GB2029015A

GB2029015A - A process and an apparatus for rapidly and quantitatively assessing the oxidisability of food products

Info

Publication number: GB2029015A
Application number: GB7928186A
Authority: GB
Original assignee: Societe des Produits Nestle SA; Nestle SA
Current assignee: Societe des Produits Nestle SA; Nestle SA
Priority date: 1978-09-01
Filing date: 1979-08-14
Publication date: 1980-03-12
Also published as: CH624771A5; FR2435037A1; FR2435037B1; GB2029015B

Abstract

For rapidly and quantitatively assessing the oxidisability of food products containing fats, the product is oxidised by irradiation with ultra- violet light in the presence of oxygen and the quantity of oxygen absorbed during oxidation or the quantity of at least one compound released as a result of the oxidation of the product is determined. An apparatus for this purpose comprising at least one chamber capable of receiving at least one sample of the product, at least one ultra-violet source (UV source) for irradiating the sample and thermally insulated from the chamber and means for detecting the concentration of oxygen dissolved in an aqueous liquid sample or means for detecting volatile compounds released from dry samples or from oils.

Description

SPECIFICATION A process and an apparatus for rapidly and quantitatively assessing the oxidisability of food products This invention relates to a process for rapidly and quantitatively assessing the oxidisability of food products containing fats and to an apparatus for carrying out this process.

In the food industry, the keeping properties of products are crucially important. At present, the keeping test extend over a period at least as long as that for which the products are required to retain most of their properties. This period may be several weeks or several months or even one year or longer in cases where the factor limiting the keeping properties of the products is the degradation of the fats present in them. Now, where it is a question of optimising a production process or of selecting the starting materials for example, it is extremely troublesome to have to wait for months or longer to know the outcome or comparative tests. Accordingly, there is an imperative need in the food industry for a process and an apparatus which enable reliable keeping tests to be carried out over a period much shorter than that for which the product is required to keep.

An object of the present invention is to satisfy, this requirement.

The process according to the present invention is characterised in that the product is oxidised by irradiation with ultra-violet light in the presence of oxygen and the quantity of oxygen absorbed during oxidation or the quantity of at least one compound released as a result of oxidation of the product is determined.

It has been found that there is a narrow correlation between the normal autoxidation of a food product under normal keeping conditions and autoxidation accelerated by irradiation with ultra violet (UV) light and that the relative qualities of various samples are revealed by well as reliably by tests accelerated by UV light as by normal long term tests.

The mechanism behind the autoxidation of fats is well known. Similarly, it is known that exposure to light accelerates the onset of the reaction.

However if a method of studying food products based on this principle has hitherto never been contemplated, it is because, as in cases where attempts have been made to accelerate autoxidation by increasing the temperature, the course of the reaction and the resulting products may differ considerably according to whether ageing is natural or artificial.

The reaction process comprises three phases: first initiation, second propagation and third the production of inactive reaction products. These inactive reaction products include aldehydes and ketones responsible for the rancid odour also hydrocarbons characteristic of the degraded fatty acids. Among these hydrocarbons are, for example, ethane, butane, pentane and octane.

It has been found that, although the range of hydrocarbons released as a result of the oxidation of a food product containing fats varies according to the conditions under which oxidation has taken place, it is nevertheless possible to rely on the quantity of at least one of the hydrocarbons released, particularly pentane, for effectively assessing the oxidisability of the product. In other words, for equal UV irradiation conditions, the respective quantities of a hydrocarbon released by various specimens or samples or starting materials or similar products are valuable criteria for assessing their keeping properties.

In cases where the products to be examined are aqueous liquids, it is easier to determine the quantity of oxygen absorbed during oxidation. In this case, it is possible to rely on the variations in the concentration of oxygen in the liquid. If the oxygen present in dissolved form in the liquid is not available in quantities sufficient to accelerate oxidation during UV irradiation, its initial concentration may be increased by bubbling.

