Notes on Comminution and Digestability

Notes on Comminution and Digestability
Eben van Tonder
11 Sptember 2021


I am now 52 years old and have two teeth missing and one is cracked which the dentist suggested I don’t have anything done too until I replaced the two missing ones with implants. It goes with the territory of being 50+! 🙂 I discovered that it is much harder for me to eat meat with two molars missing and one that I cant use to chew. I recently bought a liquidiser and started to make fruit and vegetable smoothies for lunch and supper. A world opened up for me!

After making smoothies morning, evening and at night for a few weeks now, I understand much more about bowl cutting. In fact, my first lesson teaching bowl cutting techniques will in the future start with a liquidiser. After a week, I had good cause to suspect that I digest my food much better than before which made me re-look at the relationship between comminution and digestibility. More broadly, I ask the question what are the different factors influencing digestibility from the perspective of food preparations.

I give the notes based on extensive quotes from two studies.

The Li Study (2017) on Pork Digestability

The first study I referenced was that of Li (2017) where they compared digestability when pork products were prepared by cooking, emulsifying in emulsified-type sausage, dry-cured and stewed pork. The pH was adjusted to 2 after which gastric pepsin and trypsin were used to digest. After incubation at 37oC for 2 hours, pH was adjusted to 7.5. The pork sausages were cooked at 72oC. “The in vitro digestibility was expressed as the percentage of the difference in protein contents before and after digestion.” (Li, 2017)

The results of the Li (2017) study is as follows:

Pork products were made with pork longissimus dorsi muscles from the same carcasses.

  • The four products evaluated showed a “significant differences in protein digestibility.”
  • Highest digestability was emulsion-type sausage at both conditions.
  • The lowest digestability was stewed pork, after pepsin digestion alone, followed by trypsin digestion,
  • Cooked pork and dry-cured pork had similar digestability after pepsin digestion. Dry-cured pork was lower in digestibility than cooked pork after trypsin digestion.

I give their methods of preparing the experiments with each relevant discussion point grouped together for each of the 4 preparation methods.


Preparation: Stewed pork was prepared according to the following formulations: pork muscle was vertically divided into strips (5 cm width) and cooked. The stewing was done as follows: Pork strips were blanched in boiling water for 5 min, chilled and cut into 5 × 5 × 5 cm cubes. The cubes were pan-fried (180 °C) for 5 min with soybean oil (10 g kg−1 of meat) on a pot-induction surface. The cubes were fried and turned twice at an interval of 60 s (skin side not fried) and then cooked in boiling water (water/meat: 1/4) for 5 min. After that, the cubes were stewed at 100 °C for 150 min. Eight replicates were applied for each product.

Discussion: “For stewed pork, long-time cooking may induce proteins to oxidation and aggregation that affects proteolytic susceptibility.” In the present study, stewed pork was cooked at higher temperature and for much longer time than the other three pork products, which may cause a higher level of protein oxidation and aggregation, and lower digestibility. The difference in digestibility between Bax et al. (2012) and the present study could be attributed to distinct cooking time (0.5 h vs. 1.5 h).

Dry Cured Pork

Preparation: Dry-cured pork was prepared as follows: curing with 5% salt and sun-drying for one month. The dry-cured pork was softened in hot water and cooked to the center temperature of 72 °C.

Discussion: For dry-cured pork, salting and drying are two critical steps, during which protein surface hydrophobicity increases and dehydration, oxidation and aggregation occur as well.

Cooked Pork

Preparation: Cooked pork was prepared according to the following steps: pork muscle was cut vertically into 15 × 10 × 5 cm pieces that were packed in retort pouch and directly cooked in water bath till the center temperature reached 72 °C.

Discussion: Cooking temperature has a distinct influence on the proteolytic susceptibility of myofibrillar proteins to digestive enzymes. At 70 °C, moderate denaturation happens to meat proteins with the exposure of more protein cleavage sites accessible to digestive enzymes. However, protein oxidation and aggregation would increase at 100 °C or higher temperatures, which is a condensing effect (Promeyrat, Bax, Traore, Aubry, Sante-Lhoutellier & Gatellier, 2010), but meat protein overall digestibility would be improved at high temperature (Bax et al., 2012).

