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Upon the completion of this module students should be familiar with:

  • the main chemical components of fruits and vegetables and their properties;
  • postharvest physiology and biochemistry of fruits and vegetables, particularly, ripening and respiration;
  • the characteristics of climacteric and nonclimacteric commodities;
  • factors influencing the ripening and storage-life of fruits and vegetables;
  • major causes of postharvest loss of fruits and vegetables and their prevention;
  • principles of canning, freezing, drying and pickling of fruits and vegetables.

Upon successful completion of this topic, students should be able to:

  • identify the major components of fruits and vegetables; and
  • appreciate the importance of fruits and vegetables as sources of vitamin A, C and folic acid.

Proteins

Fruits and vegetables contain about 1-5% protein. Protein content of vegetables, especially legumes, is generally higher than that of fruits. Even so, neither fruits nor vegetables are a significant contributor of protein in our diet. Many of the proteins are enzymes that are involved in the maturation, ripening and senescence of fruits and vegetables.

Review questions

  • What can be used as a general guide to the content of folic acid in vegetables?
  • In ripe fruits, what compound has starch been converted to?

Upon successful completion of this topic, students should be able to:

  • the different physiological stages of fruits and vegetables including growth, maturation, ripening and senescence;
  • the major physiological and biochemical events that occur in postharvest fruits and vegetables, especially respiration, and their influences on the quality and storage life of produce;
  • the differences between climacteric and non-climacteric commodities;
  • the effect of ethylene on the ripening of climacteric fruits;
  • the influence of storage temperature on the rate of respiration and other metabolic activities of postharvest fruits and vegetables;
  • the effect of temperature on the storage life of postharvest produce;
  • the methods for cooling produce;
  • the effect of storage atmosphere, especially the concentrations of oxygen and carbon dioxide, on the storage life and quality of fruits and vegetables; and
  • controlled and modified atmosphere storage of produce.

In addition to reducing the rate of respiration, CA and MA have been shown to:

  • reduce the rate of natural ethylene production, and reduce the sensitivity of fruits to ethylene-stimulated ripening;
  • CA or MA with low oxygen levels lowers the rate of chlorophyll degradation and improves the retention of green colour in green vegetables;
  • CA or MA with high CO2 concentrations inhibits the breakdown of pectin substances and retains the fruit firmness for a longer period;
  • improve the retention of flavour in certain produce; and
  • CA or MA with increased CO2 inhibits the growth of a number of decay organisms and hence reduces rotting of produce

Review questions

  • Why is the respiration rate of a fruit a good indicator of its storage life?
  • What are the major differences between climacteric and non-climacteric fruits?
  • Would it be a good practice to harvest non-climacteric produce unripe and then ripen it with the aid of ethylene? Explain your answer.
  • Would a reduction in storage temperature from 25°C to 20°C have the same effect in extending the storage life of produce as a reduction from 20°C to 15ºC?
  • How can ethylene be removed from the atmosphere surrounding fruit stored in a plastic bag?

Proteins in Fruits and Vegetables

Mammalian muscle flesh contains an estimate of 75% water, 19% protein, 2.5% of intracellular fat, 1.2% of carbohydrates and 2.3% of soluble nonprotein substances which can include, nitrogenous compounds and inorganic substances. Muscle proteins are either water-soluble substances referred to as sarcoplasmic or in salt concentrates referred to as myofibrillar proteins. There are a wide variety of sarcoplasmic proteins, the majority which are glycolytic enzymes responsible in glycolytic pathways as observed in energy conversion pathways. Major abundant myofibrillar proteins include myosin and actin which are essential in connective tissues functions. Fat meat is often the adipose tissue which is used by the animals to store energy in form of fats that is glycerol esters combined with fatty acids or in intramuscular fat which has high quantity amounts of phospholipids and cholesterols, (Li,  Xu & Zhou, 2012).

Water holding retention in fresh meat is an important aspect when freshness aspect is being desired as it plays a crucial role in both the yield and meat quality. The underlying principle of water loss or drip is largely influenced by both amounts of space present in the cells of the muscle and the pH of the tissue. There are other factors which can affect water holding capacity and dripping factors in meat. They include, a process using techniques applied, cuts orientation of the meats, temperature level decline after preparation, storage temperature and freezing rates, (Sharedeh, Gatellier, Astruc & Daudin, 2015).

Water holding capacity in meats often refers to as the ability of post-mortem meat to retina water throughout the phases even after external forces are applied. In this regard, there are physical and biochemical factors in meat which have an effect on the water holding capacity in meats. Net charge effect of meat has an effect on water retention. When meat ph is at isoelectric point, the net charge of proteins is 0, thus a number of negative and positive charges are equal. These charges are attached together and this results in the reaction in the water levels which gain attraction in meats. Further repulsion of like charges can occur in the myofibril structures which lead to space reduction. The reduced net protein charges often lead to lowered levels of water holding capacity due to lower available charges in proteins for water binding and allow compaction of proteins which force water to go to the free compartment hence its losses, (Lawrie, & Leward, 2016).