In cases where the products to be examined are either dry products in divided form, particularly in the form of powders, platelets, grains or flakes, or oils, it is preferably the oxygen present in the space surrounding the product which is used to feed the reaction and it is possible to measure the quantity of one or more hydrocarbons released into this space in consequence of the reaction.

The present invention also relates to an apparatus for carrying out the process. This apparatus is characterised in that is comprises at least one chamber intended to receive at least one sample of the product, at least one ultra-violet light source (UV source) for irradiating the sample thermally insulated from the chamber and means for detecting the concentration of oxygen dissolved in an aqueous liquid sample or means for detecting volatile compounds released from dry samples or oils.

In order to obtain reliable and reproducible results, it is necessary to protect the product from the heat given off by the UV source. Since, on the other hand, there is no need to cool the UV source which has to be hot to develop its full power and to emit the desired UV light, it is advisable to separate the UV source from the irradiation chamber by a filter which removes the heat and allows the UV rays through. This filter may be in the form of a double wall of quartz in which a cooling liquid, particularly water, may circulate. It may also be necessary to heat-stabilise the chamber in order to keep the product at as constant a temperature as possible. This may be done for example by at least partly delimiting the chamber by a double jacket in which a liquid of given temperature may be circulgted.

With regard to the UV source itself, it is preferred to use a source emitting ultra-violet rays of long wavelength in the range from about 300 to 400 nm. With regard to the means used for detecting the oxygen absorbed or the volatile compounds released, it is possible to use an oxygen electrode for the former and a gas-phase chromatograph for example for the latter.

Thus, in a first embodiment of the apparatus according to the invention intended for assessing the oxidisability of aqueous liquid food products, particularly animal or vegetable milks for example, the UV source is arranged inside a double envelope of quartz dipping into the irradiation chamber and the detection means are formed a polarographic electrode likewise dipping into the chamber.

In one preferred variant of this embodiment, the irradiation chamber is delimited by a glass flask having three openings, including a central opening, in its upper surface, a double-walled quartz tube containing the UV source dips vertically into the flask through the central opening and the polarographic electrode dips vertically into the flask through one of the other openings.

By contrast, in a second embodiment of the apparatus intended for assessing the oxidisability of dry food products in divided form, particularly milk powder, various fluqrs or freeze dried meat for example, and also oils, the UV source is arranged outside the irradiation chamber from which it is separated by a double wall of quartz and the detection means are formed by a tubular circuit connecting the chamber to a gas-phase chromatograph.

In one preferred variant of this second embodiment, irradiation chamber is delimited by a vertical double-jacketed metal cylinder, the double wall of quartz is formed by two parallel horizontal discs covering the chamber and the UV source is arranged above the two discs.

Other variants comprising several irradiation chambers for examining several samples are also possible.

The accompanying drawings illustrates by way of example two embodiments of the apparatus according to the present invention and also test results in the form of a graph.

In this drawings Figure 1 is a diagrammatic front view of a first variant of the apparatus.

Figure 2 is a diagrammatic vertical section through a second variant of the apparatus.

Figure 3 diagrammatically illustrates the detection circuit of the second variant.

Figure 4 is a graph showing the results of the tests described in Example 1 below.

Figure 5 is a graph showing the results of the comparative tests described in Example 2 below.

Figure 6 is a graph showing the results of the tests described in Example 3 below.

The apparatus illustrated in Figure 1 is intended for assessing the oxidisability of aqueous liquid food products. It consists of a- glass flask 1 having three upper openings or necks 2, 3, and 4.

Through the central opening 3, a quartz tube 5 with a double wall 6,7 dips into the interior of the flask which forms the irradiation chamber 8.

Cooiing water 9 is able to circulate between the inner wall 7 and the outer wall 6 of the tube. A UV source 10, a high-pressure mercury vapour bulb, is arranged at the end of the tube 5 inside the inner wall 7. A polarngrapliic electrode 11 for measuring the concentration of the oxygen dissolved in the aqueous liquid to be examined dips into the interior of the flask through the opening 2. The liquid to be examined is introduced into the flask through the opening 4 which is then closed with a stopper 12. The liquid has to fill the entire space 8.

