Homogenization of Ketchup: Physico-Chemical and Technological Aspects

The influence of homogenization on the flow properties of ketchup was investigated using Heake RV-20 rotational viscosimeter.

Homogenization allowed ketchup apparent viscosity to be increased (by 40% on average), the extent of the increase depending on temperature and homogenization pressure. All products, either homogenized or not, showed no thixotropy, recluster or syneresis phenomena during storage. In the vegetable preserve industry, homogenization is always used in the case of fruit juices, while it is not used for tomato derivatives (pastes, puées, ketchup).

Fruit and vegetable juices and concentrates are processes consisting of a dispersing stage usually called "serum" and a dispersal stage consisting of solid particles with different shapes and sizes, called "pulp" (1). In these processes viscosity depends on the percentage of "fibrous" material (2) and in particular, as observed by Kertesz and Laconti (3), in the case of a homogenized puree, it is considerably influenced by the number and size of the particles. Similar results were also obtained by Trifirô and others (4) and they could be due, as supposed by Luh (5), to an increase of the surface of the particles of pulp following homogenization. The rheological behavior of liquid foodstuffs is often described in literature as non Newtonian (6,7); generally dependent on time (6,8) (the apparent viscosity, at a constant temperature, depends on the duration of the creep gradient and/or previous creep "histories") and with a more or less high thixoropy.

The latter is defined as a reversible isothermic transformation of the sol-gel type; in other words we can say that, with a constant creep gradient, the apparent viscosity decreases in time. This type of behavior is often interpreted as a consequence of a partial alignment fo the particles in the direction of the flow, or rater as partial breakdown of the structure during the flow. After certain time of "pause", variable according to the type of product, the apparent viscosity returns to the original values; for this reason the thixotropy represents a measure of the velocity with which the bonds the bonds between the particles break during the flow and therefore forms a particularly important parameter to predict possible variations in apparent viscosity of the product during storage.

On the basis of these considerations the objectives of this works were:

Materials and Methods

Homogenization

For the various tests of homogenization of ketchup a two stage pilot homogenizer type NS3011 from Niro Soavi.

A sample of ketchup (about 30 kg or 66 lbs) underwent two stage homogenization (1st stage 240 bar or 3480.90 PSI, 2nd stage 40 bar or 580.15 PSI) at a temperature of 65 +/-2°C (or 149 +/- 35.6°F); all the other samples, also about 30 kg (or 66 lbs) each, were homogenized in conditions resulting from linking two temperatures (21 +/- 2°C or 69.8 +/- 35.6°F, 65 +/- 2°C or 149°F +/- 35.6°F) with the following pressures: 70 / 80, 200 / 210 and 260 / 270 bars (or 1015.26 / 1160.30, 2901 / 3046, 3271 / 3916 PSI).

The formulation used (about 280 kg or 617 lbs) and the process chart are shown in figure 1.

Figure 1 - Formulation and diagram of ketchup processing
Processing diagram of ketchup		Formulation of Ketchup
mix preparation preheating homogenization sterilization and filling treatment in steam tunnel cooling storage at room temperature		Ingredients Quantity vinegar 24 liter (6.34 gallon) salt 4 kg (8.81 lbs) sugar 24 kg (52.91 lbs) conc. Hot-break {12 - 14 Bx} 190 kg (418.87 lbs) conc. Cold-break {26 - 30 BX} 36 kg (79.36 lbs)

Analytical Determination

• total solids, by vacuum drying at 70°C (or 158°F) for 4 hours (9);
• Bostwick flow index, calculated on the same product and expressed in cm of flow after 30 seconds;
• percentage of pulp (in weight), centrifuging 20 g (or 0.044 lb) of a sample at 16000 * g for 15 seconds;
• total pectins (10);
• serum proteins (11);

Results and Discussion

The chemical and physical characteristics of the ketchup used for the homogenization tests are shown in table 1.

