RHEOLOGICAL AND SENSORY PROPERTIES OF POPULAR GREEK FOODSTUFFS: A REVIEW

Abstract

The rheological and sensory properties that constitute the quality benchmark of Greek foodstuffs are identified in a critical fashion and prevailing practices are discussed in relation to home-made and industrial production. Formulation modifications aimed at enhancing the appeal of products in terms of organoleptic characteristics and nutritional impact are also presented. Olive oil occupies an important position in the Greek diet and it is characterised by a unique flavour. Accurate determination of flavour is carried out using the sensory wheel, which unveils the grade of the product. Among foods with soft-solid characteristics, traditional Feta cheese has a prominent place and it is well known for a salty, slightly acidic taste. Ageing within a year results in textural changes from a soft and elastic consistency to a hard and brittle body. Full and low fat Greek yoghurts are consumed widely locally and abroad. The mouthfeel of the product changes from that of a week gel to a viscous fluid at a fat content of 10% and 2%, respectively. Finally, emulsion-type products range from commercial salamis to traditional sausages, which satisfy culinary requirements for a ‘rubbery' or ‘spreadable' texture depending on the favourite pastime dinner. It is hoped that the treatise will unveil the appetising features of Greek foodstuffs, which are increasingly recognised by the international consumer.

BACKGROUND

The Greeks used a wide range of foods since the dawn of history, with olive oil, dairy and meat produce being staples of their diet [1]. Olive oil production was especially important in Crete as early as 1500 B.C. and was used as a food, in cooking, in religious rites, on the body, and as a preservative for foods and beverages to exclude air. Lactic acid fermentation was known and widely utilised in the making of yoghurt. This fermentation was also used in high-salt brines for the production of cheese [2]. The Greeks also developed special grills, skewers, roasts with sauces, and others. Elaborate feasts were given in the fifth century B.C. with banquets being more organised as to time of day and dishes than in earlier periods. The culinary skills of the ancient Greek are reflected on the contemporary diet, highlights of which are presented in the following sections. The aim of this article is to identify and then review the edible properties of popular Greek foodstuffs (olive oil, cheeses, yoghurts and emulsion-type meat products) with emphasis on rheological and sensory aspects.1 ^*

Figure 2. Stress-deformation profiles for Greek Feta as a function of ageing at refrigerator temperature. Compression rate was 0.8 mm s⁻¹.

Figure 3. Development of viscoelasticity as a function of frequency of oscillation for yoghurts with 10% and 2% fat (mechanical spectrum with symbols and lines, respectively). High fat sample: G′ (□), G′′ (▵), η^* (ˆ); low fat sample: G′ (––), G′′ (—), η^* (- - -); temperature = 5°C; strain = 0.5%.

Figure 4. Stress-deformation profiles for Greek salami and traditional sausage taken at 5°C. Compression rate was 0.8 mm s⁻¹.

OLIVE OIL

Olive oil is the principal source of fat in Greece and is characteristically used in place of animal fats or other fatty materials typical of northern European diets [3]. Olive oil contains a large proportion of monounsaturated fatty acids, it is relatively low in saturated fatty acids and is a good source of natural nutrient and non-nutrient antioxidants such as α-tocopherol and 3,4-dihydroxy-phenethyl alcohol (hydroxytyrosol) and as well as other functional constituents (e.g. squalene). These features make it preferable to animal fats and other vegetable oils from the standpoint of health [4].

Olive oils are marketed in various grades with different chemical, physical and organoleptic properties. The best grades are virgin olive oil and mixtures of virgin olive oil with refined olive oil. These grades are protected by trade standards set by the International Olive Oil Council and also by strict regulations of the Commission of the European Union. Olive oil is appreciated for its flavour. Therefore, the organoleptic score is an important determinant of the quality of olive oil. It is not always easy to identify all the critical physicochemical parameters related to sensory quality and preference. There have been cases where various panels perceived olive oil in a similar way but the description applied to each sample differed.

To obtain a basis for an agreed vocabulary for the sensory profile, a sensory wheel was developed (Figure 1). This wheel is synthesized of smell and taste, look and feel attributes [5]. Feel attributes are thick, sticky, dry, rough and pungent. The last attribute is the most important and it is considered to describe the sensation of cooling, throat catching and astringent. Throat catching is described as leaving a burning sensation in the back of the throat after swallowing. Such attributes are considered when oils are scored according to regulation 2568/91 of the European Commission [6].