The apparatus illustrated in Figure 2 is intended for assessing the oxidisability of dry food products in divided form (for example powders, granulates or flakes) and oils. It consists of a cylinderical housing 20 of which the lower part 21 which has a double jacket 22 accommodates the irradiation chamber 8. Water 9 is able to circulate in the double jacket 22, keeping it at a selected temperature in the range from 0 to 100 C. The following stage of the cylinder forms a heat filter 23. It is made of two quartz discs 24 and 25 between which water 9 is able to circulate, removing the heat given off from a UV source 10.

The UV source 10 is arranged in the head 26 of the cylinder 20 above the heat filter 23. In this case, too, the UV source is formed by a highpressure mercury vapour lamp. The product 27 to be examined is arranged on the bottom of the irradiation chamber 8 where it may be stirred by means of the magnetic rod 28. The composition of the gases released and present in the head space 29 situated above the product 27 in the irradiation chamber 8 is analysed by means of a gas-phase chromatograph (ct Figure 3) connected to the head space 29 by a tubular circuit 30).

Figure 3 shows how the gas-phase chromatograph 31 followed byan integrator 32 is connected to the head space 29 of the irradiation chamber by the tubular circuit 30. In this circuit, a pump 33 for circulating the gas to be analysed is engaged just before each analysis so that the composition of the gas is the same in the circuit as in the head space. A sample of the gas is removed by actuating a valve 35 which isolates a coil 34 hitherto connected in series in the circuit 30 and which connects it on one side to a nitrogen (N2) source and on the other side to a circuit 36 of a chromatograph. The contents of the coil are then introduced into the circuit of the chromatograph by purging with nitrogen. The signals resulting from the analysis of the gases are quantitatively integrated by the integrator 32.

The following Examples are intended to illustrate the process according to the invention and its practical application.

EXAMPLE 1 A whole cow's milk (A), a skimmed cow's milk (B), and a soya milk (C) are examined by means of the apparatus illustrated in Figure 1. The UV source is a 70 W high-pressure mercury vapour lamp (make: HANAU TQ 150) which emits light having a wavelength of 320 nm. The three-necked flask is completely filled with the milk to be examined.

The concentration of the oxygen dissolved in the milk is measured during irradiation by means of the polarographic electrode. At the same time, samples are taken and subjected to tasting by trained tasters. The results shown by a graph in Figure 4 are obtained. This Figure show that the cow's milks (curves A and B) oxidise very quickly whilst the soya milk (curve C) appears to be distinctly less prone to oxidation. The arrows are placed on the vertical of that period of irradiation after which the tasters detected an alteration of the milk. They enable the organoleptically detectable degree of oxidation to be read off from the curves. A very clear correlation is observed between the oxidisabiiity -high or low- of the milks and the period -- respectively short or longs after which an alteration was detected.This alteration was noticed as a "burnt", "cooked" or "rancid" tast or odour.

EXAMPLE 2 (Comparison) Various flakes of cereals are produced by the same standard method, but using grains of different varieties of wheat. The standard method comprises: - grinding the grains in a hammer mill and passing them through a 2 mm mesh sieve, - mixing with water so that the mixture has a dry matter content of 35% by weight, - drying on rolls at 170 to 1 750C, - grinding and sifting through a 2 mm mesh sieve, - packaging in air in aluminium cans 107 mm in length and 75 mm in diameter (50 g of flakes per can).

Flakes are prepared from grains of three different wheats, namely a low-quality wheat (ai, a variety code-named "PROBUS 22.26" (b) and a variety code-named "PROBUS 9.5" (c). They are introduced into cans which are hermetically seaied and stored at 300C. From the 25th day of storage, the head space of various cans is analysed at intervals of a few days in order to follow the oxidation of the flakes. In fact the quantity of pentane accumulated in the head space is determined for each can by means of a gas-phase chromatograph. The results obtained are shown by graph in Figure 5. The quantity of pentane measured is indicated in integration units (us).