In all homogenizing tests, both "cold" (T = 21°C or 69.8°F) and hot (T = 65 +/- 2°C or 149 +/- 35.6°F), the dry residue, the pectins and the serum proteins, calculated on single samples, did not show appreciable differences with respect to the parameters in table 1. The most important variation was the increase in the percentage of sedimentable pulp. The results of the different homogenization tests can be summarized like this:

• Homogenization modifies the structure of the product (increase of the percentage of sedimentable pulp)
• the Bostwick flow index, or the consistency of the product, is dependent on the percentage variation of the sedimentable pulp; for example, with an increase of pulp about 9%, in case of cold homogenization, the Bostwick index goes from 4.6 to 3.4 cm
• the final consistency of the homogenization pressure: in fact, independently of the homogenization temperature used, a first level where, with an increase in the homogenizing pressure, there is a proportional increase in the consistency of the product 9decrease of the Bostwick index) up to a maximum, while for a higher pressures there is a slight decrease in the consistency (although there is a later, even if limited, increase in the percentage of sedimentable pulp). This behavior is not easy to interpret; it could depend on a difference in geometry of the pulp particles;
• the products homogenized at higher temperature produce a higher viscosity and this effect is very evident in the product treated at 70/80 bar (or 1015.26/1160.30 PSI); the cold homogenized sample shows an increase incosistenty of about 26% compared with the 37% of the hot homogenized sample;
• lastly, with regards to the influence of one or two stage homogenization, the best results were obtained with a two stage homogenization (1st stage 240 bar (or 3480.90 PSI), 2nd stage 40 bar (or 580.15 PSI)) at a temperature of 65°C (149°F), with an increase of about 45% in the final consistency.

Behavior During Storage

From a rheological aspect, the ketchup behaves like pseudoplastic fluids with quite an evident flow limit (t o), however the principle of power (t = k. gn) describes the flow behavior of these products in a satisfactory way (0.978 <= R2 <= 0.994). The flow index (n) varies from 0.26 to .31, the consistency index (k) from 14.20 to 24.70 N.sn/m2; n results independent from the homogenizing pressure, while k increases with the increase of pressure. All the products, including those not homogenized, show very limited hystersis areas (flow curves), usually at higher creep gradients, probably due to the separation of the watery phase by centrifugation during measurement; this is indicative of the absence of thixotropy in these products ("small" hystersis areas), and it can be affirmed that homogenization, even though alterating "structure" of the product, does not cause behaviors of thixotropic kind.

And so it was useless to define an eventual "recovery time" of the substance since the product did not result dependent on time, the thixotropy being almost absent; this is confirmed by the measurement of the apparent viscosity against time, reported for the sample homogenized at 130/140 bar (or 1885.49/2030.53 PSI) at a temperature of 65 +/- 2°C (or 149 +/- 35.6°F) (fig 3).

The absence of thixotropy in the homogenized products was indicative of a small possibility that they would show variations in the apparent viscosity during storage. It is noted however that in several products the recluster of the pulp particles can happen, in the specific case of the homogenized products, with the final result of a smaller "complessive surface" and consequently a smaller capacity of "interaction" between the particles and therefore with a constant loss of viscosity or consistency during storage. For these reasons, the variations of viscosity within nine months were followed using rheological measurements. The results (fig. 3) exclude variations in viscosity during this period; this not only confirms that the products are not thixrotropic, but above all, that the advantages offered by homogenization (increase in the viscosity) are maintained in time.

The last aspect studied regards the eventual presence of syneresis.

Without objective methods, the visual "sensation" indicates that the phenomenon, more or less intense immediately after the preparation of the ketchup, tends to disappear during storage.

Conclusion

Homogenization allows the increase of 40% on average in the apparent viscosity of the ketchup, decrease depending on the processing temperature and homogenizing pressure. In all cases the homogenized products are not thixotropic nor do they present reclustering of the particles during storage; for this reason the advantages of homogenization are maintained with the passing of time.

Furthermore, in the experimental condition used, the phenomena of synersis shown immediately after the preparation of the ketchup tend to disappear during storage.