Threshold values degradation products

Many of these food and pharmaceutical products are heat sensitive that is, the finished product may be damaged or destroyed if it is exposed to too great a temperature over an extended period of time. Even common products like tomato catsup and penicillin spoil or lose their efficacy when exposed to ambient temperatures for long periods of time. The chemical reactions that limit shelf life are strongly temperature dependent. Some biological products may be handled at elevated temperatures in dilute solutions, but may degrade at the same temperature once the concentration exceeds a certain threshold value. 

Figure 55.13 shows another example of areal-time process control for the heat pump dryer to reduce nutrient degradation. Experiments have been carried out with hypodermic thermocouple needles to measure the transient temperature profiles of food products (Chou et al., 1997). These measured values make it possible to tune the drying air temperature to prevent the internal product tanperature from reaching a threshold value, hence reducing thermally induced nutrient degradation. 

Some of the compounds identified in YEs which are formed either by the thermal degradation of thiamine or on the interaction of thiamine degradation products with other components are shown in Fig. 2. They include aliphatic sulfur compounds, furans, thiophenes and thiazoles. 2-Methyl-3-furanthiol and 2-methyl-3-thiophenethiol have been identified in YEs 9,13 14) and are well known thermal degradation products of thiamine (29). As well as possessing meaty aromas and low odor threshold values 34), these compounds are key precursors of several other sulfur-substituted furans and thiophenes, including the derivatives in Fig. 2. Most possess meaty aromas at low concentrations and several have been identified in YEs (see Tables I and III). 

There is one characteristic that heavily limits the use of PLLA for commercial applications and that is the low crystallization rate. This results in a mostly amorphous product after processing. The minimum crystallization half time, t, for pure PLLA is of the order of several minutes, this is an extremely long time since the cooling time after the general processing methods is around 1 s. It has been described that the mechanical properties and degradation kinetics of the semi-crystalline PLLA are different from those of completely amorphous PDLLA. Mechanical and thermal properties of PLLA become almost constant when its molecular weight is above a threshold value of 70,000 Da (10). 

The compound 4-hydroxy-2,5-dimethyl-3(2H)-furanone (furaneol, I in Formula 4.67) is the corresponding degradation product from the 6-deoxy-L-mannose (rhamnose) (Formula 4.69). Furaneol can also be formed from hexose phosphates under reducing conditions (cf. 4.2.4.4.6) and from C-3 fragments (cf. Formula 4.47). With a relatively low odor threshold value, furaneol has an intensive caramel-hke odor. It is interesting that furaneol is also biosynthesized in plants, e. g., in strawberries (cf. 18.1.2.6.9) and pineapples (cf. 18.1.2.6.10). 

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