Figure 1. The sensory wheel of virgin olive oil (with permission from reference [5]).

Pungent mouthfeel is related to the presence of polyphenols, a complex mixture, which is usually obtained from the oil by extraction with methanol-water. This polar fraction contains phenolic compounds such as tyrosol and hydroxytyrosol (free or in the form of various aglycons), hydroxycinnamic acids, oleoeuropein (a glucoside), elenolic acid and others [7].

Bitter, pungent or astringent perceptions are unpleasant for non-habitual consumers, who are not accustomed to virgin olive oils. For habitual consumers these perceptions can also be quite unpleasant depending on intensity. Determinants of pungency and bitterness are the type of cultivar, the state of the maturity of olives and the procedure/equipment used for the extraction of olive oil. Therefore, it is suggested that different crashing machines and kneading conditions should be used for olives of certain cultivars and drupes which are not fully blackened (yielding oils with a high total polyphenol content) than for olives which yield “sweet” oils with a low polyphenol content [8].

TRADITIONAL GREEK CHEESES

Cheese has always been a major constituent of the Greek diet. In the old days, this was also out of necessity since cheesemaking was an efficient way to preserve milk. Through the centuries, Greek cheesemakers developed several distinct types of cheese. These are distinguished for their appealing organoleptic properties thus making them a favourite ingredient of the Greek diet. A total of 22 kg is consumed per capita which is the highest of the world [9].

Greek cheeses are highly regarded for their outstanding flavour and texture, which are related to some extent to the unique features of the milk used in their preparation. Traditional Greek cheeses are included in a Code, which details specifications for more than 25 types considered as products of a protected designation of origin [10]. Besides Feta, the list includes Telemes, Kasseri, Kefalotiri, Kefalograviera, Mizithra, Manouri, etc.

Feta

Among Greek cheeses, Feta has a unique place. It is a traditional cheese from sheep and goat milk. It is a type of white cheese kept in brine and in big cheese plants is made using commercial rennet. Cottage Feta, on the other hand, is made using rennet from abomasa of lambs which is considered to impart pleasant flavour and pepperish taste to the product. Although the main volume of produce is made with pasteurised milk, gourmet Feta is still produced using unpasteurised milk which contributes to rich flavour and fast ripening. Typical compositions of Feta found in the Greek market include: 52.9% moisture, 26.17% fat, 16.71% protein, 2.94% sodium chloride, 0.17% lactose, with the pH being around 4.4 [11].

The main sensory attribute of Feta which is recognizable around the world is that of a salty, slightly acidic taste. Feta-like cheeses are now produced in northern European countries mainly from cow's milk which, however, should not be compared to traditional Greek Feta since it is difficult to imitate the original characteristics. These are related to a series of complex factors such as the breed of sheep and goats, the climate, the soil, the flora of the region where Feta cheese is produced and the experience of the cheese makers accumulated through centuries from generation to generation [12].

Thus, sheep milk is rich in solids, mainly fat (∼ 7.5%) and proteins (∼ 6%), whereas the fat and protein content in goat milk is 4.5% and 3.5%, respectively. Milk fat and proteins are the carriers of components contributing to taste and flavor and, clearly, these variations are used profitably in the manufacture of different types of cheese. Traditionally, Feta made from goat milk is harder with intense flavour as compared to that made from sheep milk [13]. Mixing of sheep with goat milk with no more than 20 or 30% of the latter is not uncommon and produces good quality Feta.

Sheep and goat milk are deficient in carotenes thus giving Feta a white colour whereas cow's milk contains carotenes in sufficient quantities to give the product a yellowish colour [14]. A further difference between Feta and other cheeses emanates from the high content of caproic, caprylic and capric acids found in sheep and goat milk [15]. Consequently, Feta cheese has a distinct flavor sometimes described as ‘piquant pepper'. Nevertheless, attempts have been made outside Greece, and especially in Denmark, to make cheeses in brine from cow's milk, but these largely failed to reproduce the ‘benchmark' quality of traditional Greek Feta.

