disc) abalone stored at 0[degrees]C for a few days become more palatable, judging from the high levels of AMP, glutamic acid and glycine," though no sensory data was available to support this thesis.

Sales et al. (1999) examined meat quality aspects of South African abalone H. midae under various postmortem treatments, pH of muscles stored at 7[degrees]C did not change significantly until 17 h postmortem after which it started to decrease; whereas for muscle stored at 16[degrees]C this trend occurred after 13 h. This was in contrast to similar studies with H. rubra muscle at 12[degrees]C, where onset of pH decline only occurred after 3040 h (James & Olley 1974). The study with H. midae also included a treatment of meat immediately frozen at -20[degrees]C (Sales et al. 1999). After cooking by immersion in a water bath at 75[degrees]C for 50 min, the meat from this latter treatment was more tender than meat initially stored at 7[degrees]C or 16[degrees]C and subsequently also frozen, which according to the authors, indicated a delay or absence of rigor by a low temperature.

Post-mortem changes in ATP concentrations are also likely to be directly linked to changes in abalone texture. In their study of H. discus, Gao et al. (2003) observed structural changes within chilled abalone meat over 72 h, and ascribed this to the decomposition of ATP related compounds and an associated aggregation of protein molecules contained by muscle fibers. Using light microscopy, and image processing and analysis, they found the distance between myofibrils decreased gradually with time, both in longitudinal- and cross-sections of tissue, due to myofibril detachment from the myocommata. To a lesser extent, a deterioration in collagen fibrils were also thought to contribute to structural changes, though the collagen content of meat did not change significantly.

Wells et al. (1998) raised an important point regarding the sampling and analysis of ATP metabolites, noting that these metabolites are labile and their concentrations may change rapidly, e.g., during the time taken to remove the animals from the substratum, to the time samples are excised and snap-frozen for analysis.

Processing

During the last few years the market for live abalone (predominantly Asia) has rapidly expanded, though more than half the product exported from Australia is in a processed form (i.e., frozen or canned; ABARE, 2005). Olley and Thrower (1977) reported that if the volume of residual blood in the abalone foot is high (due to inadequate draining), then sheets of "ice" may form in the blocks of frozen abalone, and the exporter may be accused of watering the product. From this early literature, it was difficult to generalize on the effects of freezing; Japanese and Australian experience suggested it was easy to freeze abalone before rigor, whereas Korean research suggested this was difficult because of ATP breaking down rapidly in the muscle (Olley & Thrower 1977). Song (1973); cited by Olley & Thrower 1977) examined different rates of freezing and a taste panel assessment concluded texture and taste were best with liquid nitrogen frozen products. Olley and Thrower (1977) suggested that the method of thawing may be more important in maintaining an optimal (high) pH and moisture content than the rate of freezing or storage temperature if the product is to be used for canning. Lester and Bottrill (1984) also found that Australian abalone tended to crumble on cooking if it had been previously subjected to rapid thawing and that the rate of freezing did not seem to be important. However, after surveying the major Australian abalone growers and several processors, the authors of this review found no consensus on methods for thawing of abalone (e.g., recommendations ranged from overnight at 4[degrees]C, to rapid thawing at room temperature).

The thermal processing (i.e., temperature x time) associated with the canning of abalone has a profound effect on yield and quality of the final product. Moreover, the initial quality of the abalone also influences the quality and yield of the final canned product. Warne and Brown (1984) advocated storage of fresh abalone at a temperature between 5-8[degrees]C, as lower temperatures induce anaerobic glycolysis which reduces flesh pH, and leads to an increased loss of weight upon canning. The weight loss and tenderisation associated with canning is predominantly related to lost water and water-soluble components and conversion of collagen to gelatine, respectively. Hatae et al. (1996) examined biochemical, textural and taste changes after boiling abalone meat for up to 6 h. After 15 min, ATP levels reduced to 0, whereas there was a concomitant doubling of AMP (i.e., from 1-2.4 [micro]mol x [g.sup.-1]), which the authors suggested may have attributed to an increase in the umami taste of cooked abalone. Also, levels of FAAs in the meat rapidly increased after 15-30 min cooking and, thereafter, gradually decreased. After 6 h cooking, the meat weight was only 60% of the original weight. A strong correlation was found between the collagen content and texture during cooking (as measured by a breaking stress assay). Overall, the sensory component (20-member taste panel) of the study concluded that 3 h was optimal for the umami taste, body, overall body taste acceptability and tenderness. Further, Hatae et al. (1996) suggested the changes in taste after 3 h may also have been associated with higher concentration of oligopeptides in meat at this time.

Chiou and Lai (2002) compared the taste of cooked (10 min; steamed), small abalone and their taste panel preferred abalone that had been previously fed an artificial diet, compared with abalone fed on Gracilaria sp., because they were sweeter and had a slightly stronger umami taste. They concluded that the taste differences were associated with compositional differences in glutamic acid, glycine and AMP which they also had inferred to be the major taste determinants in fresh abalone (Chiou et al. 2001).

