Etude des facteurs d'intensification de l'échange de chaleur dans le refroidissement des fruits et légumesInvestigating the factors for intensifying fruit and vegetable cooling

Increasing the rate of fruit and vegetable cooling is of economic significance for the following reasons: biochemical and microbiological processes are quickly suppressed; surface mass-exchange with ambient air is slowed down; power consumption is reduced, and the freezing output of cooling equipment and installations is increased.It was assumed that the fruits and vegetables in crates were arranged regularly and covered with thin paper of a negligible small thermal resistance in order to eliminate the infiltration of cold air within the layers. Crates were arranged on a pallet so that they touched horizontally. Thus, the packages form plates above and below the cooling air flow. Heat-exchange with a one-dimensional temperature field is established.It was found that intensification of cooling can be achieved by increasing the speed of cooling air as well as by stopping the process at a higher final temperature. Increasing the speed up to 5 m s^?1 shortens the time of the process by a factor of two compared with the low speeds (0.3–0.5 m s^?1) which are normally used. The product can be cooled until the temperature in the layer centre reaches 5°C, and the surface temperature about 2°C, which is low enough. Afterwards the cooled product is transferred to a freezing chamber or to a van-freezer. This factor results in a further halving of the cooling time. The combined effect of the two factors shortens the duration of the process by about 75% and increases the output of the cooling equipment from 3.5 to 4 times.     

Transfert de chaleur et de masse au cours du refroidissement des denrées périssables à bord des navires-durée minimale de refroidissement des cargaisons de fruits et légumes (dans les cales ou à l'intérieur des conteneurs)

Perishable goods (mainly fruit) are usually loaded aboard ships without precooling¹. The first phase of transport is therefore to cool the cargo down to the contractual carrying temperature as rapidly as possible. A priori, this implies that the ship's equipment can do this and in particular that the air distributing arrangement has been fitted for this purpose. The paper mainly deals with homogeneous cargoes having an even air permeability.In this Paper, the author adopts the conditions (extremely favourable) where the goods are directly exposed to the cooling fluid (with or without any protection against their drying out). These are conditions resulting in the minimum cooling time. The same calculation is applicable to any homogeneous cargo provided that the air distribution is properly balanced including that inside the packages (cases, boxes, etc). It may be locally applicable as well with the same conditions taking into account an unbalanced cooling air distribution system.The problems of cooling goods in large packages which the cooling air moves around is not dealt with: it leads moreover to prohibitive cooling durations and this case is to be avoided. This case is roughly approximated by a similar problem: the cooling of bodies with comparable sizes and shapes (plates, cylinders, spheres) with equivalent apparent conductivity which are suddenly put in a cold air flow maintained at an even and steady temperature. With such optimistic assumptions (where the heating is assumed to be negligible), the core and surface temperatures can be determined from the set of curves by Jacob⁹.The minimum cooling duration contemplated is the time needed to achieve a temperature in the whole cargo not too far from the carrying temperature. That temperature may exceed the latter by 2 or 3°C. In order to obtain it, all the basic cargo features are taken into account: breathing and, if necessary, desiccation as well as the rate of air circulation in horizontal and vertical ventilation.The functioning capabilities of the ship's equipment are also considered, on the one hand when starting and during the cooling down, if the available refrigerating capacity does not allow a step function as an initial condition, and, on the other hand, when the cooling air distributing system brings about a well known unbalance in the overall air flow distribution.     

Blanchiment et qualité des légumes et des fruits surgelés. Revue. 3. Aspects nutritionnels et hygiéniques, conclusions

The third and final section of a bibliographic study (see Int. J. Refrig.7 (1984) 384–388 and 8 (1985) 48–53), this article deals with the effects of blanching on vitamins and soluble nutrients as well as the sanitary state of quick frozen fruit and vegetables. Blanching and hydrocooling cause important losses in the soluble nutrients of products, consequently they cause extensive water pollution. The advantages produced by steam blanching are totally lost during hydrocooling. The hygienic quality of frozen vegetables is closely related to the bacterial counts of water and the cleanliness of equipment situated upstream of the blancher. The replacement of blanching by a less aggressive chemical treatment, as regards frozen vegetable quality, may be a solution for the future, but it will be some time before it can be applied to industry.

Résumé

Troisième et dernier volet d'une étude bibliographique (voir Rev. Int. Froid 7 (1984) 384–388 et 8 (1985) 48–53, l'article traite des effets du blanchiment sur les vitamines et les nutriments solubles ainsi que sur l'état sanitaire des fruits et légumes surgelés. Le blanchiment et le refroidissement à l'eau font perdre au produit des quantités importantes de nutriments solubles. Corollairement, ils engendrent une trés forte pollution des eaux. L'avantage donné par le blanchiment à la vapeur est entièrement annulé lors du refroidissement dans l'eau. La qualité hygiénique des légumes congelés est étroitement tributaire de la pureté bactérienne de l'eau et de l'état de propreté des équipements situés en aval du blancheur. Le remplacement du blanchiment par un traitement chimique moins agressif vis-à-vis de la qualité des légumes congelés représente peutêtre une solution d'avenir, mais elle ne semble pas applicable avant longtemps dans l'industrie.     