Greek cheeses are organoleptically evaluated according to the International Dairy Federation Standard 99a: 1987. Consistency and texture (body) attributes are: hard, firm, coarse, lumpy, curdy (flaky), crumbly, granular (grainy), gritty, mealy, chalky, corky, short, brittle, tough, sticky, long, springy, smooth, soft (weak), hoop side soft, pasty, smeary, thin (watery), dripping, spongy, layered, uneven.

In order to carry out sensory analysis, Feta cheese samples are cut into pieces 3 × 3 × 2 cm in size and placed in a white place where they are tempered at 18°C. Organoleptic evaluation is carried out after 60, 120 and 240 days of ripening [16]. Maximum attainable scores for body and texture are 40, for flavour (odour and taste) 50 and for appearance 10. From the grading system it is clear that texture and flavor are more important parameters than appearance. Good Greek Fetas may receive from the panel a total score above 85, with an excellent cheese achieving the maximum score of 100 [17], [16]. A similar approach has also been reported for Kefalograviera, a traditional hard Greek cheese [18].

Health Conscious Feta Manufacturing

Today the recommended intake of total fat in a ‘Western diet' is in the ballpark of 20 per cent of the body's total energy [19]. In view of this, low-fat Feta cheese has been prepared and its organoleptic properties were assessed as a function of ageing. The amount of fat in the reduced-calorie product varied from 1.5 to 6.0% which constitutes a substantial reduction compared to the content of the traditional formulation (26.17% fat in the above). It was found that ageing did not affect the sensory properties of Feta cheese at each level of fat [17]. Reduction of fat content from 6.0 to 1.5% in the formulation was perceived unfavorably by the trained taste panel. Still, Feta cheeses with 6.0 and 4.5% fat content scored on aggregate in excess of 85 out of 100, mainly due to superior flavour, which made them good products.

A second modification of the traditional Feta formulation focuses on the partial replacement of salt (NaCl) with KCl. A typical Western diet reflects a sodium intake of 4 to 5 g, which is up to 35 times higher than the minimum adult requirement (30 mg) and is associated with increased incidents of hypertension [20]. Up to 50% substitution of KCl for NaCl produced Feta cheese with appearance, body and flavour characteristics comparable to those of the full-salt formulation [16]. Rheological properties were also studied using compression testing of samples left to age for 60, 120 and 240 days. Increasing levels of KCl produced slightly softer and shorter (i.e. more brittle) samples although, statistically, differences were not significant. Regretfully, values of force to fracture are given in kg which makes it difficult to compare results from different laboratories.

Rheological Profile Analysis of Feta

Recently, rheological profile analysis [21] in the form of compression testing was defined in terms of characterising features of the stress/deformation profile of dairy products such as yoghurts, cheeses and spreads (22,23). These rheological properties are highly correlated with the textural perceptions:

•	The maximum stress (σ m ), which is the point where the stress goes through a maximum value.
•	The maximum strain (ε m ), which is the strain at the maximum stress (σ m ).
•	The inflection point stress (σ i ), which is the stress at the inflection point of the curve where the stress goes through a minimum value following a sharp decrease at a strain larger than the maximum strain (ε m ).
•	The ratio of the inflection point stress to the maximum stress (σ i /σ m ).

The analysis has now been applied to Greek Feta at different stages of ageing. The soft cheese was 110 days old with the hard one being 300 days old. A convenient compression rate of 0.8 mm s⁻¹ was chosen which corresponds to about 2′′ min⁻¹. Figure 2 reproduces the stress-deformation profile of the two samples compressed at refrigeration temperature (about 5°C).

The rheological characteristics of soft Feta at 110 days of storage are σ _m ≈ 2.2 kPa, ε _m ≈ 17%, σ _i ≈ 0.7 kPa and σ _i /σ _m ≈ 0.32. The high values of yield strain argue for a rather elastic structure which fractures catastrophically thus giving a low stress ratio. By contrast the σ _i /σ _m ratio of a spreadable cheese (e.g. full fat Philadelphia) is between 0.95 and 1.0 [22]. Clearly, aggregation of the milk proteins with time (300 days) reinforces the strength of the network which is now capable of achieving at fracture stress values of about 7 kPa. Fracture is particularly sharp and occurs early in the deformation cycle (ε _m = 6.8%), a result which demonstrates the brittle nature of mature Feta cheese.