Gao et al. (2001) investigated the effects of cooking on the rheological properties and structural changes in abalone meat. They found that during cooking (3h; boiled), abalone meat shrank, lost water and water-soluble components as drip, and decreased in weight. From light and scanning electron microscopy cross-sections of abalone meat, they concluded that the structure of cooked abalone meat differed significantly from that of raw abalone meat and that rheological properties such as elasticity, viscosity and texture changed greatly with heat treatment. This study confirmed that in raw meat the difference in rheological properties resulted from a difference in collagen fibril structures.

Gao et al. (2002) continued this work and investigated the type of cooking method on the changes in abalone meat. The adductor muscle of H. discus was boiled or steamed for 1, 2 or 3 h. Structural changes in the myofibrils were greatest in the boiled abalone meat compared with the steamed meat, when viewed under light and scanning electron microscopy. When the heating time was increased from 1-3 h, the theological parameter, [E.sub.0] (elastic modulus) of boiled abalone meat decreased gradually with increased heating time, whereas the [E.sub.0] of steamed meat was reduced when heated for 2 h. Gao et al. (2002) concluded that the difference in rheological properties between boiled and steamed abalone meats was due mainly to the denaturation level of myofibrils when heated for 1 h, as well as the changes in water and solid content during heating.

Chiou et al. (2004) looked at the importance of both temperature and time on heat-induced changes in the quality in abalone. They investigated the chemical, physical and sensory changes of small abalone meat during cooking at 80[degrees]C and 98[degrees]C for 10-120 rain. Cooking at 98[degrees]C showed relatively high decreases in moisture, weight, AMP and free fatty acids, and changes in color compared with that cooked at 80[degrees]C. The meat cooked at 80[degrees]C was also characterized by an increase in cutting force, whereas that cooked at 98[degrees]C had a cutting force similar to raw meat.

Another potential method for tenderization is through enzyme treatment. Sanchez-Brambila et al. (2002a) assessed the use of papain solutions for tenderizing Haliotis fulgens and H. cracherodii prior to canning, but found that this did not improve their texture, although a similar treatment had improved the texture of the whelk Astrea undosa (Sanchez-Brambila et al. 2002b).

MEASURING QUALITY AND ASSOCIATED ATTRIBUTES IN ABALONE

Meat quality is perceived subjectively by the consumer as a combination of characteristics that determine the level of acceptability. However, because of regional differences in the preferences of consumers, it is difficult to obtain a standard scale for the subjective evaluation of meat quality (Sales et al. 1999). Therefore, objective measurements for the evaluation of color, tenderness and texture have been developed for assessing meat quality in general (Lawrie 1991). However, such systems do not appear to be in widespread use in the abalone industry. It would clearly be advantageous to have rapid, cheap, reliable and industry-accepted objective measurement systems available for many sectors within the abalone industry, as an addition to the widely used subjective measures. For growers, it would allow a better understanding of specific management practices on measureable quality and so facilitate a more consistent product reaching market. For wholesalers, it would allow a more efficient, less subjective quality discrimination and allow pricing according to specified (measureable) quality standards. Objective measures of quality would also benefit industry breeding programs, by allowing rapid and high-throughput of quality measures of selectively bred stock, thus supporting breeding decisions (Norris & Cunningham 2004).

Earlier scientific reports on abalone quality utilized sensory panels for assessment of quality by aroma, flavor, texture and mouthfeel, for example. The toughness (or tenderness) of muscle as food is often evaluated in terms of penetration depth or more commonly, as the force required to penetrate into, or elongate the muscle. More recent studies on abalone quality have used measurements of texture by instrumental analysis. Several studies have used objective texture measurements in conjunction with trained sensory panels.

James and Olley (1971) found that taste panel texture measurements correlated with the pH of canned Australian abalone. They also extended the use of the maturometer, i.e., an instrument measuring the amount of force required to punch holes in abalone slices, for measuring the texture of canned abalone and showed significant correlation to sensory assessment of texture. Hatae et al. (1996) scored 8 sensory parameters relating to quality of cooked abalone, i.e., odor (strength and preference), hardness, elasticity, preference of texture, umami taste, body of taste and preference of taste. Hatae et al. (1996) also evaluated changes in texture by evaluating the breaking stress using a Rheoner RE-3305 on 4 mm-thick vertical abalone slices.

A more comprehensive survey was undertaken by Sanchez-Brambila et al. (2002a) who scored the following 18 sensory attributes for canned abalone:

* Texture characteristics: springiness, cohesiveness, hardness and chewiness

* Basic taste: sweet, sour, salty and bitter

* Flavor/aroma: briny, decaying vegetation, metallic, crustacean, fishy, cardboardy

* Aftertaste/mouthfeel: metallic, astringent, oily, starchy

In their study, they assessed texture parameters instrumentally, using a texture analysis (Model TAXT2, Texture Technologies) and a table-top Warner-Bratzler shear device. Both the adductor and opercular foot muscles (1 cm-thick slices) were evaluated. Of significance, in both studies the instrumental measurements correlated with the sensory texture measurements ([R.sup.2] = 0.64 0.74) (Sanchez-Brambila et al. 2002a).