Effects of salt on meat have been investigated and studies have shown lower salt soluble protein indicating higher extractability especially in breast meat, (Barbut , Zhang & Marcone, 2005), while elevated sarcoplasmic denaturation of protein incubated at increased temperatures, (Zhu, Ruusunen, Gusella, Zhou & Puolanne , 2011). With meats especially that of chicken, water holding capacity characteristics have shown to change tremendously during the first 24-hour post-mortem. An investigation by Zhuang and Savage, (2012), showed that water holding capacity was higher at 24 hours post-mortem.

Hence this study sought to identify how to determine procedures utilized for conducting water holding capacity in meats and further to establish effects of ph and salt contractions factors on water holding a capacity of meat.

Water Holding Retention in Fresh Meat

In conducting this experiment, ph solutions will be required, 0.1 M HCl, 0.1M NaOH solution, 3.0% NaCl solution, 0.5% Na triphosphate solution, ph meter, centrifuge and plastic centrifuged tubes.

A total of 8 centrifuged tubes were labeled and each tube weight measurement was done. Lean meat was prepared and minced, and then 10 g of the minced meat was transferred to each centrifuge tube. In each tube, 10 mL of same ph solution was added and mix thoroughly. Ph measurements were undertaken after this stage.  0.1m of HCL and 0.1M of NaOH was added in order to adjust the ph to the set target measurements for each tube. When the ph of the tube was reached, the solution made up to 40mL. Tubes 7 and 8 relevant electrolyte of either 3% NaCl or 0.5% Na triphosphate was used to make the solution 40Ml.

The following volumes and ph targets were obtained;

Tube 1 - ph 4.0

Tube 2 - ph 4.5

Tube 3 - ph 5.0

Tube 4 - ph 5.5

Tube 5 - ph 6.0

Tube 6 - ph 7.0

Tube 7 - 3.0% NaCl

Tube 8 - 0.5% Sodium triphosphate

After a period of 15 minutes, the tubes were centrifuged for a period of five minutes at 3000rpm then the supernatant was decanted into a beaker. The sediment myofibrils were weighed and volumes of supernatant were taken and recorded.

Experimental step

Ph 4.0

Ph 4.5

Ph 5.0

Ph 5.5

Ph 6.0

Ph 7.0

3%NaCl

0.5%Na triphosphate

 

Tube 1

Tube 2

Tube 3

Tube 4

Tube 5

Tube 6

Tube 7

Tube 8

The weight of the tube (g)

13.09

13.08

13.21

13.12

13.03

13.11

13.14

13.13

The weight of the minced meat

10.61

10.88

10.12

10.08

10.95

10.27

10.20

10.75

Volume of 0.1 M HCl required to adjust the Ph (ml)

2.6

2.3

1.1

0.6

-

-

-

--

Volume of 0.1 M NaOH required to adjust ph (ml)

0.6

1.1

Weight of myofibril pellet (g)

19.91

13.93

12.09

11.02

13.29

12.99

13.99

15.31

Volume of supernatant (ml)

19.91

25.97

28.90

30.59

28.31

28.02

26.21

25.19

WHC

1.87

1.28

1.19

1.09

1.31

1.26

1.37

1.42

WHC %

187%

128%

119%

109%

131%

126%

137%

142%

Table 1Shwoing water holding capacity calculation at different ph rates

The above results show that the minced meat is affected by water holding capacity and its tenderness due to the changing ph. When the ph levels were brought to lower levels of 4.0, water holding capacity of the meat increased, while when the meat was brought to its isoelectric point at the angle of myofibrilla proteins after acidic or basic treatment at around 5.5 pHs, there is gain in weight from the previous acidic to basic forms, (Santoz et al., 2016). This process of the reversibility suggest that tenderness of meat may be a factor of proteolysis, rather it could lead to further destruction of myofibrillar structures which were disrupted t the acidic and basic ph after electric repulsion, (Ke, 2006).

Hence muscle ph plays a key role in water holding capacity of meat. The decrease in ph due to the progressive post-mortem of meat leads to reduced charges of meat, reduces the net protein charge, hence diminishing the water holding capacity. An increased decline in ph leads to the accelerated water holding capacity and reduced weight of sediment meat, (Zhou et al., 2014).

In the conversion of muscle to meat, there is build up of lactic acid in the tissues which lead o rescued meat ph. When the isoelectric ph has been attained in the meat, major proteins, the net charge of the protein is  (Zhang et al., 2018 Zhang et, which signifies that there is an equal number of negative and positive charges. These unlike charges attract each other and lead to water levels reduction which are responsible for holding the proteins observed in the table. As there is an occurrence of repulsion of like charges, net charges of myofibril reach zero which is concurrent to reduction of myofibril structure enabling packing of structures together, this yield space reduction in the myofibril.

Factors Affecting Water Holding Capacity and Dripping in Meat

The isoelectric point in this experiment is determined at the point when the water holding capacity is minimized. This point reflects the equal charges on the negative and positive sides. From the results, the minced meat has an isoelectric point of ph 5.5. this is the point where the there is diminished water holding capacity of the meat.