FULL AND LOW FAT GREEK YOGHURTS

Yoghurt is a popular food in Greece in its traditional or commercial form. Both cow and sheep milk can be used for its production. Traditional Greek yoghurts are characterised by a crust on the surface of the container, which is formed by some proteins of the serum and fat [24]. To obtain the crust it is necessary not to homogenise the milk. This is the main difference between the traditional Greek yoghurt and the so-called European-type yoghurts.

Home-made yoghurt is considered to be the produce of cottage industry. It is prepared by boiling the milk, reducing the temperature to 35–43°C, adding a proportion of the yoghurt made the day before, and incubating at 35–40°C. The final product is sold in plastic or clay containers. Industrial practices are more or less the same [25]. The main difference lies in large-scale production and the addition of a sour-milk culture for incubation to occur after filling into plastic containers for set yoghurts while incubation takes place in large vats for stirred yoghurts. Traditional Greek yoghurts tend to be slightly more acidic than usual with their pH ranging from 4.0 to 4.4.

“Strained” yoghurt is another traditional product and is available in a full-fat or a low-fat formulation. In the past, it was produced by cooling the curdle below 10°C with water being strained off from the product by depositing in cloth bags. Today, “natural” straining has been replaced by the modern techniques of filtration and centrifugation [26]. Strained yoghurt is used in the preparation of “tzatziki”, a traditional yoghurt dip with garlic flavour which is very popular with the locals and visitors alike.

Traditional evaluation of the organoleptic properties of yoghurts involves the use of a scale with grading from 1 to 5 for structure and body, colour and appearance, and flavour [27]. Evaluation of structure and body is based on homogeneity, syneresis and the easiness to cut the product by spoon (spoonable yoghurts). Penetrometers and viscometers are also used to evaluate the structure and flow properties of set and stirred yoghurts [28]. Typically, viscosity is measured at 5°C using Brookfield or Haake type viscometers with values ranging between 12,000 and 40,000 cP (1 cP = 1 mPa s).

These quotations, however, describe adequately neither the structural properties nor the shear-rate dependence of viscosity of yoghurts. Today, there is an increased realisation that oscillatory methods can extract the elastic and viscous contribution to the mechanical behaviour of a foodstuff as the structure changes with production time, temperature, shear rate, etc [29]. By ‘sweeping' the imposed time or frequency (time = 1/frequency), mechanical spectra are obtained from which the nature of the foodstuff can be analysed to identify the most important textural features. Figure 3 reproduces the outcome of oscillatory analysis on Greek yoghurts.

The full fat product also contains 4.8% proteins from cow milk and 3.5% sugars. The protein-fat matrix is capable of forming a thick body with gel-like properties. Thus the elastic component of the network, G′, remains well above the viscous component, G′′, over the experimental frequency range (from 0.1 to 100 rad s⁻¹), with the viscoelastic ratio, tan δ = G′′/G′, being about 0.24. Both traces of the elastic and viscous modulus show frequency dependence which is characteristic of “weak gels” [30].

The low fat yoghurt has an increased content of sugars (5.3%) but lower levels of milk protein (3.9%). In addition, it contains 0.1% gelatin to impart shining to the product. The reduced levels of fat and protein result in a weak structure that flows readily and exhibits signs of syneresis. The value of G′ at frequency of 0.1 rad s⁻¹ is about 0.59 kPa as compared to 1.92 kPa for the full-fat product. In both cases, the complex dynamic viscosity (η^*) declines dramatically with increasing frequency of oscillation thus demonstrating the shear-thinning nature of the samples. Due to this, viscosity values should be quoted at a specific shear rate and in the case of the full fat product, η^* = 20 kPa s at 0.1 rad s⁻¹.

POPULAR EMULSION-TYPE MEAT PRODUCTS

Small-scale farms are found in abundance in Greece and practise at large animal- and environmental-friendly rearing thus producing beef and pork which is fattened without drugs. Primary cuts are used for steaks, chops and oven roasts but secondary cuts or meat by-products are processed to turn them into restructured meat which is sold as a high-value product [31]. These are known as emulsion-type meat products and in the likes of frankfurters, cooked salamis and traditional sausages are widely consumed in Greece.