Emulsion Sausages

Preparation: Emulsion-type sausage was prepared according to the following formulation: pork muscle and back fat at a ratio of 4 to 1, salt (1.8%) and tripolyphosphate (0.4%). Meat and fat were chopped using a high-speed chopper during which salt and tripolyphosphate were mixed, and the batter was stuffed into 48-mm-diameter plastic casings. The sausages were cooked till the centre temperature reached 72 °C.

Discussion: Although emulsion-type sausage, dry-cured pork and cooked pork were cooked at the same temperature (70 °C), an emulsifying system was formed during the preparation of emulsion-type sausage, and the fat droplets around muscle fibres would decrease protein oxidation and aggregation (Youssef, Barbut, & Smith, 2011), and thus increase the digestibility.

Conslusion from Li (2017) Study

The Li (2017) study introduced me to factors impacting digestibility such as oxidation and aggregation along with the fascinating effect of fat on this process in fine comminuted products. Further, cooking times and temperatures and their impact on aggregation and compacting as temperatures rich 100oC. I have an interesting story about this. A few months ago I was testing an emulsion type sausage and the cooking pot malfunctioned, boiling the sausage at 100oC for half an hour. When I opened the cooking pot and discovered this the natural hog casings I used were all cooked off but amazingly the sausages aggregated and compacted to such an extent that the sausages were all still almost perfectly intact.

Casing cooked off at 100o C for 30 minutes. A good example of aggregation.

Casing cooked off at 100o C for 30 minutes. A good example of aggregation.

When I repeated this at 72oC I did not nearly have the same “compacting” effect. The products were made with 45% MDM and 10% product which I made from beef hide. The sausages that I made with the same inclusion of MDM but 10% product I made from collagen did not have the same dramatic effect of compacting and aggregation. By adding these materials I obviously moved outside a direct application of the Li (2017) study, but the observations are nevertheless fascinating and the points of aggregation and compacting are well demonstrated.

Farouk (2019) Study on Beef-Centric Meals Digestability

I give the different aspects they investigated and their conclusions.

Animal Age

Beef protein was highly digestible regardless of the age of animal from which the meat was collected (4-day-old calf, 18- to 24-month-old bull, or 6-year-old cow).

Rigor State

The time of sampling of LD muscle (prerigor, from 50 min through 200 min postmortem) had little influence on the digestibility of beef proteins, and this was not markedly affected by rigor at 48 h.

Ultimate pH

The proteins of high ultimate pH meat digested faster than their low ultimate pH equivalent. Densitometry measurements of each gel lane were used to calculate digestion efficiencies and rates for each of the treatment combinations. Physiological and biochemical mechanisms underpinning the greater digestibility of high ultimate pH beef have been discussed by Farouk et al..

Muscle/ Meat Cuts

While there were few differences up to 5 min, by 60 min supraspinatus appeared more digested (fewer and fainter protein bands), suggesting that this cut might be faster and more thoroughly digested.

Mincing/Particle Size

There was little effect of particle size on the digestibility of cooked proteins in meat. Exposure of meat proteins to pepsin activity in vitro should have been much greater for the finely milled substrate, yet this did not markedly influence the rate or extent of proteolysis. Note that the samples were milled and not finely chopped in a bowl cutter or similar. Particle size was therefore still relatively large,

Organ Meats

The structure and composition of organ meats is substantially different from muscle meat, and this has consequences for digestion. For instance, the protein content of the heart, kidney and spleen from prime steers was 10–27% less on a fresh-weigh basis. Digestion commenced at a significantly faster pace for the kidney and liver compared to muscle, when evaluated as the relative digestibility of T5. The lower molecular weight and globular nature of the kidney and liver proteins likely contribute to their faster in vitro digestibility.

Discussion: Animal organ meat, sometimes referred to as offal or the fifth quarter of a carcass, is only a minor contribution to typical Western diets for a variety of reasons, thus missing out on its potential culinary and nutritional values. Connective tissue substances are resistant to in vitro digestion with pepsin and so lowered the total relative peptic digestibility of calf beef compared to the older cattle. They were more digested (less intense) in prerigor bull beef than in 48 h postrigor, in less collagenous cuts compared to higher rcontaining cuts, and in finely ground milled meat compared to coarsely smashed meat.