Deviations of less than 1 05 Ul are not significant.

Each curve illustrated the variation with time of the oxidation of the canned flakes emanating from one of the above varieties of wheat a), b) and c). It can be seen that the variety c) gives flakes which oxidise distinctly less quickly than those prepared from the variety b), whereas the flakes obtained from the low-quality a) deteriorate quickest. It will also be noted that this systematic study covers a period of at least one to two months which is the period for which the flakes are expected to keep satisfactorily.

EXAMPLE 3 Various flakes of cereals are produced in the same way and from the same varieties of wheat mentioned in Example 2, except that they are not canned, but instead are directly examined by means of the apparatus illustrated in Figures 2 and 3. The UV source is a 70 W high-pressure mercury vapour lamp (make: HANAU Q 81) which emits light having a wavelength of 320 mm. The temperature of the irradiation chamber is kept at 300C. The results shown by the graph in Figure 6 are ohtained. The quantity of pentane measured is indicated in integration units and the irradiation time in hours. It can he seen that the relative qualities of the various flakes become clearly apparent in less than 5 hours and that it is not even desirable to continue the test any longer because otherwise the curves might show interruptions. It can be seen that the curves of Figure 6 have substantially the same relative position as the curves of Figure 5 (Example 2) and that, accordingly, it would even be possible quantitatively to assess the oxidisability of the canned flakes over several months from the quantitative results obtained in a few hours by means of the process and an apparatus according to the invention.

Claims

1. A process for assessing the oxidisability of food products containing fats, which comprises oxidising the product by irradiation with ultraviolet light in the presence of oxygen and determining the quantity of oxygen absorbed during oxidation or the quantity of at least one compound released as the result of the oxidation of the product.

2. A process as claimed in claim 1, in which the food product in an aqueous liquid, the oxygen is present in dissolved form in the liquid and the variation in the concentration of the dissolved oxygen is measured.

3. A process as claimed in claim 1, in which the food product is either a dry product in divided form or an oil, the oxygen is present in the space surrounding the product and the quantity of at least one hydrocarbon released into this space as a result of the oxidation of the product is measured.

4. An apparatus for assessing the oxidisability of food products containing fats, comprising at least one chamber capable of receiving at least one sample of the product, at least one ultra-violet light source (UV source) for irradiating the sample thermally insulated from the chamber and means for detecting the concentration of oxygen dissolved in an aqueous liquid sample or means for detecting volatile compounds released from dry samples or from oils.

5. An apparatus as claimed in 4 for assessing the oxidisability of aqueous liquid food products, wherein the UV source is arranged inside a solubie envelope of quartz dipping into the chamber, means are provided to enable a cooling liquid to circulate in the double envelope, and the detection means are formed by a polarographic electrode likewise dipping into the chamber.

6. An apparatus as claimed in claim 5, wherein the chamber is delimited by a glass flask having three openings, including a central opening, in its upper surface, a double-walled quartz tube accommodating the UV source dips vertically into the flask through the central opening and the polarographic electrode dips vertically into the flask through one of the other openings.

7. An apparatus as claimed in claim 4 for assessing the oxidisability of dry food products in divided form or of oils, wherein the UV source is arranged outside the chamber from which it is separated by a double wall of quartz, means being provided to enable a cooling liquid to circulate in the double wall, and the detection means are formed by a tubular circuit which connects the chamber to a gas-phase chromatograph.

8. An apparatus as claimed in claim 7, wherein the chamber is delimited by a double-walled vertical metal cylinder, the double wall of quartz is formed by two parallel horizontal discs covering the chamber and the UV source is arranged above the two discs.

9. A process for assessing the oxidisability of food products containing fats substantially as described with particular reference to Example 1 or Example 3.

10. An apparatus for assessing the oxidisability of food products containing fats substantially as described with particular reference to Figure 1 or Figure 2 and 3 of the accompanying drawings.