A typical formulation of a cooked-emulsion product contains up to 55% beef or pork meat, 20% pork fat, 2% salt (NaCl), 0.5% polyphosphates, and 0.5% spices [32]. In the case of salamis, pigskin gelatin, egg and milk proteins, or starch may be added to improve the stability of the emulsion. Figure 4 shows the rheological profile analysis of salami obtained at 5°C using a similar compression rate, as for Feta cheese (0.8 mm s⁻¹). Clearly, sample deformation requires application of substantial force with the network rupturing at about 8.7 kPa (maximum stress, σ _m ). This occurs late in the compression cycle thus unveiling a rubbery structure with a maximum strain (ε _m ) value of 40% (contrast with the brittle nature of mature Feta in Figure 2). Furthermore, the rubbery fracture is catastrophic and results in a low ratio of inflection to maximum stress (σ _i /σ _m = 0.18).

Traditional sausages are prepared according to local recipes and their characteristics are dictated by the local habits of the consumer. Amongst them, raw (fermented) sausages are a special delicacy [33]. They are made of uncooked skeletal muscle of pork or beef, lard and spices, with the meat being usually coarsely ground. Following staffing of the mixture, the sausage is kept at 18 to 21°C in air-conditioned rooms with relative humidity of 75 to 90% thus causing its partial dehydration (weight loss of 20–35%). Ripening is completed in 20 to 40 days [34]. The process is accelerated by souring with the addition of glucono-delta-lactone which drops the pH and causes shrinkage in the protein gel. Sausages can be fried or baked at any stage during ripening and when they become firm and sliceable are preserved at room temperature and consumed raw.

The main criteria used in the quality assessment of sausages are the texture of the casing, the connectivity of the meat and its juiciness [35]. The meat mass should be moderately soft with a well-moulded “plastic” texture and should withdraw easily by pressing with the thumb. Proper dehydration and good stuffing can go a long way towards achieving these properties. A good example of the textural properties of raw sausage is depicted in Figure 4. In contrast with the cooked salami, the network is quite soft and spreads easily during deformation thus producing the textural profile of a “plastic” dispersion. Following cooking, the texture is slightly hard and connective but disintegrates easily during mastication. Thus, difficult-to-masticate pieces due to admixture of cartilage and hard connective tissue should be avoided.

Juiciness is affected by the quality and quantity of fat, water content, the degree of grinding, and the properties of the muscle tissue. In traditional sausages, 35% fat is used to obtain satisfactory juiciness. “Grainy” fat is preferred. This should not liquefy during comminution which leads to a slight rise in temperature of the mincer. Soft fat drips during cooking leading to very dry sausages (hay-like). Unacceptably dry sausages are also obtained if lean meat is used due to loss of water during frying or cooking. Taste panellists assess the organoleptic properties of traditional sausages using a scale from 1 to 5. Defects examined by the panel are: very soft, fatty texture, very wet, very elastic, very dry (lack of juiciness), very hard, too grainy and brittle, and presence of hard particles.

As in the dairy products described in the preceding sections, attempts have been made to partially replace fat with vegetable oils and meat protein with soya bean or lupin seed proteins. The former attempt was met with some success in terms of the emulsion stability but firmness, lightness of internal colour and flavour intensity was reduced in the sample containing vegetable oils [32]. Use of lupin protein up to a level of 2% in frankfurters did not affect the colour, texture and sensory characteristics of the product [36]. At higher contents of the protein, however, the acceptability of the product was reduced due to an unpleasant and rather bitter taste.

CONCLUSIONS

Readings of shear moduli, dynamic complex viscosity, compression testing and sensory evaluation were employed to build up a database of the quality benchmark of popular Greek foodstuffs. Olive oil, Feta cheese, yoghurts, salamis and sausages have unique organoleptic characteristics pertaining to the Greek climatic conditions, the soil, the flora of the region, experience of producers and culinary habits of the consumer. The review constitutes an attempt to identify the textural properties and mouthfeel of traditional and health-conscious recipes for future reference and comparison with local produce and foreign imitations.

ACKNOWLEDGMENTS

The authors are grateful to Dr John Ambrosiadis, Aristotle University for assistance with the section on cooked meat emulsions.

Notes

^*Although the paper reviews the textural properties of popular Greek foodstuffs, it is difficult to obtain good-quality rheological references on the subject and, for this reason, some basic compression-testing and dynamic-oscillation data were generated in this laboratory (Figures 2-4.