NaCl and alkaline phosphates have an effect on the water holding capacity. There was an increased level of water holding capacity. Diffusion of sodium and phosphate ions in the minced meat offers additional moisture which prevents drying of the meat, (Lorenzo, Dominguez, Fonseca, & Gomez  2014). Sodium Chloride solution promotes swelling of protein structures which leads to increase int eh repulsive electrostatic force thus allowing the expansion of filament matrix. The phosphates offer neutralization and cross-linking between the myosin and action, enabling the dissociation of myosin. Salts has always been used to increase the water holding capacity of meat, it improves the water binding capacity of the meat, (Puolanne , Ruusunen & Vainionpää, 2001).

Water holding capacity in meats is preferred and desired so as o allow the meat to remain in its freshness for a long duration of time. It is a crucial property as it offers desirable qualities and properties of meat. It is an advantages aspect of meat processing as meat tends to be of high humidity due to water compaction in the myosin segments.

Cured meat reactions revolve around the addition of nitrite or nitrate with other ingredients in the meat such as sugar, spices and even salt. Most commonly nitrite is added to meat, the nitrite compound causes a formation of meat myoglobin formation, which causes the formation of brown color in meat. Eventually, the brown cured meat forms nitro sylhemocrome which forms a pink colored pigment. The pink colour, however, fades off when the meat is exposed to light and air.

Conclusion 

From this experimental study, it is evident that water holding capacity in meats is very crucial. Ph and salt concentration in meat products play key roles in enhancing and reducing water holding capacity in meats. When meat is at isoelectric ph, water holding capacity tends to decrease water content in meats. Addition of salts to meat tends to an improved water holding capacity in the myofibril of meat hence improving the overall meat quality.

References 

Barbut, S., Zhang, L., & Marcone, M. (2005). Effects of pale, normal, and dark chicken breast meat on microstructure, extractable proteins, and cooking of marinated fillets. Poultry Science, 84(5), 797-802.

Domínguez, R., Gómez, M., Fonseca, S., & Lorenzo, J. M. (2014). Effect of different cooking methods on lipid oxidation and formation of volatile compounds in foal meat. Meat science, 97(2), 223-230.

Ke, Shuming, "Effect of pH and salts on tenderness and water -holding capacity of muscle foods" (2006). Doctoral Dissertations Available from Proquest. AAI3215890.
https://scholarworks.umass.edu/dissertations/AAI3215890

Lawrie, R. A., & Ledward, D. A. (2006). The structure and growth of muscle: Lawrie’s meat science.

Li, S., Xu, X., & Zhou, G. (2012). The roles of the actin-myosin interaction and proteolysis in tenderization during the aging of chicken muscle. Poultry science, 91(1), 150-160.

Lorenzo, J. M., Fonseca, S., Gómez, M., & Domínguez, R. (2015). Influence of the salting time on physico-chemical parameters, lipolysis, and proteolysis of dry-cured foal "cecina". LWT-Food Science and Technology, 60(1), 332-338.

Puolanne, E. J., Ruusunen, M. H., & Vainionpää, J. I. (2001). Combined effects of NaCl and raw meat pH on water-holding in cooked sausage with and without added phosphate. Meat Science, 58(1), 1-7.

Santos, C., Moniz, C., Roseiro, C., Tavares, M., Medeiros, V., Afonso, I., ... & da Ponte, D. J. (2016). Effects of Early Post-Mortem Rate of pH fall and aging on Tenderness and Water Holding Capacity of Meat from Cull Dairy Holstein-Friesian Cows. Journal of Food Research, 5(2), 1.

Sharedeh, D., Gatellier, P., Astruc, T., & Daudin, J. D. (2015). Effects of pH and NaCl levels in a beef marinade on physicochemical states of lipids and proteins and on tissue microstructure. Meat science, 110, 24-31.

Zhang, X., Wang, W., Wang, Y., Wang, Y., Wang, X., Gao, G., ... & Liu, A. (2018). Effects of nanofiber cellulose on functional properties of heat-induced chicken salt-soluble meat protein gel enhanced with microbial transglutaminase. Food Hydrocolloids.

Zhou, F., Zhao, M., Zhao, H., Sun, W., & Cui, C. (2014). Effects of oxidative modification on gel properties of isolated porcine myofibrillar protein by peroxyl radicals. Meat science, 96(4), 1432-1439.

Zhu, X., Ruusunen, M., Gusella, M., Zhou, G., & Puolanne, E. (2011). High post-mortem temperature combined with rapid glycolysis induces phosphorylase denaturation and produces pale and exudative characteristics in broiler Pectoralis major muscles. Meat Science, 89(2), 181-188.

Zhuang, H., & Savage, E. M. (2012). Postmortem aging and freezing and thawing storage enhance ability of early deboned chicken pectoralis major muscle to hold added salt water. Poultry science, 91(5), 1203-1209.

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