Meat Accompaniments

An online survey of menus from New Zealand and Australia restaurants revealed that the most common accompaniments served with red meat were potato, onion, mushroom, tomato, rice, noodle, bean, and carrot. This is varied slightly by country and markedly by cuisine. For instance, in New Zealand, noodle, rice, and bean were more popular at Asian restaurants, while potato, mushroom, and tomato were more popular with European cuisine.

SDS PAGE separation of proteins and peptides in the digesta of beef cooked with the top five accompaniments plus pumpkin showed that meats from all three age categories of animals (4-day-old calf, 18- to 24-month-old bull, or 6-year-old cow) were most digestible when cooked with mushroom, whereas digestion was least efficient when the meats were cooked with rice and potatoes. Based on relative digestibility calculation and averaging over all animal ages, the rank order of protein digestibility was found to be mushroom > pumpkin > onion = tomato > rice > potato.

Bull meat cooked with mushrooms was very effective in promoting digestion through the gastric and intestinal phases. In contrast, meat cooked by itself did not digest completely even after 240 min.

Enhanced digestion from cooking with mushroom (and pumpkin) could be due to the presence of endogenous proteolytic enzymes in these vegetables that were not present in the other accompaniments.

Conclusions from the Farouk (2019) Study

Prerigor, low collagen supraspinatus muscle finely ground prior to cooking would be judged more digestible than the alternatives in these experiments.

Sustainable production of animals as a source of food demands that we make full use of every carcass. Unlocking the potential of the less familiar cuts and promoting their inherent benefits is an important role for nutritional research. Beef organ meats/ofals such as liver and kidney were more digestible than muscle meat from the same carcass.  This suggests new opportunities for organ meat as a versatile ingredient, perhaps by formulating highly digestible animal protein foods for infants with less developed GIT or for elder consumers with compromised GIT function.  The soft texture and minimal myofribril content of the liver and kidney also offer functionality. These could be a valuable resource for the 1st and 3rd age consumer groups who struggle with chewing and swallowing muscle meat.

Well-informed combining can also produce beneficial biochemical synergies. For instance, consuming orange juice that contains ascorbic and citric acids will enhance the bioavailability of ferric iron in plant foods. It is possible that some accompaniments affect the digestion of food and so might be chosen to optimise benefits for a particular consumer or to better suit an occasion.

Regarding the effect of cooking, it is important to note that cooking meat on its own has variable effects on meat digestibility depending on both temperature and time. For instance, peptic digestibility of beef is lowered, and pancreatic digestibility is enhanced when meat is cooked quickly to 100°C, with longer cooking at the same temperature reducing overall susceptibility of meat proteins to proteolytic enzymes; cooking pork mildly at 70°C enhanced peptic digestion, while at 100°C slowed peptic digestion. In the present study, the combined meat and accompaniments were cooked at 100°C; cooking at this temperature with some of the accompaniments improved the digestibility of muscle meat from animals of all ages; Mushroom affected the bull beef. Note that even the resistant proteins near 42–40 kDa were digested by T30. A zymogram of the enzymes in extracts of accompaniments revealed proteolytic enzymes in mushroom and pumpkin. These enzymes may be contributing to digestibility. Mushroom and pumpkin are known to contain proteolytic enzymes, but their effects on wholetissue digestion had not been demonstrated.

Within the parameters of the present study, beef was observed to be more digestible or digested faster when it came from an older animal, at prerigor, and when it had high ultimate pH or contained less collagen content. Some beef organ meats were more digestible than beef muscle. Digestibility improved when meat was cooked with vegetables that contain proteolytic enzymes and diminished slightly with carbohydrate-rich or starchy foods such as rice and potatoes.


Factors Affecting the Digestibility of Beef and Consequences for Designing Meat-Centric Meals. Farouk, M. F., Wu, G., Frost, D. A., Staincliffe, M., and Knowles, S. O..

Li, L., Liu, Y,. Zou, X., He, J., Xu, X., Zhou, G., Li, C.. 2017. In vitro protein digestibility of pork products is affected by the method of processing. Food Research International 92 (2017) 88–94; Elsevier.

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