The impact of water on food thermodynamics and physics and, therefore, on its quality is more important than the one of any other food chemical component. When fundamentals of chemical kinetics are applied, the rates of the reactions responsible of food quality decay can be described as a function of food composition and of other external elements interacting with foods. Among them, water activity and water content have been widely used to determine the role of water in the kinetic reactions of deterioration. Recently, researchers have found limitations in using the water activity parameter. According to them, the role of water in foods can be better described by evaluating the role, in stability of the quality attributes, of the non-equilibrium states of amorphous food products. Following this approach, the dynamics of changes are described in kinetics terms and can be more efficiently predicted by the glass transition temperature more than by the water activity. The glass transition, which is a second order transition in amorphous materials from the glassy to the rubbery state, is primarily dependent on water, which is a plasticizer and is responsible for the physical state of multiphase systems (as foods are) together with temperature. The subject of the role of water in the decay of food quality is presented in this paper, according to the principles of food material science | L'impatto dell'acqua sulla termodinamica e sulla fisica dell'alimento e, quindi, sulla sua qualità è maggiore di quello di ogni altro componente chimico. Applicando principi fondamentali di cinetica chimica, le velocità delle reazioni responsabili del decadimento della qualità possono essere espresse in funzione della composizione e di fattori esterni al prodotto. L'attività dell'acqua o il contenuto in umidità sono stati ampiamente utilizzati per determinare il ruolo dell'acqua nelle reazioni cinetiche di degradazione. Scuole più recenti attribuiscono al concetto di attività dell'acqua alcune limitazioni: il ruolo dell'acqua negli alimenti è discusso in relazione allo stato di non-equilibrio dei prodotti alimentari amorfi nella stabilità degli attributi di qualità. Secondo questo approccio la dinamica dei cambiamenti è descritta in termini cinetici e può essere efficacemente predetta dalla temperatura di transizione vetrosa, più che dall'attività dell'acqua. La transizione vetrosa, transizione di secondo ordine da uno stato vetroso allo stato gommoso dei materiali amorfi, è innanzitutto influenzata dall'acqua quale plasticizzante del sistema, che, quindi, insieme alla temperatura, determina lo stato fisico delle fasi di sistemi multifase quali sono gli alimenti. Il tema del ruolo dell'acqua nel decadimento della qualità degli alimenti viene presentato in questo lavoro secondo i principi della scienza dei materiali alimentari

There has been developed a two-stage method for cooling jars with preserved food after sterilization. The first stage comprises cooling preserves up to t 75-80 deg. C in the air flow, the air velocities being 2.75, 3.7, 4.8, and 5.8 m/sec. The time of cooling reduced as the air velocity increases to be 11, 9, 8.5 and 8 min, respectively. The average speed of cooling gradually increases from 1.82 deg. C/min to 2.5 deg. C/min. The second stage comprises continuing cooling to apply a water film of 60-65 deg. C to the jar surface at 5-10 sec interval. During the process a jar is with a certain frequency is overturned down from the bottom to the cap. The average speed of cooling product is 3.33 deg. C/min at cooling air velocity of 2.75 m/sec and it is gradually increasing to reach 5.4 deg. C/min. Experimentally range of optimum cooling air velocities has been determined to be 4.8-5.8 m/sec, cooling time being decreased by 0.6 min only. A mathematical model has been has been developed describing the two-stage air and water vaporizing cooling time of stewed fruit jars depending on some factors. Relative error in comparing target values with experimental values did not exceed 8%. The developed method is recommended to be used at food canning industry enterprises and for designing continuously operating devices. | Разработан двухэтапный способ охлаждения банок с консервированными продуктами после стерилизации. На первом этапе (до t 75-80 град. С) охлаждение консервов производят в потоке атмосферного воздуха при различных скоростях охлаждающего воздуха – 2,75; 3,7; 4,8 и 5,8 м/с. Продолжительность охлаждения с увеличением скорости охлаждающего воздуха снижается и составляет соответственно 11,0; 9,0; 8,5 и 8,0 мин. При этом средняя скорость охлаждения продукта постепенно увеличивается с 1,82 град. С/м до 2,5 град. С/мин. На втором этапе охлаждение продолжается с нанесением на поверхность банки водяной пленки температурой 60-65 град. С с интервалом 5-10 с, при этом в процессе охлаждения банку с определенной частотой переворачивают с донышка на крышку. Средняя скорость охлаждения продукта составляет 3,33 град. С/мин при скорости охлаждающего воздуха 2,75 м/с и постепенно увеличивается, достигая 5,4 град. С/мин. Экспериментально определен интервал оптимальных скоростей охлаждающего воздуха (4,8-5,8 м/с), при котором продолжительность процесса охлаждения сокращается всего на 0,6 мин. Составлена математическая модель, описывающая продолжительность двухэтапного воздушно-водоиспарительного охлаждения банок с компотами в зависимости от ряда факторов. Относительная погрешность при сопоставлении расчетных значений с опытными не превышала 8%. Разработанный способ рекомендуется для применения на предприятиях консервной промышленности и при проектировании аппаратов непрерывного действия.