Several other studies have evaluated textural properties by measuring breaking stress using a plunger (4 mm diameter) on 4 mm-thick slices of abalone (Olaechea et al. 1993) and breaking strength of blocks of abalone meat (9 [cm.sup.2] x 1 cm height) using a flat device (10 x 1 mm) on an Instron Universal Testing Instrument (Sales et al. 1999). A good linear relationship has been found with measured values and amount of collagen in the tissue (Olaechea et al. 1993). Similarly, a MIRINZ tenderometer (AgResearch MIRINZ, Hamilton, New Zealand) was applied to measure the shear force of tissue pieces from juvenile H. iris (Allen et al. 2006). Gao et al. (2001, 2002, 2003) conducted rheological experiments by means of measuring the stress-relaxation curves and rupture strength of vertical and cross-section samples of abalone meat using a tensipressor (Model TTP-50BX) equipped with cylindrical plungers of 0.36 cm and 0.27 cm in diameter, respectively. In raw abalone meat stored at 4[degrees]C over 72 h, they found correlations between the changes in the objective measurements provided by the tensipressor (e.g., elastic modulus, relaxation time and viscosity) and those provided by microscopic examination of meat structure, with subsequent image processing and analysis. Chiou et al. (2004) analyzed 1 cm x 1 cm thick vertical slices of abalone meat using a Texture Analyzer (TA-XT2) with a knife-type plunger. The force to cut through the surface of the 1 cm thick meat was recorded and regarded as the cutting force to assess the textural change.

Over the last decade, Near-IR Reflectance Spectroscopy (NIRS) has increasingly found widespread application in industry for quality control measurements of food products, because of its high throughput, low cost of analysis per sample, and in the case of some parameters, high correlation to conventional "wet chemistry" analyses. For example, this technology has been used by industry and researchers to accurately predict the content of pigment (astaxanthin) in Atlantic salmon (Solberg 2004), and moisture, fat, and specific fatty acids in meat from salmon, beef and pork (Prevolnik et al. 2004, Solberg 2004). NIRS has also been tested as a means to predict organoleptic properties (e.g., tenderness, juiciness, flavor, texture, chewiness and acceptability) of a range of meat products (Prevolnik et al. 2004). However, NIRS has only demonstrated a limited ability to predict meat quality, though Prevolnik et al. (2004) suggested the main reason for this is related to the reliability of the method to which it is calibrated. To our knowledge NIRS technology has not been rigorously tested on abalone products, but it may prove useful given the unique textural properties of abalone (e.g., high collagen, low fat, and highly variable glycogen content) compared with other meat products.

CONCLUSIONS

The quality of abalone, as defined by taste and texture, can vary significantly according to species, season, diet, health and physiological condition, culture environment and genetic factors (Chiou et al. 2001, Hatae et al. 1995, Wells et al. 1998). Further, harvesting and processing methodologies can have an even more profound effect on meat quality. Existing information suggests that AMP and related metabolites and free amino acids influence the taste of abalone, and collagen content is linked to texture (toughness) of abalone. The importance of pH in postmortem meat texture quality is also outlined.

From this review, it is clear that further understanding is required around the definition of abalone quality and how to measure it, compared with other muscle products such as salmon and red meats. Some knowledge of the postmortem biochemistry and harvesting and processing effects on abalone is documented but further research and understanding is required, particularly into the processing effects such as freezing and thawing. The development of a rapid and cheap method for detecting taste/texture factors (e.g., NIRS) or a more direct measure of quality is necessary. Other future research needs include how the environment and/or diet can influence these quality factors and whether they can be manipulated to alter taste, for example, will benefit the industry in producing a more consistent product.

It should also be re-emphasised that the importance of the factors that affect quality vary according to the specific end-product to be marketed as well as the target market and consumer group. Hence these factors need to be taken into account and understood, in any definition of product quality.

ACKNOWLEDGMENTS

The authors thank Graeme Dunstan, Louise Ward, Robin Shorthose and Nick Savva for their review of an earlier version of this manuscript.

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MALCOLM R. BROWN, (1,3) * ANITA L. SIKES, (2,3) NICHOLAS G. ELLIOTT (1,3) AND RON K. TUME (2,3)

(1) CSIRO, Division of Marine and Atmospheric Research, GPO Box 1538, Hobart 7001, TAS., Australia; (2) Food Science Australia, P.O. Box 3312, Tingalpa DC 4173, QLD, Australia; and (3) CSIRO Food Futures Flagship

* Corresponding author. E-mail: malcolm.brown@csiro.au Handling and Transport
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