Etude de l'intensification des échanges de chaleur par convection dans des canaux de section triangulaire et rectangulaire

Most of the research work applied to the intensification of heat transfer through convection in turbulent flow, has been devoted to local heat flow and local friction losses. Intensification is obtained by means of turbulators inside smooth channels (fins, steps).In the re-sticking zone of the boundary layer, the heat transfer is maximum and the friction factor is minimum, the latter being maximum in the free flow zone. Maximum values are more than ten times higher than those observed in an undisturbed analogous stream.As early as 1958, those results have been applied to the construction of heat exchangers by which the heat transfer coefficient rises faster than the hydraulic resistance (discovery No 242).Such heat exchangers (water-air) are built from ribbed fin-plates and cross-tubes. The ribs act as turbulators inside the channels which are allowed between plates and tubes.The present paper deals with the influence of geometrical shape and distribution of the turbulator on thermal and hydraulic efficiencies of heat transfer surfaces.The results are presented as graphs where:Nu,Nu_R = Nusselt number for channels with and respectively, without, turbulators; ε,ε_R=friction loss factor for channels with, and respectively, without, turbulators; l'/d = pitch of trottling=ratio between the length of smooth piece l' and its hydraulic diameter d; d*/d=rate of throttling=ratio between hydraulic diameter of the throttled section d*, and that of the smooth piece d.Experimentations have been performed with 16 different combinations of l'/d and d*/d, in channels of the same triangular section and Reynolds number varying between 400 and 5500; in channels with different triangular sections; in channels with rectangular sections.Main results are: the heat transfer intensification depends on the parameters l'/d and d*/d; the maximum intensification depends on the shape of the channel section, the highest values being obtained with triangular shaped channels; the most efficient turbulators

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consist of two dimensioned surfaces generated along the channel radius.New radiators for farm tractor-engines have been developed. Their volume and weight are half those of conventional radiators. Their construction does not require any significant modification of the traditional manufacturing technique of such type of equipment and their utilization in dusty environments does not raise any particular difficulty.

Résumé

La réalisation de saillies et de cavités transversales dans la paroi des canaux d'air à section triangulaire ou rectangulaire entraîne une intensification des échanges de chaleur. Cette intensification peut être quantifiée par le nombre caractéristique (Nu/Nu_R)/(ξ/ξ_R) dont la valeur est supérieure ou égale à 1 pour des valeurs du rapport Nu/Nu_R inférieures à 2,15. L'intensification des échanges de chaleur dépend des paramétres géométriques caractéristiques du canal d'air (l'/d) et (d*/d).L'utilisation de plaques-ailettes nervurées dans la construction des radiateurs pour moteurs de machines agricoles entraîne une diminution de deux fois et plus du volume et de la masse de ces appareils, c'est la profondeur qui est réduite.Une grande résistance à l'encrassement et une technique simple de fabrication permettent d'envisager une modernisation rapide du parc de machines agricoles automotrices.     

Transport de fruits et légumes frais à température dirigée

Controlled temperature transport is necessary to preserve the qualities of fresh fruit and vegetables. Refrigeration should be applied as soon as possible after harvest and possibly, under certain conditions, in the vehicle during transport. This paper describes different types of vehicles and devices for producing refrigeration during transport. The particular case of ready to use vegetables (minimal processing) is discussed.     

Blanchiment et qualité des légumes et des fruits surgelés. Revue. I. Introduction et aspects enzymatiques

The first section of a three-part bibliographic study, this article successively examines the role, advantages and drawbacks of blanching, the enzymatic systems of foodstuffs, the localisation of peroxidase and catalase enzymes, methods of controlling blanching, the validity of tests, the regeneration of peroxidasic activity, the relation between temperature and the duration of blanching. Reasons are given as to why it is not possible to establish schedules for blanching, similar to what exists for sterilisation. It is shown that significant residual peroxidasic activity may not have undesirable consequences on maintaining the quality of a good number of quick frozen vegetables stored at − 18°C.

Résumé

Premier volet d'une étude bibliographique qui en comporte trois, l'article examine successivement: le rôle, les avantages et les inconvénients du blanchiment; les sytèmes enzymatiques des denrées, localisation des peroxydases et des catalases, méthodes de contrôle du blanchiment, validité des tests, régénération de l'activité peroxydasique, relation température et durée de blanchiment. Il est expliqué pourquoi il n'est pas possible d'établir des barêmes de blanchiment comme il existe des barêmes de stérilisation. Une activité peroxydasique résiduelle significative peut être sans conséquences défavorables sur le maintien de la qualité de bon nombre de légumes surgelés conservés à − 18°C.     

Blanchiment et qualité des légumes et des fruits surgelés. Revue. 2. Aspects sensoriels

The second section of a three-part bibliographic study (see Int. J. Refrig. 7 (1984) 384–388), this article deals with biochemical and physiocochemical mechanisms responsible for changes in the various components of the sensorial quality of frozen fruit and vegetables with and without prior blanching: texture, flavour and aroma (the origin of losses, the consequences of pre-cooking or blanching), colour (alteration of natural pigments, enzymatic browning, rupturing of chloroplasts). Solutions are proposed for the purpose of limiting undesirable effects due to thermal treatment. Blanching, which is indespensable for stabilizing the quality of several quick frozen vegetables, can be considered as a ‘necessary evil’.     

Analysis of advantages and limitations of absorber-generator heat exchangeAnalyse des avantages et des limites de l'échange de chaleur de l'absorbeur-générateur

In the last few years increased interest has been shown in absorption heat pumps utilizing advanced cycles. One particular advanced cycle which works with very large solution volumes is the so-called absorber-generator heat exchange cycle. We will show in this Paper that the theoretical advantages of this cycle can be severely diminished by limitations which are due to the characteristics of the solution field of the working pair used. These limitations are also important if absorption processes are to be used to realize the Lorenz process.     

Blanchiment et qualité des légumes et des fruits surgelés. Revue. I. Introduction et aspects enzymatiquesBlanching and quality of frozen vegetables and fruit. Review. I. Introduction and enzymatic aspects

The first section of a three-part bibliographic study, this article successively examines the role, advantages and drawbacks of blanching, the enzymatic systems of foodstuffs, the localisation of peroxidase and catalase enzymes, methods of controlling blanching, the validity of tests, the regeneration of peroxidasic activity, the relation between temperature and the duration of blanching. Reasons are given as to why it is not possible to establish schedules for blanching, similar to what exists for sterilisation. It is shown that significant residual peroxidasic activity may not have undesirable consequences on maintaining the quality of a good number of quick frozen vegetables stored at − 18°C.

Résumé

Premier volet d'une étude bibliographique qui en comporte trois, l'article examine successivement: le rôle, les avantages et les inconvénients du blanchiment; les sytèmes enzymatiques des denrées, localisation des peroxydases et des catalases, méthodes de contrôle du blanchiment, validité des tests, régénération de l'activité peroxydasique, relation température et durée de blanchiment. Il est expliqué pourquoi il n'est pas possible d'établir des barêmes de blanchiment comme il existe des barêmes de stérilisation. Une activité peroxydasique résiduelle significative peut être sans conséquences défavorables sur le maintien de la qualité de bon nombre de légumes surgelés conservés à − 18°C.     

Heat and mass transfer analysis in air-cooling of spherical food productsTransfert de chaleur et de masse dans le refroidissement par air des aliments sphériques

The one dimensional transient heat conduction equation, in spherical co-ordinates, is solved with convective surface boundary condition, during air-cooling. The enthalpy potential concept is used to include the cooling effect of desiccation. A calculation scheme is proposed in which, up to half the cooling time, the calculation is made with both heat and mass transfer from the product surface, thereafter it is continued with heat transfer only. The calculated temperatures for apples and potatoes are compared with the measured values available in the literature, and good agreement is observed.     

Etude quantitative de la structure à l'état congelé de muscle et de steak haché

The structure of frozen meat is studied after freeze-drying, with the technique of image analysis. The variable parameters are the freezing rate, the type of meat and the effect of grinding and mixing or not with pork fat.After freezing, the samples were cut (10 μ thick), freeze-dried, fixed and stained with Regaud's haematoxylin.Using a Quantimet 720 we measured the total volume occupied by ice crystals, the distribution in size of ice crystals and the specific area of freeze-dried samples. These results were compared with those obtained by two physical methods, mercury intrusion porosimetry and the volumetric method of krypton adsorption.The comparison of stereological and physical methods suggests a spindle-shaped model for the ice-crystals in frozen whole meat.Concerning the technological parameters, the results show that grinding involves an increase of the size of ice crystals and that mixing pork fat with ground meat induces a decrease in the size of ice crystals in the pieces of meat.

Résumé

On étudie la structure de la viande congelée, aprés lyophilisation, par analyse des images. Les variables sont la vitesse de congélation, le type de viande et l'influence du hachage et de l'addition ou non de tissue adipeux de porc.Aprés congélation, les échantillons ont été coupés (10 μm d'épaisseur), lyophilisés, fixés, puis colorés à l'hématoxyline de Regaud.En utilisant un Quantimet 720 nous avons mesuré le volume total occupé par les cristaux de glace, la répartition de leurs dimensions et la surface spécifique des échantillons lyophilisés. On a comparé ces résultats à ceux obtenus par deux méthodes physiques, la porosimétrie au mercure et la méthode volumétrique d'adsorption de krypton.La comparaison des méthodes stéorologiques et physiques améne à penser que les cristaux de glace du muscle de viande congelé entier ont une forme en fuseau.Quant aux paramétres technologiques, les résultats montrent que le hachage implique une augmentation de la taille des cristaux de glace et que le mélange de tissu adipeux de porc avec la viande hachée provoque une diminution de la taille des cristaux de glance dans les morceaux de viande.